THE AXIAL AND APPENDICULAR SKELETON EXCLUDING THE SKULL
Return to Index STATION 1: SKELETAL TISSUES Examine the microscope slides of hyaline cartilage, on demonstration. It would be useful to do a quick sketch of the slides and label them from the diagrams on the wall. Notice the nuclei of discrete cells in lacunae (spaces) embedded in a matrix of polysaccharides. These cells (chondrocytes) on the underside of the perichondrial membrane secrete the matrix. No blood vessels or nerves penetrate the matrix. All nourishment is by means of diffusion through the matrix, so cartilage seldom acquires much thickness although it grows by internal expansion. There are drawings of the different types of cartilage on display. They have different amounts of calcification, which is the addition of calcium salts, and differ in their fiber content. Cartilage is derived from mesenchyme and forms a matrix for bones, ends of bones, intercostals from ribs, nose, ears etc. Name the three major types of cartilage. General connective tissue is dispersed throughout the body as mesenchyme, adipose (fat) and areolar tissue. Fibrous tissue makes up tendons, which join bone and muscle and ligaments, which join bones. Bone can either be dermal or replacement. Dermal bone forms directly in the skin from mesenchyme. Osteoblasts form a calcium phosphate matrix and deposit salts, they then become osteocytes. Dermal bones are plate like, but can become thicker or grow at the edges. Membrane bones are deep bones that also do not have a primary cartilage matrix. Give some examples of dermal bones seen in this lab. Replacement bones (endochondral bones) are formed by replacing embryonic cartilage with bone. New cartilage grows mainly at the ends and at epiphyses to lengthen the bone. To widen it, the periosteal membrane directly secretes perichondrial bone in strips. The axial skeleton was derived from sclerotome, the appendicular skeleton from myotome. Give some examples of replacement bones seen in this lab. Examine the microscope demonstration of dry ground bone. The forming of this complex structure is called ossification. Refer to the drawing of bone on demonstration and identify Haversian Canals (Osteons), Canaliculi, Lacunae, and Bone Matrix. The Haversian Canals contain blood vessels, lymphatics and nerves in life, and run longitudinally in the bone. The Lacunae contain the bone cells or Osteocytes. The Lacunae are connected one to another and to the Haversian Canals by small canaliculi and larger Volkmann’s canals. The matrix appears white and is composed of mineral salts, chiefly calcium phosphate. Examine the slides of transverse and longitudinal sections through a long bone (endochondral) in a chick embryo. The sheet of fibrous connective tissue surrounding the outside of the bone, called the periosteum is seen in the chick slides. Compare these slides with the diagrams on demonstration, and with the sections through the femur on display. What is the material in the cavity of living bones and what does it do? Examine the demonstration material of young and old long bