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BIO 342  
Comparative Vertebrate Anatomy  
Lecture Notes 2 - Vertebrate Skeletal Systems


  • inorganic components of bone comprise 60% of the dry weight (largely calcium hydroxy-appetite crystals) & provide  the compressive strength of bone. The organic component is primarily collagen, which gives bone great tensile strength.
  • provides support and movement via attachments for soft tissue and muscle, protects vital organs, is a major site for red marrow for production of blood cells, and plays a role in the metabolism of minerals such as calcium and phosphorus.
  • There are two basic structural types of bone, compact and spongy. Compact bone forms the outer shell of all bones and also the shafts in long bones. Spongy bone is found at the expanded heads of long bones and fills most irregular bones. .

Bone formation begins with a blastema (any aggregation of embryonic mesenchymal cells which will differentiate into tissue such as muscle, cartilage, or bone). These cells then develop into either FIBROBLASTS or OSTEOBLASTS. Fibroblasts form collagen; osteoblasts form bone cells. Together, these form MEMBRANE BONE (bone deposited directed in a blastema).  

Intramembranous ossification is the process of membrane bone formation. This process give rise to:

  • bones of the lower jaw, skull, & pectoral girdle
  • dentin & other bone that develops in the skin
  • vertebrae in some vertebrates (teleosts, urodeles, & apodans)

Endochondral ossification is the process in which bone is deposited in pre-existing cartilage, & such bone is called REPLACEMENT BONE.


Skeletal elements:

  • Dermal skeleton
    • skin of most living vertebrates has no hard skeletal parts but dermal bone elements are usually present in the head region
    • early vertebrates (ostracoderms) had so much dermal bone they were called 'armored fishes'


    • after ostracoderms, fish continued to develop much bone in skin but that bone has become 'thinner' over time
  • Endoskeleton
    • Somatic - axial & appendicular skeletons
    • Visceral - cartilage or bone associated with gills & skeletal elements (such as jaw cartilages) derived from them


Dermal bone of fishes:

  • Basic structure includes lamellar (compact) bone, spongy bone, dentin, &, often, a surface with a layer of enamel-like material
  • Evolutionary 'trend' = large bony plates giving way to smaller, thinner bony scales

Also check:

1 = lamellar bone, 2 = spongy bone, 3 = dentin, 4 = enameloid, & 5 = fibrous plate (collagen)  


Tetrapods - retain dermal elements in the skull, jaws, & pectoral girdle

Somatic skeleton = axial skeleton (vertebral column, ribs, sternum, & skull) + appendicular skeleton

 Vertebral column:

  Vertebrae - consist of a centrum (or body), 1 or 2 arches, plus various processes

  • Amphicelous
    • concave at both ends
    • most fish, a few salamanders (Necturus), & caecilians  
  • Opisthocoelous
    • convex in front & concave in back
    • most salamanders
  • Procelous
    • concave in front & convex in back
    • anurans & present-day reptiles
  • Acelous
    • flat-ended
    • mammals
  • Heterocelous
    • saddle-shaped centrum at both ends
    • birds


Vertebral arches:

  •  Neural arch - on top of centrum
  •  Hemal arch (also called chevrons) - beneath centrum in caudal vertebrae of fish, salamanders, most reptiles, some birds, & many long-tailed mammals  

Vertebral processes:

  • projections from arches & centra
  • some give rigidity to the column, articulate with ribs, or serve as sites of muscle attachment  

Transverse processes - most common type of process; extend laterally from the base of a neural arch or centrum & separate the epaxial & hypaxial muscles

Diapophyses & parapophyses - articulate with ribs

Prezygapophyses (cranial zygapophyses) & postzygapophyses (caudal zygapophyses) - articulate with one another & limit flexion & torsion of the vertebral column  

Vertebral columns:

  • Cartilaginous fishes
    • do not have typical fish vertebral columns
    • vertebrae include neural arches (cartilaginous dorsal plates) & dorsal intercalary plates are located between successive arches
  •  Teleosts
    • well-ossified amphicelous vertebrae
    • notochord persists within each centrum (but constricted)
    • neural arch associated with each centrum & hemal arches in tail (caudal) vertebrae  
  • Chondrosteans (sturgeons & paddlefish) & modern lungfishes
    • incomplete centra
    • notochord is not constricted
    • cartilage deposited in notochord sheath provides structural support  

Diplospondyly = 2 centra and 2 sets of arches per body segment; occurs in some fish (including sharks)

Agnathans - only skeletal elements associated with the notochord are paired, lateral neural cartilages

Vertebral columns of tetrapods

  • Cervical region
    • Amphibians - single cervical vertebra; allows little head movement
    • Reptiles - increased numbers of cervical vertebrae (usually 7) & increased flexibility of head
    • Birds - variable number of cervical vertebrae (as many as 25 in swans)
    • Mammals - usually 7 cervical vertebrae
    • Reptiles, birds, & mammals - 1st two cervical vertebrae are modified & called the atlas & axis
      • atlas - 1st cervical vertebra; ring-like (most of centrum gone); provides 'cradle' in which skull can 'rock' (as when nodding 'yes')
      • axis - 2nd cervical vertebra
    •   Transverse foramen (#6 in above caudal view of a cervical vertebra)
      • found in cervical vertebrae of birds & mammals
      • provides canal for vertebral artery & vein  
  • Dorsal region
    • Dorsals - name given to vertebrae between cervicals & sacrals when all articulate with similar ribs (e.g., fish, amphibians, & snakes)
    •  Crocodilians, lizards, birds, & mammals - ribs are confined to anterior region of trunk
  • Sacrum & Synsacrum
    • sacral vertebrae - have short transverse processes that brace the pelvic girdle & hindlimbs against the vertebral column
      • Amphibians - 1 sacral vertebra
      • Living reptiles & most birds - 2 sacral vertebrae
      • Most mammals - 3 to 5 sacral vertebrae
    • Sacrum - single bony complex consisting of fused sacral vertebrae; found when there is more than 1 sacral vertebra (see examples below):
    • Synsacrum
      • found in birds
      • produced by fusion of last thoracics, all lumbars, all sacrals, & first few caudals
      • fused with pelvic girdle
      • provides rigid support for bipedal locomotion


  • Caudal region
    • Primitive tetrapods - 50 or more caudal vertebrae
    • Present-day tetrapods
      • number of caudal vertebrae is reduced
      • arches & processes get progressively shorter (the last few caudals typically consist of just cylindrical centra as shown below)
    • Anurans - unique terminal segment called the urostyle (section of unsegmented vertebral column probably derived from separate caudals of early anurans)
    • Birds - last 4 or 5 caudal vertebrae fused to form pygostyle (see drawing above)
    • Apes & humans - last 3 to 5 caudal vertebrae fused to form coccygeal (or tail bone)


  • Unlike most tetrapods today, vertebral column of earliest tetrapods did not consist of 1 bone/body segment.
  • Crossopterygian vertebrae consisted of an hypocentrum (a large, wedge-shaped piece) plus 2 pleurocentra (smaller, intersegmental pieces). This type of vertebra is called a rachitomous vertebra.
  • The 'trend' in vertebra evolution has been for pleurocentra to increase in size (and, of course, for the hypocentrum to decrease in size). This trend is apparent in this diagram:

Cross-hatched areas = hypocentrum; black areas = pleurocentrum.

Ribs - may be long or short, cartilaginous or bony; articulate medially with vertebrae & extend into the body wall

  • A few teleosts - have 2 pair of ribs for each centrum of trunk (dorsal rib separates epaxial & hypaxial muscles)  
  • Most teleosts -  ventral ribs only
  • Sharks - dorsal ribs only
  • Agnathans - no ribs
  • Tetrapods - ribs usually articulate with vertebrae in moveable joints (see above drawing)
    • Early tetrapods - ribs articulated with every vertebra from the atlas to the end of the trunk
    • Later tetrapods - long ribs limited to thoracic region
      • Thoracic ribs - most composed of a dorsal element (vertebral rib) & a ventral element (sternal rib)
      • Sternal rib - may be ossified (birds) or remain cartilaginous (mammals); usually articulate with sternum (except 'floating ribs')
      • Uncinate processes - found in birds; provides rib-cage with additional support

Sternum - strictly a tetrapod structure &, primarily, an amniote structure.

  • Amphibians - no sternum in early amphibians &, among present-day amphibians, only anurans have one
  • Amniotes
    • sternum is a plate of cartilage & replacement bone
    • sternum articulates with the pectoral girdle anteriorly & with a variable number of ribs

Useful link:

BIO 342  
Comparative Vertebrate Anatomy  
Lecture Notes - Skeletal System II (Skull)

The Vertebrate Skull consists of: 

  1 - neurocranium (also called endocranium or primary braincase)

  2 - dermatocranium (membrane bones)

  3 - splanchnocranium (or visceral skeleton)  


1 - protects the brain

2 - begins as cartilage that is partly or entirely replaced by bone (except in cartilaginous fishes)  

    •  Cartilaginous stage:
      • neurocranium begins as pair of parachordal & prechordal cartilages below the brain
      • parachordal cartilages expand & join; along with the notochord from the basal plate
      • prechordal cartilages expand & join to form an ethmoid plate
      • Cartilage also appears in the
        • olfactory capsule (partially surrounding the olfactory epithelium)
        • otic capsule (surrounds inner ear & also develops into sclera of the eyeball)
      • Completion of floor, walls, & roof:
        • Ethmoid plate - fuses with olfactory capsules
        • Basal plate - fuses with otic capsules
      • Further development of cartilaginous neurocranium = development of cartilaginous walls (sides of braincase) &, in cartilaginous fishes, a cartilaginous roof over the brain

Cartilaginous fishes - retain a cartilaginous neurocranium (or chondrocranium) throughout life

Bony fishes, lungfishes, & most ganoids - retain highly cartilaginous neurocranium that is covered by membrane bone

Cyclostomes - the several cartilaginous components of the embryonic neurocranium remain in adults as more or less independent cartilages


Other bony vertebrates - embryonic cartilaginous neurocranium is largely replaced by replacement bone (the process of endochondral ossification occurs almost simultaneously at several ossification centers)  

Neurocranial ossification centers: 

1 - occipital centers

  • cartilage surrounding the foramen magnum may be replaced by as many as four bones:
  • Mammals - all 4 occipital elements typically fuse to form a single occipital bone (pictured below)
  • Tetrapods - neurocranium articulates with the 1st vertebra via 1 (reptiles and birds) or 2 (amphibians and mammals) occipital condyles (see human skull below)  

2 - Sphenoid centers form:

  • basisphenoid bone (anterior to basioccipital)
  • presphenoid bone
  • side walls above basisphenoid & presphenoid form:
    • orbitosphenoid
    • pleurosphenoid
    • alisphenoid


3 - Ethmoid centers tend to remain cartilaginous & form

  • anterior to sphenoid
  • cribiform plate of ethmoid & several conchae (or ethmoturbinal bones)

The ethmoid region is clearly visible within the bisected skull above. In most mammals, the nasal chamber  
is large & filled with ridges from the ethmoid bones called the turbinals or ethmoturbinals. These bones are covered with  
                  olfactory epithelium in life and serve to increase the surface area for olfaction (i.e., a more acute sense of smell).  
                  Another ethmoid bone, the cribiform plate, separates the nasal chamber from the brain cavity within the skull.

4 - Otic centers - the cartilaginous otic capsule is replaced in lower vertebrates by several bones:

  • prootic
  • opisthotic
  • epiotic  
  • One or more of these may unite with adjacent replacement or membrane bones:
    • Frogs & most reptiles - opisthotics fuse with exoccipitals
    • Birds & mammals - prootic, opisthotic, & epiotic unite to form a single petrosal bone; the petrosal, in turn, sometimes fuses with the squamosal to form the temporal bone


DERMATOCRANIUM - lies superficial to neurocranium & forms:

1 - bones that form the roof of the brain & contribute to the lateral walls of the skull

2 - bones of the upper jaw

3 - bones of the palate(s)

4 - opercular bones

 Basic pattern of "roofing bones":

  • crossopterygians - a series of paired & unpaired bones along mid-dorsal line of skull (below left)
  • labyrinthodonts - unpaired bones lost & a series of paired bones resulted (nasals, frontals, parietals, & dermoccipitals) (below right)
  • Fontanels = 'soft spots'
    • occur when neurocranium is incomplete dorsally (e.g., teleosts & tetrapods)
    • can be felt in head until the membranes under the skin have ossified

Bones of the upper jaw

  • Pterygoquadrate (palatoquadrate) cartilage - 1st upper jaw that vertebrate embryos develop
    • Cartilaginous fishes - palatoquadrate is the only upper jaw that develops
    • Bony vertebrates - the palatoquadrate becomes covered with dermal bones (premaxillae & maxillae) that make up the adult upper jaw  

Palatal bones - the floor on which the brain rests is at the same time the roof of the oral cavity in fishes & amphibians (primary palate)

  • Sharks - cartilaginous
  • bony vertebrates - membrane bones form
  • Birds, mammals, & some reptiles - a secondary ('false') palate develops creating a horizontal partition that separates the oral cavity into nasal & oral passages. The secondary palate is formed from processes of the premaxillae, maxillae, and palatines.

Opercular bones

  • Operculum = fold of the hyoid arch that extends back over the gill slits in holocephalans & bony fishes
  • Tetrapods - no vestiges of opercular bones remain

BIO 342  
Comparative Vertebrate Anatomy  
Lecture Notes 4 - Skeletal System III


  • skeleton of the pharyngeal arches
    • Fishes - skeleton of the jaws & gill arches
    • Tetrapods - skeleton modified for new functions

Fish visceral skeleton - consists of 7 sets of paired cartilages in the 7 visceral arches & a series of mid-ventral cartilages (basihyal & basibranchials) in the pharyngeal floor


Bony fishes

  • visceral skeleton resembles that of sharks except that bone is added
  • caudal ends of the cartilaginous pterygoquadrate undergo endochondral ossification & become the quadrate bones. The remainder becomes the palatine & pterygoid bones. The posterior tip of Meckel's cartilage becomes an articular bone. (See

    Feeding movements in many bony fishes -> cranial kinesis (see Figure 9.23, p.351 of text)

    Cranial kinesis:

  • movement between the upper jaw and braincase
  • advantages:
    • provides a way to change the size and configuration of the mouth rapidly
    • optimize biting and rapid feeding.
  • disadvantages: lose force, difficult to optimize apposition of occlusive surfaces.

See also: Shark Jaw Movement


  • visceral skeleton unlike that of jawed fishes
  • Hagfishes - no identifiable pterygoquadrate or Meckel's cartilage

Jaw suspension of fishes

  • The jaw-hyoid complex of fishes requires bracing against some support to function effectively, and the nearest one is the neurocranium (endocranium).
  • Types of suspensions:
    • autostyly  (below left) - hyomandibula play no role in bracing the jaws (lungfish & tetrapods)
    • amphistyly (below middle) - jaws & hyomandibula both braced directly against the braincase (extinct sharks)
    • hyostyly (below right) - mandibular cartilage is braced against the otic capsule; jaws braced against hyomandibula (sharks & present-day bony fishes)


TETRAPODS - With life on land (& pulmonary respiration), the visceral skeleton underwent substantial modification. Some structures were lost & others remained to perform new functions.

  • Pterygoquadrate (palatoquadrate) cartilage = embryonic upper jaw cartilage
    • Amphibians, reptiles, & birds - posterior end undergoes endochondral ossification & becomes the quadrate (which articulates with the articular bone of the lower jaw)


    • Mammals - dentary (lower jaw) articulates with the squamosal of skull (quadrate separates from the rest of the palatoquadrate & becomes the incus of the middle ear)
  • Meckel's cartilage
    • Reptiles - largely ensheathed by dermal bones (as in the above turtle)
    • Birds & mammals - few or no remnants in adult lower jaw (&, in mammals, the articular, formed by ossification of the tip of Meckel's cartilage, projects into the middle ear cavity & becomes the malleus)


  • Arch II  = Hyomandibular cartilage:
    • Sharks - interposed between quadrate region & otic capsule
    • Tetrapods - no longer articulates with quadrate & ossifies to become part of the stapes (columella)
  • Arches III ---> V become part of hyoid apparatus
  • Arches VI & VII - not present in tetrapods


Hyoid apparatus of tetrapods

  • consists of a body & 2 or 3 horns (cornua)
  • anchors tongue, provides attachment for some extrinsic muscles of larynx, & is site of attachment of muscles that aid in swallowing

Lower jaw

  • may have originated as part of a visceral arch, as in sharks (mandibular cartilage)
  • in bony vertebrates, mandibular cartilage is reinforced & largely replaced by a series of dermal bones



BIO 342  
Comparative Vertebrate Anatomy  
Lecture Notes 5 - Skeletal System IV  
Appendicular Skeleton

Appendicular skeleton

  • consists of pectoral & pelvic girdles plus skeleton of fins & limbs
  • Some vertebrates have no appendicular skeleton (e.g., agnathans, apodans, snakes, & some lizards) & in others it is much reduced.

Pectoral girdles:

1 - brace for anterior appendages

2 - consist of membrane & replacement bones (in bony vertebrates)

3 - Early fishes had 3 replacement bones (coracoid, scapula, & suprascapula) and a series of dermal bones (clavicle, cleithrum, supracleithrum, and post-temporal)

4 - Later bony fishes (ganoid fish) - tendency for reduction in number and size of replacement bones

5 - Tetrapods - tendency for reduction in number of dermal bones


Bony fishes - pectoral girdles of living bony fishes have reduced coracoid & scapula (replacement bone) but large cleithrum & supracleithrum (dermal bone). A posttemporal bone (dermal) connects the supracleithrum to the skull.


 Cartilaginous fishes - no dermal bone 


Tetrapods - early ones had pectoral girdle similar to those of early bony fishes, but lost posttemporal & acquired interclavicle    (which still occurs in several amniotes, e.g., alligator, birds, & monotremes)

Clavicle & coracoid - one or both typically brace scapula against sternum (as in birds; below)

Scapula - present in all tetrapods with even vestiges of anterior limbs, e.g., turtles & birds & mammals  

Pelvic girdles

  • brace posterior paired appendages
  • no dermal components (unlike pectoral girdle)

Fishes - pelvic girdle consists of 2 cartilaginous or bony plates (ischiopubic plates) that articulate with the pelvic fins



Tetrapods - pair of cartilaginous plates form in embryos & each ossifies at 2 centers to form: pubis & ischium. An additional blastema gives rise to the ilium.  

Frogs & toads

  • ilia elongated & extend from sacral vertebra to urostyle
  • joint between ilium & sacral vertebra (sacroiliac) is freely moveable (& moves when a frog or toad jumps)



  • ilium & ischium expanded to accommodate musculature needed for bipedal locomotion
  • girdle is braced against lumbar & sacral vertebrae
  • pubic bones are typically reduced (long but thin); the limited pubic symphysis provides a larger outlet for eggs

Mammals - ilium, ischium, and pubis unite to form the innominate bone (the 2 innominates = pelvic girdle)


  • All jawed fishes (except eels) have pectoral & pelvic fins
  • Fins are used primarily for steering ('rudders')
  • Types of fins:
    • lobed fins - found in sarcopterygians
    • fin fold fins (see diagram below)
      • found in cartilaginous fish
      • consist of 1 to 5 basal cartilages plus several rows of radials
    • ray fin - tendency to lose proximal components of fin skeleton (see diagram below)


  • Starting with amphibians, vertebrates typically have 4 limbs. However, some have lost one or both pairs &, in others, one pair is modified as arms, wings, or paddles
  • typically have 5 segments:
    • Anterior limb
      • brachium (upper arm) - consists of humerus
      • antebrachium (forearm) - consists of radius & ulna
      • carpus (wrist) - consists of carpals
      • metacarpus (palm) - consists of metacarpals
      • digits - consist of phalanges
    • Posterior limb
      • femur (thigh) - consists of femur
      • crus (shank) - consists of tibia & fibula
      • tarsus (ankle) - consists of tarsals
      • metatarsus (instep) - consists of metatarsals
      • digits - consist of phalanges


Some vertebrates lack both pairs of limbs

  • caecilians (apodans)
  • most snakes
  • snake-like lizards

Some vertebrates have forelimbs only:

  • manatees & dugongs
  • dolphins (see diagram below)
  • cetaceans (vestigial elements may be embedded in body wall)
  • sirens (salamander)

Early tetrapods - limbs were short & first segment extended straight out from the body . This posture persists among lower tetrapods (e.g, see the alligator below), but, in birds & mammals, there has been a rotation of the appendages so that the long axis of the humerus & femur more nearly parallels the vertebral column.

  • Arm & forearm
    • Upper arm = humerus
    • Forearm = radius & ulna
    • Manus (or hand)
      • Wrist - 3 rows of carpal bones:
        • proximal row = radiale, ulnare, intermedium, & pisiform
        • middle row = 3 central carpals (centralia)
        • distal row = 5 distal carplas numbered 1 through 5 (starting on thumb, or radial, side)
      • Palm - metacarpals
      • Digits
        • each consists of a series of phalanges
        • general ‘formula' (starting at thumb) = 2,3,4,5,3

Adaptive modifications of the manus  
  1 - Flight

  • Birds - loss of digits & bones plus fusion of some bones
  • Bats - 5 digits; elongated metacarpals (II through V) & phalanges support the patagium
  • Pterosaurs - 4th digit elongated to support patagium


2 - Swimming - increase in number, & size, of phalanges  

3 - Terrestrial locomotion (walking & running):

      •   Plantigrade
        • flat-footed
        • all bones of manus and/or pes on the ground
        • amphibians, most reptiles (see alligator photo above), & some mammals (insectivores, monkeys, apes, humans, & bears)
      • Digitigrade
        • 1st digit is reduced or lost
        • manus & pes are elevated
        • rabbits, rodents, & many carnivores
      • Unguligrade
        • reduced number of digits
        • walk on tips of remaining digits
        • claws become hooves


As the fastest North American mammal, pronghorn antelope (unguligrades) can reach speeds of 60 miles per hour.  
At high speed they cover the ground in strides of 14 to 24 feet, and are known to run for long distances at speeds of 30 to 40 miles per hour.

4 - Grasping

    • opposable thumb
      • saddle joint at base of thumb where it meets palm
      • thumb at wider angle from index finger
      • strong thumb muscles

Posterior limbs - bones are comparable to those of forelimbs except that a patella (‘kneecap') develops in birds & mammals

Origin of fins:

 1 - Fin fold hypothesis - fins derived from continuous fold of body wall


 2 - Gill arch hypothesis - fins derived from the last 2 gill arches (very unlikely)

 3 - Fin spine hypothesis - fins derived from tissue attached to spines (that may have evolved to provide protection from predators)



Origin of limbs - derived from paired fins of ancient fishes:

Used by permission of John W. Kimble  

The first tetrapods, Labyrinthodont amphibians (right), probably evolved from a Crossopterygian ancestor (left).  
When the fresh water pools in which these fish lived became stagnant, they may have crawled up the bank to breath air using  
primitive lungs. As the lobed fins of these fish evolved into stronger limbs, the first tetrapods appeared.

Comparison of paired anterior fins of lobe-finned fishes (A-D) and limbs of early tetrapods (E, F)

A. Sterropterygion, B. Sauripterus,   
C. Panderichthys, D. Eusthenopteron,   
E. Ichthyostega, F. Acanthostega 

Here we have a possible explanation for the formation of a new morphological feature - limbs with digits - from the paired fins of sarcopterygian (lobe finned) fishes. Some of these fish (notably Eusthenopteron) have bones in their paired fins that are very similar to the bones of tetrapod limbs. Specifically, they have a single bone (similar to the humerus or femur) followed by paired bones (similar to the radius and ulna or fibula and tibia of tetrapods). Scientists think Eusthenopterons used their limbs to walk on the sea (or lake or river) bed.   
What Eusthenopteron lacks are digits - having fin rays instead (although Sauripterus is a closely related fish that does have 8 digits just like the earliest amphibians - see the illustration). However, the fossil record supplies us with examples of tetrapods that are quite similar to Eusthenopteron - Acanthostega and Icthyostega. 


BIO 342  
Comparative Vertebrate Anatomy  
Lecture Notes 6 - Muscular System

Vertebrate muscles:

  • striated vs. smooth
  • voluntary vs. involuntary
  • skeletal vs. non-skeletal

Skeletal muscle (left) & Smooth muscle (right)

Skeletal muscle = muscles attached to the skeleton that are striated & voluntary

Non-skeletal muscle = muscles not attached to the skeleton; most are smooth & involuntary  

Vertebrate Muscles:

1 - Skeletal, striated, voluntary muscles

    • axial
    • appendicular
    • branchiomeric (homologous to the branchial/ pharyngeal muscles from fishes to mammals, striated muscles, innervated by cranial nerves)
    • integumentary

2 - Non-skeletal, smooth, chiefly involuntary muscles

3 - Cardiac muscle

4 - Electric organs

Skeletal muscles have muscular & tendinous portions:

  • Muscle - consists of skeletal muscle cells (which, in turn, consist of myofibrils and myofilaments)
  • Tendons - extensions of a muscle's tough connective tissue sheath (fascia & epimysium) that anchor a muscle to its origin & insertion
    • Origin = site of attachment that is relatively fixed
    • Insertion = site of attachment that is normally displaced by contraction of the muscle

Used with permission of John W. Kimball

Names of skeletal muscles are based on:


Human muscles were named several hundred years ago, & many of these names are still used. Based on similarities of origins & insertions, these names were subsequently used for the, apparently, corresponding muscles of other vertebrates. However, origins & insertions are not reliable criteria for determining homology because natural selection has sometimes favored 'shifts' in muscle position. More reliable criteria for determining homologies are:

    • embryonic origin
    • nerve supply


Axial Muscles:

  • include the skeletal muscles of the trunk & tail
  • extend forward beneath the pharynx as hypobranchial muscles & muscles of the tongue
  • are present in orbits as extrinsic eyeball muscles (check slide 27 in this powerpoint presentation)
  • are metameric (most evident in fish and aquatic amphibians where the axial muscles are used in locomotion; in other tetrapods, metamerism is obscured due to presence of paired appendages responsible for locomotion on land)
  • are segmental because of their embryonic origin; arise from segmental mesodermal somites


Axial musculature of an aquatic salamander, Necturus maculosus. The layers of lateral hypaxial musculature are exposed from superficial to deep in the cranial to caudal direction. The number of external oblique layers varies between one and two in this species (the figured specimen exhibits two). Abbreviations: oes, M. obliquus externus superficialis; oep, M. obliquus externus profundus; oi, M. obliquus internus; ta, M. transversus abdominis (Brainerd and Simons 2000).

Trunk & tail muscles of fish:

Axial musculature consists of a series of segments (myomeres) separated by myosepta

    • Myosepta serve as origins & insertions for segmented muscles
    • Myomeres are divided into dorsal & ventral masses by a horizontal septum that extends between the transverse processes of the vertebrae:
      • Epaxials = above the septum
      • Hypaxials = below the septum
    • Middorsal & midventral septa separate the myomeres of the 2 sides of the body. The midventral septum is called the LINEA ALBA.

Trunk & tail muscles of tetrapods

  • Tetrapods, like fish, have epaxial & hypaxial masses, & these retain some evidence of metamerism even in the highest tetrapods.
  • Modifications:

1 - epaxials are elongated bundles that extend through many body segments & that are located below the expanded appendicular muscles required to operate the limbs

2 - hypaxials of the abdomen have no myosepta & form broad sheets of muscle

3 - hypaxials are oriented into oblique, rectus, & transverse bundles

Epaxials of tetrapods:

  • lie along vertebral column dorsal to transverse processes & lateral to neural arches
  • extend from base of the skull to tip of the tail
    • Urodeles & some lizards - epaxials are obviously metameric & are referred to as the dorsalis trunci (see salamander above)
    • Higher tetrapods - superficial epaxial bundles form long muscles that extend over many body segments; deep bundles are still segmented

Longest bundles:

1- longissimus group

          • lies on transverse processes of vertebrae; includes the longest epaxial bundles
          • subdivisions include:
            • longissimus dorsi
            • longissimus cervicis
            • longissimus capitis

2 - iliocostalis group

          • lateral to longissimus & spinalis
          • arises on ilium & inserts on dorsal ends of ribs or uncinate processes

3 - spinalis group

          • lies close to neural arches
          • connects spinous processes or transverse processes with those several vertebrae anteriorly

Shortest bundles - intervertebrals

        • remain segmented
        • connect processes (spinous, transverse, & zygapophyses) of adjacent vertebrae

Hypaxials of tetrapods:

1 - Muscles of lateral body wall:

    • oblique (external & internal), transverse, & rectus muscles

2 - Muscles that form longitudinal bands in roof of body cavity (subvertebral muscles)

Oblique & transverse muscles:

  • Early amphibians & reptiles
    • ribs developed in myosepta along entire length of the trunk
    • urodeles still have myosepta the length of the trunk, but ribs no longer form in all of them
  • Modern amniotes
    • myosepta & ribs are restricted to the thorax (so abdominal muscles are not obviously segmented)
    • hypaxials form 3 layers: external oblique, internal oblique, & transverse (in the thorax region: external & internal intercostals, which play an important role in respiration, & transverse muscle)

1 - External intercostal muscles, 2 - Internal intercostal muscles, 3 - Ribs, 4 - Intercartilaginous muscles,  
5 - Sternum, 6 - Subcostal muscles, & 7 - Vertebral column  

Rectus muscles:

  • weakly developed in most fish; 'stronger' in tetrapods
  • support ventral body wall & aid in arching the back
  • in mammals - rectus abdominis (typically extends from the anterior end of the sternum to the pelvic girdle)

Subvertebral muscles:

  • underneath & against transverse processes of vertebrae
  • includes the psoasiliacus in the lumbar region & the longus colli in the neck
  • less developed in the thorax & none in the tail


1 - short epaxials perform same function as in fish (side-to-side movements of vertebral column)

2 - short & long bundles arch & support the vertebral column

3 - most anterior bundles = attach to & move the skull


1 - Aquatic urodeles = used chiefly for swimming

 Diagram showing the onset and duration of lateral hypaxial muscle EMG activity relative to maximum body bending in two Ambystoma tigrinum during swimming. Values are means S.D. for alpha-burst (filled bars) and beta-burst (open bars) onset and duration times. OES, m. obliquus externus superficialis; OEP, m. obliquus externus profundus; OI, m. obliquus internus; TA, m. transversus abdominis. *Denotes the side of the body on which the electrodes were implanted (Bennett et al. 2001).


2 - Terrestrial urodeles = assist in locomotion

3 - Other tetrapods = reduced in volume compared to fish (because of shift in mode of locomotion); now support contents of abdomen, assist in respiration (especially intercostal muscles), & assist epaxials in bending vertebral column (rectus muscles)

Hypobranchial & tongue muscles:

  • Fish
    • hypobranchials extend forward from pectoral girdle & insert on mandible, hyoid, & gill cartilages
    • hypobranchials strengthen floor of pharynx & assist branchiomeric muscles in elevating floor of mouth, lowering jaw, & extending gill pouches
  • Tetrapods
    • hypobranchials stabilize & move hyoid apparatus & larynx
    • the tongue of amniotes is a 'sac' anchored to hyoid skeleton & filled with hypobranchial muscle

The neck muscles ending in "hyoid" are associated with the hyoid apparatus, whereas those  
beginning or ending with "thyro" are attached to the larynx. These muscles are hypobranchia  
and function in movement of the hyoid apparatus, larynx and/or floor of the mouth.

Appendicular muscles - move fins or limbs

  • Extrinsic - originate on axial skeleton or fascia or trunk & insert on girdles or limbs
  • Intrinsic - originate on girdle or proximal skeletal elements of appendage & insert on more distal elements

Fish - appendicular muscles serve mostly as stabilizers; intrinsic muscles are limited in number & undifferentiated

Tetrapods - appendicular muscles are much more complicated than in fish

    • greater leverage required for locomotion on land
    • jointed appendages (as opposed to fins) require complex muscles

Extrinsic appendicular musculature

  • Dorsal group of the forelimbs, e.g., trapezius and latissimus dorsi, arise on:

1 - fascia of trunk in lower tetrapods

2 - skull, vertebral column, & ribs to a point well behind the scapula in higher tetrapods & converge

on the girdle & limb

  • Ventral group, e.g., pectoralis, arises on sternum & coracoid, & converge on limb

RESULT = pectoral girdle & limb are joined to trunk by extrinsic appendicular muscles as illustrated in this diagram:

The 'muscular sling' of tetrapods. Appendicular muscles of the forelimbs suspend the anterior body of tetrapods from the shoulders. Some of these muscles are axial muscles (rhomboideus & serratus ventralis), some are branchial muscles (trapezius), & some arise from the forelimb musculature itself (pectoralis).

The pelvic girdle requires no such muscular anchoring because it is attached directly to the vertebral column. As a result, the volume of extrinsic muscle is relatively small in posterior limbs.

Extrinsic appendicular muscles:

1 - most develop from hypaxial blastemas in the body wall

2 - referred to as secondary appendicular muscles because it was not their original function to operate appendages

3 - chief extrinsic muscles of forelimbs of tetrapods include: scapular deltoid, latissimus dorsi, rhomboideus, serratus ventralis, & pectorals

Intrinsic appendicular muscles:

1 - form from blastemas within the limb bud

2 - called primary appendicular muscles

Appendicular muscles:

Amphibians - much more complex than in fish

Reptiles - more numerous & diverse than in amphibians; better support of body & increased mobility of distal segments of the limbs

Birds - intrinsic musculature is reduced; pectoralis (downstroke muscle) & supracoracoideus (upstroke muscle) are enlarged

Mammals - similar to reptiles but more diverse

Branchiomeric muscles:

1 - associated with the pharyngeal arches

2 - series of skeletal & smooth muscles

3 - adductors, constrictors, & levators operate jaws plus successive gill arches

Muscles of the Mandibular Arch:

    • Squalus & other fish - operate the jaws (adductor mandibulae & intermandibularis)
    • Tetrapods
      • muscles of 1st arch still operate jaws
      • adductors of mandible:
        • masseter & temporalis (see diagram below)
        • pterygoid
        • digastric

Muscles of the Hyoid Arch:

    • move hyoid arch
    • aid in hearing (stapedial muscle)
    • assist in moving lower jaw (e.g., digastric)

Muscles of 3rd & successive arches:

    • Squalus - constrictors above & below gill chambers plus levators (including the cucullaris) that compress & expand the gill pouches
    • Bony fish - muscles reduced; operculum plays important role in respiration
    • Tetrapods - muscles further reduced; primary muscles include:
      • stylopharyngeus (Arch III) - used for swallowing
      • intrinsic muscles of the larynx or 'voicebox' (remaining arches)
      • cucullaris - gives rise to trapezius, cleidomastoid, & sternocleidomastoid muscles of amniotes

Integumentary muscles:

Extrinsic integumentary muscles (e.g., platysma)

    • originate (usually) on the skeleton & insert on the underside of the dermis
    • striated
    • move skin of amniotes

Intrinsic integumentary muscles (arrector pili muscles)

    • entirely within the dermis
    • found in birds & mammals
    • mostly smooth muscles

Electric organs:

1 - consist of a number of electric discs (up to 20,000) piled in either vertical or horizontal columns

2 - each disc (electroplax) is a large coin-shaped cell

3 - evolved several times in a variety of fish (good example of convergent evolution)

Several species of fish have evolved an electric organ in their tail that produces a continous electric signal  
that propogates through the water. These fish have specialized receptors on their skin surface that can "feel" electricity.  
Objects in their environment, such as rocks, plants and pirhanas disturb the flow of their electric signal through the water.  
This disturbance will affect how strong the electric field is on a patch of skin near the object. In other words, a non-conductive  
object such as a rock will cast an electric "shadow" on the skin. The skin receptors near the object sense these disturbances  
and then increase or decrease their signal rate (depending wether the local electric field increased or decreased in intensity). These  
signals are then sent up to specialized regions of the brain that collate all the information and compute a coherent "picture" of the  
fish's environment (see

Functions of electric organs:

1 - defense

2 - communication

3 - locating prey (electrolocation)

BIO 342  
Comparative Vertebrate Anatomy  
Lecture Notes 7 - Digestive System

Digestive tract - ‘tube’ from mouth to vent or anus that functions in:

  • ingestion
  • digestion
  • absorption
  • egestion

 Major subdivisions include the oral cavity, pharynx, esophagus, stomach, small & large intestines, and cloaca. Accessory organs include the tongue, teeth, oral glands, pancreas, liver, & gall bladder.  

Differences in the anatomy of vertebrate digestive tracts is often correlated with the nature & abundance of food:

  •  readily absorbed (e.g., hummingbirds) vs. requiring extensive enzymatic activity (e.g., carnivores)
  • constant food supply (e.g., herbivores) vs. scattered supply (e.g., carnivores)

The embryonic digestive tract of vertebrates consists of 3 regions:

                  1 - midgut - contains yolk or attached yolk sac

                  2 - foregut - oral cavity, pharynx, esophagus, stomach, & small intestine

                 3 - hindgut - large intestine & cloaca

Mouth & oral cavity. The oral cavity begins at the mouth & ends at the pharynx. Fish have a very short oral cavity, while tetrapods typically have longer oral cavities. The mammalian mouth is specialized to serve as a suckling and masticatory organ (with muscular cheeks).

  •  Palate = roof of the oral cavity
    • primary palate - internal nares lead into the oral cavity anteriorly
    • secondary palate - nasal passages are located above the secondary palate and open at the end of the oral cavity

Teeth are derivations of dermal armor.

  • Placoid scales - show gradual transition to teeth at the edge of the jaw
  • Composition of teeth - primarily dentin surrounded by enamel
  • Vary among vertebrates in number, distribution in the oral cavity, degree of permanence, mode of attachment, & shape

Toothless vertebrates are found in every class of vertebrates and include agnathans, sturgeons, some toads, turtles, birds, & baleen whales.

A right whale swims at or near the surface of the water with its mouth open.  
Water and food enter through a gap in the front baleen plates, and  
food is caught in the matted baleen fringes inside.

Toothed vertebrates:

  • Fish - teeth are numerous & widely distributed in the oral cavity & pharynx
  • Early tetrapods - teeth widely distributed on the palate; most amphibians & some reptiles still have teeth on the vomer, palatine, & pterygoid bones
  • Crocodilians, toothed birds, & mammals - teeth are limited to the jaws


    1 - have tended toward reduced numbers & distribution

    2 - most vertebrates (through reptiles) have succession of teeth

    3 - most vertebrates (except mammals) replace teeth in ‘waves’ (back to front; every other tooth)

    4 - mammals generally develop 2 sets of teeth: milk (deciduous) teeth & permanent teeth  

Morphological variation in teeth:

  • vertebrates other than mammals - all teeth are shaped alike (homodont dentition)
  • mammals - teeth exhibit morphological variation: incisors, canines, premolars, & molars (heterodont dentition)
    • incisors = cutting
    • canines = piercing & tearing
    • premolars & molars = macerating


  • Gnathostome fish & primitive amphibians - tongue is a simple crescent-shaped elevation in the floor of the oral cavity caused by the underlying hyoid skeleton & is called the primary tongue
  • Most amphibians - primary tongue (or hypobranchial eminence) + glandular field (or tuberculum impar) ('stuffed' with hypobranchial musculature)
  • Reptiles & mammals - primary tongue + glandular field (or tuberculum impar) + lateral lingual swellings (more hypobranchial muscle)
  • Birds - lateral lingual swellings are suppressed & intrinsic muscle is usually lacking

Tongue mobility:

  • Turtles, crocodilians, some birds, & whales - tongue is largely immobilized in the floor of the oral cavity & cannot be extended
  • Snakes, insectivorous lizards & amphibians, & some birds - tongue sometimes long and may move in and out of the oral cavity (see
  • Mammals - tongue is attached to the floor of the oral cavity (via the frenulum) but can still be extended out of the oral cavity

Using a keen sense of smell, anteaters are able to effectively track down  
ant nests on the forest floor. Once a nest is found, the mammal usually rips it open  
with its sharp foreclaws to expose its delectable contents. The anteater then proceeds to  
catch and eat the ants by repetitively flicking its long sticky tongue in and out of the nest.  
The giant anteater's unique tongue can measure as long as two feet (60 cm).

Functions of vertebrate tongues:

Oral glands - secrete a variety of substances including:

  • saliva
    • Lubrication and binding: the mucus in saliva is extremely effective in binding masticated food into a slippery bolus that (usually) slides easily through the esophagus without inflicting damage to the mucosa. Saliva also coats the oral cavity and esophagus, and food basically never directly touches the epithelial cells of those tissues.
    • Solubilizes dry food: in order to be tasted (by taste buds), the molecules in food must be solubilized.
    • Oral hygiene: The oral cavity is almost constantly flushed with saliva, which floats away food debris and keeps the mouth relatively clean. Saliva also contains lysozyme, an enzyme that lyses many bacteria and prevents overgrowth of oral microbial populations.
    • Initiates starch digestion: in most species, amylase is present in saliva and begins to digest dietary starch into maltose. Amylase does not occur in the saliva of carnivores.
    • Provides alkaline buffering and fluid: this is of great importance in ruminants, which have non-secretory forestomachs.
    • Evaporative cooling: clearly of importance in dogs, which have very poorly developed sweat glands - look at a dog panting after a long run and this function will be clear.
  • poison (lizards, snakes, and mammals)
  • anticoagulant (vampire bats; video)

Pharynx - part of digestive tract exhibiting pharyngeal pouches (at least in the embryo) that may give rise to slits

  • Fish - pharynx is respiratory organ
  • Tetrapods:
    • pharynx is the part of the foregut preceeding the esophagus & includes:
      • glottis (slit leading into the larynx)
      • openings of auditory (eustachian) tubes
      • opening into esophagus

     Mammals - an epiglottis is positioned over the glottis so that, when a mammal swallows, the larynx is drawn forward against the epiglottis & the epiglottis blocks the glottis (which prevents food or liquids from entering the trachea)



  • a distensible muscular tube connecting the pharynx & the stomach
  • may have diverticulum called the crop (see diagram of pigeon below)

Stomach = muscular chamber(s) at end of esophagus

  • serves as storage & macerating site for ingested solids & secretes digestive enzymes
  • Vertebrate stomachs:
    • Cyclostomes - weakly developed; similar to esophagus
    • Fish, amphibians, & reptiles - increasing specialization (more differentiated from the esophagus)
    • Birds - proventriculus (glandular stomach) and ventriculus (muscular stomach, or gizzard)
Reticulo-rumen (reticulum and rumen)

Reticulum and rumen are often discussed together since each compartment is separated by a low partition. Eighty percent of the capacity of the stomach is related to the reticulo-rumen. The contents of the reticulum and rumen intermix freely. The rumen is the main fermentation vat where billions of microorganisms attack and break down the relatively indigestible feed components of the ruminant's diet. 


After fermentation in the reticulum and rumen, food passes to the omasum. The omasum acts as a filter pump to sort liquid and fine food particles. Coarse fibre particles are not allowed to enter the omasum. Also, the omasum may be the site for absorption of water, minerals and nitrogen.


The abomasum is the true stomach and the only site on the digestive tract that produces gastric juices (HCl and the enzymes, pepsin and rennin). Ingesta only remains here for 1 to 2 hours.

The intestine is located between the stomach & the cloaca or anus & is an important site for digestion & absorption. Vertebrate intestines are differentiated to varying degrees into small & large intestines.


    Fishes - relatively straight & short intestine in cartilaginous fishes & in primitive bony fishes (lungfish & sturgeon). However, the intestine of cartilaginous fishes has a spiral valve.


    Amphibians - intestines differentiated into coiled small intestine and short, straight large intestine

    Reptiles & Birds - coiled small intestines & a relatively short large intestine (that empties into the cloaca)

    Mammals - small intestine long & coiled and differentiated into duodenum, jejunum, & ileum. The large intestine is often relatively long (but not as long as the small intestine). A cecum is often present at the junction of the small & large intestines in herbivores.


Accessory organs - Liver, gall bladder, & pancreas

  • Liver & gall bladder
    • liver produces bile which is stored in the gall bladder (cyclostomes, most birds, and some mammals, including cervids, have no gall bladder)
    • bile aids in digestion by emulsifying fats (breaking fats down into tiny particles that permits more efficient digestion by enzymes)
  • Pancreas - secretes pancreatic juice (bicarbonate solution to neutralize acids coming from the stomach plus enzymes to help digest carbohydrates, fats, and proteins) into the intestine

Ceca - blind diverticula that serve to increase the surface area of the vertebrate digestive tract

  • Fishes - pyloric & duodenal ceca are common in teleosts; these are primary areas for digestion and absorption (not fermentation chambers)
  • Tetrapods - ceca are present in some herbivores; may contain bacteria that aid in the digestion of cellulose



  • chamber at end of digestive tract that receives the intestine, & urinary & genital ducts, & opens to the exterior via the vent
  • shallow or non-existent in lampreys, ray-finned fishes, & mammals (except monotremes)
  • if no cloaca is present, the intestine opens directly to the exterior via anus


BIO 342  
Comparative Vertebrate Anatomy  
Lecture Notes - Respiratory System

Respiration is the process of obtaining oxygen from the external environment & eliminating CO2.

  • External respiration - oxygen and carbon dioxide exchanged between the external environment & the body cells
  • Internal respiration - cells use oxygen for ATP production (& produce carbon dioxide in the process)

Adaptations for external respiration:

    1 - Primary organs in adult vertebrates are external &  internal gills, swim bladders or lungs, skin, & the buccopharyngeal mucosa

    2 - Less common respiratory devices include filamentous outgrowths of the posterior trunk & thigh (African hairy frog), lining of the cloaca, & lining of esophagus

Respiratory organs:

  • Cutaneous respiration
    • respiration through the skin can take place in air, water, or both
    • most important among amphibians (especially the family Plethodontidae)
  • Gills (see Respiration in Fishes)
    • Cartilaginous fishes:
      • 5 ‘naked’ gill slits
      • Anterior & posterior walls of the 1st 4 gill chambers have a gill surface (demibranch). Posterior wall of last (5th) chamber has no demibranch.
      • Interbranchial septum lies between 2 demibranchs of a gill arch
      • Gill rakers protrude from gill cartilage & ‘guard’ entrance into gill chamber
      • 2 demibranchs + septum & associated cartilage, blood vessels, muscles, & nerves = holobranch
      • usually have 5 gill slits
      • operculum projects backward over gill chambers
      • interbranchial septa are very short or absent
    • Agnathans:
      • 6 - 15 pairs of gill pouches
      • pouches connected to pharynx by afferent branchial (or gill) ducts & to exterior by efferent branchial (or gill) ducts
  •  Larval gills:
    • External gills
      • outgrowths from the external surface of 1 or more gill arches
      • found in lungfish & amphibians
    • Filamentous extensions of internal gills
      • project through gill slits
      • occur in early stages of development of elasmobranchs
    • Internal gills - hidden behind larval operculum of late anuran tadpoles

Swim bladder & origin of lungs - most vertebrates develop an outpocketing of pharynx or esophagus that becomes one or a pair of sacs (swim bladders or lungs) filled with gases derived directly or indirectly from the atmosphere. Similarities between swim bladders & lungs indicate they are the same organs.

Vertebrates without swim bladders or lungs include cyclostomes, cartilaginous fish, and a few teleosts (e.g., flounders and other bottom-dwellers).

Swim bladders:

  • may be paired or unpaired (see diagram above)
  • have, during development, a pneumatic duct that usually connects to the esophagus. The duct remains open (physostomous) in bowfins and lungfish, but closes off (physoclistous) in most teleosts.
  • serve primarily as a hydrostatic organ (regulating a fish's specific gravity)
  • gain gas by way of a 'red body' (or red gland); gas is resorbed via the oval body on posterior part of bladder
  • may also play important roles in:
    • hearing - some freshwater teleosts (e.g., catfish, goldfish, & carp) 'hear' by way of pressure waves transmitted via the swim bladder and small bones called Weberian ossicles (see diagram below)
    • sound production - muscles attached to the swim bladder contract to move air between 'sub-chambers' of the bladder. The resulting vibration creates sound in fish such as croakers, grunters, & midshipman fish.
    • respiration - the swim bladder of lungfish has number subdivisions or septa (to increase surface area) & oxygen and carbon dioxide is exchanged between the bladder & the blood


Lungs & associated structures

  • Larynx
    • Tetrapods besides mammals - 2 pair of cartilages: artytenoid & cricoid
    • Mammals - paired arytenoids + cricoid + thyroid + several other small cartilages including the epiglottis (closes glottis when swallowing)
    • Amphibians, some lizards, & most mammals - also have vocal cords stretched across the laryngeal chamber


  •  Trachea & syrinx
    • Trachea
      • usually about as long as a vertebrates neck (except in a few birds such as cranes)
      • reinforced by cartilaginous rings (or c-rings)
      • splits into 2 primary bronchi &, in birds only, forms the syrinx at that point
    • Reptilian lungs
      • simple sacs in Sphenodon & snakes
      • Lizards, crocodilians, & turtles - lining is septate, with lots of chambers & subchambers
      • air exchanged via positive-pressure ventilation
    • Avian lungs - modified from those of reptiles:
      • air sacs (diverticula of lungs) extensively distributed throughout most of the body
      • arrangement of air ducts in lungs ----> no passageway is a dead-end
      • air flow through lungs (parabronchi) is unidirectional
    • Mammalian lungs:
      • multichambered & usually divided into lobes
      • air flow is bidirectional:

  Trachea <---> primary bronchi <---> secondary bronchi <---> tertiary bronchi <---> bronchioles <---> alveoli

BIO 342  
Comparative Vertebrate Anatomy  
Lecture Notes 9 - Circulatory System

Vertebrate Circulatory Systems:

  • transport gases, nutrients, waste products, hormones, heat, & various other materials
  • consist of heart, arteries, capillaries, & veins:
    • Arteries
      • carry blood away from the heart
      • have muscular, elastic walls
      • terminate in capillary beds
    • Capillaries
      • have very thin walls (endothelium only)
      • are the site of exchange between the blood and body cells
    • Veins
      • carry blood back to the heart
      • have less muscle in their walls than arteries but the walls are very elastic
      • begin at the end of capillary beds
    • Heart
      • a muscular pump (cardiac muscle)
      • contains a pacemaker to regulate rate but rate can also be influenced by the Autonomic Nervous System

Vertebrate Hearts: (see HHMI Biointeractive - click on Vertebrate circulatorium)

Cartilaginous fishes

  • single-circuit heart with 4 chambers: sinus venosus, atrium, ventricle, & conus arteriosus
    • the sinus venosus receives blood & is filled by suction when the ventricle contracts & enlarges the pericardial cavity
    • the atrium is a thin-walled muscular sac; an A-V valve regulates flow between atrium & ventricle
    • the ventricle has thick, muscular walls
    • the conus arteriosus leads into the ventral aorta (and a series of conal valves in the conus arteriosus prevent the backflow of blood)

Teleosts - heart is similar to that of cartilaginous fishes, except a bulbus arteriosus (a muscular extension of the ventral aorta) is present rather than a conus arteriosus (a muscular extension of the ventricle)

Used by permission of John W. Kimble  

Lungfish & amphibians - modifications are correlated with the presence of lungs & enable oxygenated blood returning from the lungs to be separated from deoxygenated blood returning from elsewhere (see HHMI Biointeractive):

  • Partial or complete partition within atrium (complete in anurans and some urodeles)
  • Partial interventricular septum (lungfish) or ventricular trabeculae (amphibians) to maintain separation of oxygenated & unoxygenated blood
  • Formation of a spiral valve in the conus arteriosus of many dipnoans and amphibians. The spiral valve alternately blocks & unblocks the entrances to the left and right pulmonary arches (sending unoxygenated blood to the skin & lungs).
  • Shortening of ventral aorta, which helps ensure that the oxygenated & unoxygenated blook kept separate in the heart moves directly into the appropriate vessels

5 = ventricle, 11 = right atrium, 12 = left atrium, 13 = conus arteriosus


    1 - Heart consists of 2 atria & 2 ventricles &, except in adult birds & mammals, a sinus venosus

    2 - Complete interatrial septum

    3 - Complete interventricular septum only in crocodilians, birds, & mammals; partial septum in other amniotes

Used by permission of John W. Kimball

Arterial channels - supply most tissues with oxygenated blood (but carry deoxygenated blood to respiratory organs). In the basic pattern:

    1 - the ventral aorta emerges from heart & passes forward beneath the pharynx

    2 - the dorsal aorta (paired above the pharynx) passes caudally above the digestive tract

    3 - six pairs of aortic arches connect the ventral aorta with the dorsal aortas  

Aortic arches of fishes - general pattern of development of arches in cartilaginous fishes:

    1 - Ventral aorta extends forward below pharynx & connects developing aortic arches. The first pair of arches develop first.

    2 - Segments of first pair are lost & remaining sections become efferent pseudobranchial arteries

    3 - Other pairs of arches (2 - 6) give rise to pre- & posttrematic arteries

    4 - Arches 2 - 6 become occluded; dorsal segments = efferent branchial arteries & ventral segments = afferent branchial     arteries

    5 - Capillary beds develop within nine demibranchs

        Result: Blood entering an aortic arch from ventral aorta must pass through gill capillaries before proceeding to dorsal aorta



    • the pulmonary artery branches off the 6th aortic arch and supplies the swim bladder (& this is the same way that tetrapod lungs are supplied)

        Aortic arches of tetrapods - embryos have 6 pairs of aortic arches:

    • but the 1st & 2nd arches are temporary & not found in adults
    • the 3rd aortic arches & the paired dorsal aortas anterior to arch 3 are called the internal carotid arteries
    • the 4th aortic arches are called the systemic arches
    • the 5th aortic arch is usually lost
    • the pulmonary arteries branch off the 6th arches & supply blood to the lungs


Further modifications of tetrapod arches:


    • Urodeles - most terrestrial urodeles have 4 pairs of arches; aquatic urodeles typically have 3 pairs (III, IV, & VI)
    • Anurans - have 4 arches early in development (larval stage); arch VI develops a pulmonary artery (to lungs) while arches III, IV, & V supply larval gills. At metamorphosis:
      • aortic arch 5 is lost
      • the dorsal aorta between arches 3 & 4 is lost, so blood entering arch 3 (carotid arch) goes to the head
      • a segment (ductus arteriosus) of arch 6 is lost so blood entering this arch goes to the skin & lungs
      • aortic arch 4 (systemic arch) on each side continue to the dorsal aorta & distributes blood to the rest of the body
      • Oxygenated blood from the left atrium & deoxygenated blood from the right are largely kept separate in the ventricle by:
        • Ventricular trabeculae
        • Spiral valve in the conus arteriosus
    • Reptiles - 3 aortic arches in adults (III, IV, & VI)
      • Ventral aorta - no spiral valve but truncus arteriosus is split into 3 separate passages: 2 aortic trunks & a pulmonary trunk. As a result:
        • pulmonary trunk emerges from the right ventricle & connects with 6th aortic arches (deoxygenated blood from right atrium goes to lungs)
        • one aortic trunk comes out of left ventricle & carries oxygenated blood to the right 4th aortic arch & to carotid arches
        • the other aortic trunk appears to come out of right ventricle &  leads to left 4th aortic arch. So, does the left 4th arch carry oxygenated blood?

         Turtles, snakes, & lizards - the interventricular septum is incomplete where right & left systemic arches (4th) leave the ventricle & trabeculae in that region of the heart form a ‘pocket’ called the cavum venosum. Oxygenated blood from the left ventricle is directed into cavum venosum, which leads to the 2 systemic arches. As a result, both the left & right systemic arches receive oxygenated blood.  (see HHMI Biointeractive)  


        Crocodilians - ventricular septum is complete but a narrow channel called the Foramen of Panizza connects the base of the right & left systemic trunks  (see HHMI Biointeractive)


   Role of the Foramen of Panizza in the crocodilian circulatory system:

  • When a crocodilian is above water and breathing air, the semilunar valve in the right aorta remains closed because of higher pressure in the left & right aorta (higher than in the right ventricle). As a result, the right aorta receives blood from the left aorta (so both aortas carry oxygenated blood) and blood from the right ventricle (low in oxygen) passes only into the pulmonary artery (and goes to the lungs).


  • When a crocodilian is under water and not breathing, right ventricular pressure increases due to pulmonary resistance (vasoconstriction of blood vessels supplying the lungs). As a result, the semilunar valve in the right aorta is now forced open so some of the blood from the right ventricle now enters the right aorta rather than the pulmonary artery. This means that, rather than going to the lungs (where there is little or no oxygen anyway because the crocodilian is under water & not breathing), some of the blood enters the systemic (body) circulation. This means that vital organs & tissues (such as skeletal muscles and the central nervous system) will get an increased blood supply and additional oxygen. This, in turn, allows a crocodilian to stay underwater longer (which is most important because many crocodilians hunt by remaining underwater and 'ambushing' prey that come for a drink or to cool off).

Oreillette droite = right atrium, Oreillette gauche = left atrium, Ventricule droit = right ventricle, Ventricule gauche = left ventricle


Secret of the crocodile heart       (Franklin, C.E., and M. Axelsson. 2000. An actively controlled heart valve. Nature 406:847)

By examining the heart of a crocodile, researchers have discovered how it is that an air-breathing creature can manage to cruise through the murk, for several hours without surfacing. The crocodile has a unique type of valve in its heart which actively controls blood flow between the lungs and the rest of the body. University of Queensland researcher, Craig Franklin, together with University of Goteborg colleague Michael Axelsson have been studying the heart of the estuarine crocodile, Crocodylus porosus. "These valves represent an absolute evolutionary novelty,” said Dr Franklin. “They are further proof of the complexity and sophistication of the 'plumbing' and general anatomy of the crocodile family," Dr Franklin said.

Unlike the passive flap-like valves of other vertebrates, the crocodile valve has cog teeth made up of nodules of connective tissue. The cog teeth mesh together, diverting blood away from lungs and into their bodies. The researchers have found that these “teeth” are controlled by the amount of adrenalin in the bloodstream."When the crocodile is relaxed, the absence of adrenalin acts to close the cog-teeth valves," Dr Franklin said. He said this mechanism may allow the crocodiles to dive for several hours without needing to resurface to breathe. The valves are situated in the crocodile's right ventricle, which pumps blood to the pulmonary artery feeding the lungs as well as to the left aorta which supplies the body. The cog-teeth valve can divert blood going to the lungs back into the body, a phenomenon known as a shunt. "In contrast, mammalian hearts are very inflexible with the blood supply to the lungs a separate activity to that feeding the body." - Abbie Thomas - ABC Science Online

        Birds & mammals - no mixing of oxygenated & unoxygenated blood; complete interventricular septum + division of ventral aorta into 2 trunks:

  • Pulmonary trunk that takes blood to the lungs
  • Aortic trunk that takes blood to the rest of the body

    Result of modifications: All blood returning to right side of heart goes to the lungs; blood returning from lungs to the left side of heart goes to systemic circulation.

Venous channels - In early vertebrate embryos, venous channels conform to a single basic pattern. As development proceeds, these channels are modified by deletion of some vessels & addition of others. The primary venous pathways include:

  • cardinals
  • renal portal
  • lateral abdominal
  • hepatic portal
  • coronary
  • pulmonary


The venous channels in sharks:

  • Cardinal streams - sinus venosus receives all blood returning to heart. Most blood enters sinus venosus via Common Cardinals. Blood from head is collected by Anterior Cardinals. Postcardinals receive renal veins & empty into Common Cardinals.
  • Renal Portal stream - Early in development, some blood from caudal vein continues forward as Subintestinal (drains digestive system); this connection is then lost. During development, afferent renal veins (from old postcardinals) invade kidneys, & old postcardinals near top of kidneys are lost; all blood from tail must now enter kidney capillaries.
  • Lateral Abdominal stream - LA vein starts at pelvic fin (where it receives iliac vein) & passes along lateral body wall; receives brachial vein, then turns, becomes Subclavian vein,  & enters Common Cardinal vein.
  • Hepatic Portal stream & Hepatic sinuses - Among 1st vessels to appear in vertebrate embryos are Vitelline veins (from yolk sac to heart). One Vitelline vein joins with embryonic Subintestinal vein (that drains digestive system) & becomes the Hepatic Portal System. Between liver & sinus venosus, 2 Vitelline veins are known as Hepatic sinuses.


Venous channels in other fishes are much like those of sharks except:

  • Cyclostomes have no renal portals
  • In most bony fishes the lateral abdominals are absent & the pelvic fins are drained by postcardinals

Venous channels of tetrapods - early embryonic venous channels are very similar to those of embryonic sharks. Changes during development include:

  • Cardinal veins & precavae - embryonic tetrapods have posterior cardinals, anterior cardinals, & common cardinals
    • Urodeles - posterior cardinals persist between caudal vein & common cardinals in adults
    • Anurans, most reptiles, & birds - posterior cardinals are lost anterior to kidneys
    • Mammals - right posterior cardinal persists (azygos); part of left posterior cardinal persists (hemiazygos)

Terminology note: Common cardinals in tetrapods are called PRECAVAE; anterior cardinals are called INTERNAL JUGULAR VEINS.  

    • Some mammals (e.g., cats & humans) lose the left precava; the left brachiocephalic carries blood from left side to right precava (sometimes called SUPERIOR VENA CAVA).
  • Postcava - Both posterior cardinals begin to develop in embryos, but only one persists & becomes the POSTCAVA. The Postcava passes directly through the liver (sort of an ‘expressway’ for blood from kidneys & the posterior part of the body to the heart). The Postcava is sometimes called the INFERIOR VENA CAVA. In crocodilians, birds, & mammals, veins from hindlimbs connect directly to Postcava.
  • Abdominal stream:
    • Early tetrapod embryos - paired lateral veins (like lateral abdominals of sharks) begin in caudal body wall near hind limbs, continue cranially, receive veins from forelimbs, & empty into cardinal veins or sinus venosus. As development continues:
      • Amphibians - 2 abdominal veins fuse at midventral line & form VENTRAL ABDOMINAL VEIN. Blood in this vessel goes into liver capillaries & abdominals anterior to liver are lost (so abdominal stream no longer drains anterior limbs).
      • Reptiles - 2 lateral abdominals do not fuse but still terminate in liver capillaries (so do not drain anterior limbs; see diagram below).
      • Birds - retain none of their embryonic abdominal stream as adults
      • Mammals - no abdominal stream in adults
  • Renal Portal system:
    • Amphibians & some reptiles - acquires a tributary (external iliac vein; not homologous to mammalian external iliac) which carries some blood from the hind limbs to the renal portal vein. This channel provides an alternate route from the hind limbs to the heart.
    • Crocodilians & birds - some blood passing from hind limbs to the renal portal by-passes kidney capillaries, going straight through the kidneys to the postcava (see diagram above)
    • Mammals - renal portal system not present in adults
  • Hepatic Portal system - similar in all vertebrates; drains stomach, pancreas, intestine, & spleen & terminates in capillaries of liver
  • Pulmonary veins - carry blood from lungs to left atrium in lungfish & tetrapods

Circulation in a mammalian fetus & changes at birth:

    In a developing fetus, blood obtains oxygen (& gives up carbon dioxide) via the placenta, not the lungs. As a result, blood flow must largely bypass the lungs so that oxygentated blood can get to other developing tissues. Getting oxygenated blood from the placenta back to the heart & out to the body as quickly and efficiently as possible involves a series of vessels & openings found only in a mammalian fetus:

  • blood (with oxygen & nutrients acquired in placenta) passes into umbilical vein
  • blood largely bypasses the liver via the ductus venosus
  • blood returns to the heart & enters right atrium, but much of the blood then bypasses the right ventricle & enters the left atrium via the foramen ovale
  • blood that does enter the right ventricle largely bypasses the pulmonary circulation via the ductus arteriosus

Major changes at birth:

   1 - Ductus arteriosus closes

   2 - Foramen ovale sealed off

   3 - Blood no longer flows through umbilical vein

Lymphatic system - found in all vertebrates; consists of lymph vessels, lymph nodes, &, in some species, lymph hearts

  • Lymph vessels
    • found in most soft tissues of the body & begin as blind-end lymph capillaries that collect interstitial  fluid
    • valves present (in birds & mammals) that prevent backflow
    • empty into 1 or more veins (e.g., caudal,  iliac, subclavian, & posterior cardinal)
  • Lymph nodes - located along lymph vessels; contain lots of  lymphocytes & macrophages (phagocytic cells)
  • Lymph hearts - consist of pulsating smooth muscle that propels lymph fluid through lymph vessels; found in fish,  amphibians, & reptiles

BIO 342  
Comparative Vertebrate Anatomy  
Lecture Notes 10 - Urogenital System

Vertebrate kidneys consist of glomeruli, tubules surrounded by peritubular capillaries, & longitudinal ducts. Variations in  
 kidney structure among vertebrates are primarily in the number & arrangement of the glomeruli & tubules.

  • Glomeruli are masses of capillaries that (along with Bowman's capsule) 'filter' the blood - the first step in eliminating waste products from the blood.
  • Kidney tubules collect the glomerular filtrate & conduct it to a longitudinal duct. Tubules consist of several segments & begin as a Bowman’s (or glomerular) capsule. A glomerulus plus its surrounding Bowman's capsule is called a renal corpuscle.
  • Longitudinal ducts = begin developing at anterior end of kidney & grow caudally until opening into the cloaca

1 = Bowman's capsule, 2 = glomerulus, 3 = afferent arteriole, 4 = efferent arteriole,  
5 = proximal convoluted tubule, 6 = distal convoluted tubule, 7 = collecting duct,  
8 = loop of Henle, 9 = peritubular capillaries


Archinephros - earliest vertebrate kidneys probably extended the entire length of the body cavity & had external glomeruli that drained the coelomic fluid

Pronephros - 1st embryonic tubules in all vertebrates; called pronephric tubules because they are the 1st to develop & are anteriorly located

  • Number - never very many (e.g., 3 in frogs, 7 in human embryos, & 12 in chicken embryos)
  • The duct that drains the pronephros is called the pronephric duct.
  • The pronephros is temporary & function only until glomeruli & tubules further back become functional.


  • formed by corpuscles & tubules that develop caudal to pronephric region; form connections with existing pronephric duct (which is now called the mesonephric duct)
  • the embryonic kidney in reptiles, birds, & mammals
  • the functional adult kidney in fish & amphibians (& sometimes called the opisthonephros)

Jawed fishes & amphibians - among males, some anterior tubules of mesonephros conduct sperm from testis to mesonephric duct. That part of the mesonephros is called the SEXUAL KIDNEY while the rest is the URINIFEROUS KIDNEY.

Amniote embryos - mesonephros functions for a short time after hatching or birth &, during that time, a new kidney called the  
  metanephros is developing



  • the adult amniote kidney
  • the number of corpuscles is large; up to about 4.5 million is some species
  • drained by a duct called the metanephric duct or ureter

Mammalian kidneys are divided into the CORTEX(#5), MEDULLA (#6), & PELVIS(#4):

  • Cortex - contains renal corpuscles & lots of capillaries
  • Medulla - contains collecting ducts and loops of Henle; divided into pyramids (#7) & columns (#2)
  • Pelvis - hollow; receives the urine (which exits the kidney via the ureter - #3)

Tubules of mammalian kidney have U-shaped Loops of Henle (avian kidney = very short loops & reptilian kidney = no loops)

Blood supply:

  • kidney is supplied by 2 or more renal arteries in reptiles & birds, & by a single renal artery in mammals (below).
  • Pathway of blood in mammalian kidney: renal artery > segmental arteries > interlobar arteries > arcuate arteries > interlobular arterioles.

Urinary bladders are found in all vertebrates except agnathans, snakes, crocodilians, some lizards, & birds (except ostriches).

  • Fish - bladders are terminal enlargements of the mesonephric ducts called TUBAL BLADDERS
  • Amphibians through Mammals - bladders arise as evaginations of ventral wall of the cloaca

Value of tetrapod urinary bladder:

  • void urine when desired rather than continuously as it is formed
  • uses of urine:
    • reproduction (e.g., providing males with information concerning the reproductive status of a female)
    • behavioral (e.g., marking territories)
    • moisten soil (some freshwater turtles use urine to soften the ground and make it easier to dig holes for egg-laying)


  • arise as paired ridges just medial to mesonephroi
  • due to fusion or failure of 1 ridge to differentiate, some vertebrates (agnathans, some female lizards & crocodilians, & most female birds) have a single testis or ovary
  • hormones cause differentiation of early gonads into either testes or ovaries


  • In some teleosts, ovaries are hollow sacs, either because the ovary develops around coelom or the ovary becomes hollow at ovulation (eggs are discharged into cavity which is continuous with the oviduct)
  • In other teleosts plus agnathans, the ovaries are compact & eggs are discharged into coelom
  • Amphibians - ovaries are hollow & eggs are discharged into the coelom
  • Reptiles, birds, & monotremes - ovaries solid but develop irregular, fluid-filled lacunae (cavities); eggs discharged into coelom
  • Mammals - ovaries compact; no large chambers or lacunae


  • usually smaller than ovaries because sperm, although numerous, are much smaller than eggs (especially eggs with yolk)
  • In mammals, testes are larger than ovaries

Translocation of testes in mammals:

  • testes descend permanently into scrotal sacs in many mammals
  • some mammals - testes lowered into scrotal sacs & retracted at will
  • inguinal canal - passage between abdominal cavity & scrotum
  • scrotal sacs - do not develop in some mammals; testes remain in abdomen

Male genital ducts:

  • Some fishes (e.g., gar & sturgeon) & amphibians - mesonephric duct transmits sperm & urine
  • Some amphibians - mesonephric duct transports only sperm; new accessory urinary duct drains the kidney
  • Sharks - mesonephric duct is used primarily for sperm transport; accessory urinary duct develops
  • Teleosts - mesonephric duct drains kidney; separate sperm duct develops
  • Amniotes - embryonic mesonephric ducts transport sperm in adults


Intromittent organs:

  • useful when fertilization is internal; introduce sperm into female reproductive tract
  • found in some fish, some birds, reptiles, & mammals
    • cartilaginous fish - appendages of pelvic fins called claspers direct sperm into female reproductive tract
    • snakes & lizards - have pair of HEMIPENES (pocketlike diverticula of wall of cloaca)
    • turtles, crocodilians, a few birds, & mammals - exhibit an unpaired erectile penis
      • penis - usually a thickening of floor of cloaca consisting of spongy erectile tissue (corpus spongiosum) with grooves to direct sperm & ending in a glans penis (sensory endings that reflexly stimulate ejaculation)
      • mammals (except monotremes) - penis extends beyond body. The embryonic corpus spongiosum becomes a tube with urethra inside & 2 additional erectile masses develop (corpus cavernosa).

Female genital ducts:

  • typically consists of a pair of gonoducts (or oviducts) that extend from ostia to the cloaca
  • different segments of ducts perform special functions
  • when internal, fertilization occurs near beginning of ducts



  • anatomy in various vertebrate groups:
    • cartilaginous fish - 2 ostia fuse to form single ostium (or osteum); shell gland secretes albumen & a shell; uterus holds eggs until laying
    • teleosts - ducts are continuous with cavity of the ovary
    • lungfish & amphibians - oviducts long & convoluted; lining secretes jelly-like material around each egg
    • crocodilians, some lizards, & nearly all birds (diagram below) - 1 coiled oviduct lined with glands that add albumen, shells, &, sometimes, pigment
    • monotremes - tract is reptilian; caudal end secretes a shell before egg passes into the cloaca
    • placental mammals - embryonic ducts give rise to oviducts, uteri, & vaginas. Adult tract is paired anteriorly & unpaired posteriorly (typically terminating as an unpaired vagina).
      • oviducts (fallopian tubes) are relatively short, small in diameter, convoluted, & lined with cilia; begin at ostium bordered with fimbria
      • uterus:
        • Marsupials - no fusion of embryonic ducts so there are 2 tracts (DUPLEX UTERUS)
        • Other placental mammals - varying degrees of fusion:
          • bipartite uterus - 2 uterine horns, a uterine body (with 2 lumens), & a single vagina
          • bicornuate uterus - 2 uterine horns, a uterine body (with a single lumen), & a single vagina
          • simplex uterus - no uterine horns & oviducts open directly into body of uterus

Vagina - fused terminal portion of oviducts that opens either into urogenital sinus or to the exterior; receives male intromittent organ

BIO 342  
Comparative Vertebrate Anatomy  
Lecture Notes 11 - Nervous System

The Vertebrate Nervous System:

    1 - receives stimuli from receptors & transmits information to effectors that respond to stimulation

    2 - regulates behavior by integrating incoming sensory information with stored information (the results of past experience) & translating that into action by way of effectors

    3 - includes billions of nerve cells (or neurons), each of which establishes thousands of contacts with other nerve cells

    4 - also includes neuroglia cells that support, nourish, & insulate neurons

Subdivisions of the Vertebrate Nervous System:

  1 - Central Nervous System - including the brain & spinal cord

  2 - Peripheral Nervous System - including cranial nerves, spinal nerves, & all branches of cranial & spinal nerves


Neurons (or nerve cells):

  • respond to stimuli & conduct impulses
  • 3 types - all with cell body & processes (axons & dendrites):
    • multipolar
    • bipolar
    • unipolar
Multipolar neuron
Bipolar neuron Unipolar neuron

Nerves = bundles of nerve cell processes; may be sensory, motor, or mixed

Spinal cord:

  • located in vertebral canal
  • anatomical beginning is the foramen magnum of the skull
  • length varies among vertebrates:
    • in vertebrates with abundant tail musculature, the spinal cord extends to the caudal end of the vertebral column
    • in vertebrates without tails or without much tail musculature, the spinal cord extends to about the lumbar region of the vertebral column
  • a cross-section of the spinal cord reveals gray matter & white matter. The gray matter consists of nerve cell bodies, while the white matter consists of nerve cell processes (axons). These processes make up ascending (sensory) and descending (motor) fiber tracts.

Used with permission of G. Mandl

Spinal nerves:

  • arise from spinal cord by dorsal & ventral roots. The dorsal root exhibits a ganglion & is sensory, while the ventral root has no ganglion & is motor.
  • early vertebrates:
    • dorsal & ventral roots did not unite
    • dorsal roots were mixed (contained both sensory & motor fibers)
    • no dorsal root ganglion
  • Rami - 2 branches of each spinal nerve:
    • dorsal ramus - supplies epaxial muscles & skin of the dorsal part of the body
    • ventral ramus - supplies hypaxial muscles & skin of the side & ventral part of the body
  • Functional types of neurons in spinal nerves (& other nerves):
    • somatic afferent - sensory from general cutaneous receptors (in the skin) & proprioceptors (in skeletal muscles, tendons, & joints)
    • somatic efferent - motor to skeletal muscles
    • visceral afferent - sensory from receptors in the viscera (smooth muscle, cardiac muscle, & glands)
    • visceral efferent - motor to smooth muscle, cardiac muscle, & glands


  • the anterior end of the embryonic central nervous system exhibits 3 primary sections:
    • prosencephalon (forebrain) - subsequently divides into the telencephalon (cerebrum) & diencephalon (epithalamus, thalamus, & hypothalamus)
    • mesencephalon (midbrain) - develops without further subdivision & forms the tectum
    • rhombencephalon (hindbrain) - subdivides into the metencephalon (pons & cerebellum) and myelencephalon (medulla oblongata)


  • Phylogenetic trend in vertebrate brains is for enlargement of forebrain:
    • increasingly complex behaviors & muscle control:
      • coordination of limb movements more complicated (e.g., bipedal dinosaurs & birds)
      • increased input of sensory information & increased output of motor responses



     Myelencephalon - consists of the medulla oblongata & its major functions include:

  • origin of cranial nerves (VII - X or VII - XII)
  • pathway for ascending & descending fiber tracts
  • contains centers important in regulating respiration, heartbeat, & intestinal motility

    Metencephalon - consists of the pons & cerebellum:

  • Pons - pathway for ascending & descending fiber tracts & origin of cranial nerves V, VI, & VII
  • Cerebellum - modifies & monitors motor output:
    • important in maintaining equilibrium
    • coordinates & refines motor action

    Mesencephalon - consists of the tectum which includes the optic lobes & auditory lobes:

  • optic lobes - receive fibers from retina; vary in size with relative importance of vision
  • auditory lobes - receive fibers from inner ear

    Diencephalon - consists of the epithalamus, hypothalamus, & thalamus:

  • epithalamus - includes pineal gland (epiphysis) that affects skin pigmentation (by acting on melanocytes) in lower vertebrates & plays a role in regulating biological rhythms in higher vertebrates
  • hypothalamus - regulates body temperature, water balance, appetite, blood pressure, sexual behavior, & some aspects of emotional behavior
  • thalamus - major coordinating, or relay, center for sensory impulses from all parts of the body


    Telencephalon - consists of the cerebrum which, in turn, consists of  2  cerebral hemispheres

  • cerebrum has 2 regions: a dorsal PALLIUM (with medial, dorsal, & lateral divisions) & a ventral SUBPALLIUM (consisting of a striatum & a septum)
  • all vertebrates have a cerebrum based on the same basic plan; major phylogenetic changes are due to loss, fusion, or enlargement of the various regions.
    • medial pallium receives olfactory information
    • dorsal & lateral pallia receive other sensory input (including visual & auditory information relayed from the thalamus)
  • agnathans, fish, & amphibians - pallia are similar


  • reptiles - pallium has 3 main divisions (medial, dorsal, & lateral) but also has a large DORSAL VENTRICULAR RIDGE (DVR), derived from lateral pallium; DVR may be higher association area
  • birds - DVR expands further; dorsal part increases in size & is called the WULST; as in reptiles, the DVR appears to serve as a higher association area


  • mammals - do not have enlarged DVR but DORSAL PALLIUM is enlarged & is called the CEREBRAL CORTEX; cortex receives & analyzes sensory information & initiates motor activity
  • subpallium:
    • septum - important part of the limbic system (regulates emotions & plays vital role in short-term memory)
    • striatum - also called basal ganglia; present in all vertebrates & controls sequence of actions in complex movements

 Cranial nerves - agnathans, most fish, & living amphibians have 10 cranial nerves; crossopterygians & amniotes have 12:

  • Olfactory nerve (I) - sensory nerve; sense of smell
  • Optic nerve (II) - sensory ‘nerve’; sense of vision
  • Oculomotor nerve (III) - motor nerve to extrinsic eye muscles
  • Trochlear nerve (IV) - motor to extrinsic eye muscles
  • Trigeminal (V) - mixed nerve; sensory from skin of head & mouth (including teeth) & motor to muscles of 1st pharyngeal arch (muscles of jaw)
  • Abducens (VI) - motor to extrinsic eyeball muscles
  • Facial (VII) - mixed nerve; sensory from lateral line of head, ampullae of Lorenzini, & taste buds; motor to muscles of hyoid arch
  • Auditory (VIII) - sensory from inner ear (balance & hearing)
  • Glossopharyngeal (IX) - mixed nerve; sensory from taste buds & lateral line; motor to muscles of 3rd arch
  • Vagus (X) - mixed nerve; sensory from & motor to heart, anterior digestive system, mouth, gill pouches 2 - 5, & lateral line
  • Accessory nerve (XI) - motor to derivatives of cucullaris muscle (cleidomastoid, sternomastoid, & trapezius)
  • Hypoglossal nerve (XII) - motor to hyoid & tongue muscles

Possibly useful mnemonics to aid in memorization of cranial nerves: “On Old Olympus Towering Top A Finn And German Viewed A Hop” or “Oh, Once One Takes The Anatomy Final, A Good Vacation Appears Heavenly.”  

Sensory Organs

Sensory receptors:

  • monitor the external & internal environment by responding to selected stimuli, then ‘translating’ those stimuli into nerve impulses
  • Types of sensory organs:
    • somatic sensory organs - provide information about the external environment
    • visceral sensory organs - provide information about the organism's internal environment
    • general sensory organs - widely distributed over the surface & interior of the body
    • special sensory organs - confined to the head (amniotes & terrestrial amphibians)

Special Somatic Receptors

  • Neuromast organs ('groove organ' below in Figure 10-4)
    • receptors in skin of fishes & aquatic amphibians that detect water currents & ‘hear’ sounds
    • occur singly, in groups, or in a linear series (e.g., lateral lines)
    • may also be modified to detect electricity (ampullae of Lorenzini)

Shark lateral line system  

The ampullae of Lorenzini are small vesicles that form part of an extensive subcutaneous sensory network system.  These vesicles are found around the head of the shark.  They detect weak magnetic fields produced by other fish at short ranges. This enables a shark to locate prey buried in the sand or to orient to nearby movement.  Each ampulla is a bundle of sensory cells innervated by several nerve fibers.  These fibers are enclosed in a jelly-filled tubule that has a direct opening to the surface through a pore.  These pores on the head of the shark are visible to the naked eye, and appear as dark spots.   
  • Neuromast organs have 2 types of cells:
    • hair cells (receptor cells) - each hair cell has several short cilia & kinocilia that project into fluid or a cupula (displacement of cupula & cilia generates nervous impulses)
    • supporting cells
  • Membranous labyrinth
    • exhibited by all vertebrates
    • fluid-filled & embedded in skull lateral to hindbrain
    • Labyrinth usually consists of 3 semicircular canals, a utriculus, & a sacculus

 Semicircular canals:

  • Hagfish - have only one (posterior)
  • Lamprey - have 2 (anterior & posterior)
  • Other vertebrates - have 3 (anterior, posterior, & horizontal)

Functions of the labyrinth:

 1 - Equilibrium

  • Dynamic equilibrium - when head moves, inertia causes a slight relative movement of fluid in at least one semicircular canal ---> deflects cupula (in ampulla) ---> nervous impulses
  • Static equilibrium - maculae (in sacculus & utriculus) tilt when head moves ---> nervous impulses

 2 - Hearing - function of ORGAN OF CORTI located in lagena (enlargement of sacculus); lagena tends to be longer in terrestrial vertebrates &, in most mammals, it’s coiled into the cochlea. The organ of Corti contains a specialized strip of neuromasts connected to the nervous system via the auditory nerve.

1-Inner hair cell, 2-Outer hair cells, 3-Tunnel of Corti, 4-Basilar membrane, 5-Reticular lamina,  
6-Tectorial membrane, 7-Deiters' cells, 8-Space of Nuel, 9-Hensen's cells, & 10-Inner spiral sulcus

[Drawing by Stephan Blatrix, from "Promenade around the cochlea"  by R Pujol, S. Blatrix, T. Pujol and V. Reclar-Enjalbert, CRIC,  
University Montpellier 1 - INSERM. URL:]


  • Outer ear of tetrapods:
    • Amphibians & most reptiles - eardrum (tympanic membrane) is on surface of the head
    • Crocodilians, birds, & mammals - eardrum is deeper in the skull at the end of an air-filled passageway called the outer ear canal (or external auditory meatus)
    • Mammals - pinna collects & directs sound waves
  • Middle ear of tetrapods - cavity plus ossicle(s):
    • Amphibians, reptiles, & birds - single middle ear ossicle (columella or stapes)
    • Mammals - 3 middle ear ossicles (malleus, incus, & stapes)
  • Inner ear = labyrinth, including lagena (or cochlea)

How do pressure or sound waves become ‘sound?’

 (Also: check out &

Pit receptors of reptiles = infrared receptors:

 1 - Labial pits

  • found in pythons (Family Boidae); nerve endings lie at the bottom of several recessed labial pits
  • permit detection of a mouse about 15 cm away

 2 - Loreal pits

  • also called facial pits; can detect temperature changes of as little as 0.001 degree C & so can detect prey several feet away
  • present in snakes in the family Crotalidae (North American rattlesnakes, copperheads, & water moccasins), also called the pit vipers

Light receptors (or photoreceptors) - vertebrates can perceive only a narrow band of electromagnetic radiation between about 350 & 760 nm; 2 types include the epiphysis (already described) & the eye

Structure of a vertebrate eye:

Accommodation is the process of focusing light on the retina & this can occur in several different ways:

  • Lamprey - contraction of corneal muscle pulls cornea against the lens & moves the lens
  • Teleosts (bony fish) - retractor muscle attached to lens (rectractor lentis muscle) moves lens posteriorly
  • Amphibians & cartilaginous fish - protractor muscle attached to lens pulls the lens forward for near vision
  • Snakes - increased pressure in the vitreous humor generated by muscles near the iris pushes the lens forward
  • Most reptiles, birds, & mammals - curvature of lens is altered by ciliary (annular) muscles


Special visceral receptors - olfactory (smell) & gustatory (taste):

  • Olfaction - involves receptors located in nasal passages; olfactory epithelium contains basal cells (replacement cells), supporting cells (secrete mucus), & olfactory receptor cells
  • vomeronasal organs - only in tetrapods but absent in most turtles, crocodiles, birds, some bats, primates, & aquatic mammals:
    • amphibians - recessed area off the main nasal cavity
    • reptiles - separate pit to which tongue & oral membranes deliver chemicals
    • mammals:
      • isolated area of olfactory membrane within nasal cavity that is connected to mouth via a nasopalatine duct
      • well-developed in monotremes, marsupials, insectivores, & many carnivores
    • function of vomeronasal organs - may be especially important in detecting conspecific odors, but also useful in prey detection
  • Gustation (taste) - taste buds, like olfactory receptors, detect chemical stimuli
    • Taste buds
      • consist of supportive cells & taste cells
      • distribution:
        • Fish - widely distributed in roof, walls, & floor of pharynx; bottom feeders & scavengers (catfish & carp) have taste buds distributed over entire surface of head & body, especially on the barbels (‘whiskers’)
        • Tetrapods - taste buds restricted to tongue, posterior palate, & oral pharynx

General Somatic Receptors - come in two categories: cutaneous receptors & proprioceptors

  • cutaneous receptors (for touch, pressure, pain, & temperature)
    • naked endings - in skin of all vertebrates; stimulated by contact
    • encapsulated endings - present in tetrapods; nerve endings wrapped in a connective tissue capsule
      • Herbst corpuscles - on beak, tongue, & palate of water birds
      • end bulbs & Ruffini corpuscles - thermal receptors in mammals
      • Pacinian corpuscles - touch & pressure receptors
  • proprioceptors - located in skeletal muscles, joints, and tendons & provide information about body position

General Visceral Receptors:

  • mostly naked endings in mucosa of the tubes, vessels, & organs of the body, in cardiac muscle,& in smooth muscle; chiefly stretch & chemoreceptors
  • some functions of general visceral receptors:
    • monitor oxygen & carbon dioxide content of blood
    • monitor blood pressure (baroreceptors in Figure 1 below)
    • monitor concentration of solutes in blood
  • similar among all vertebrates
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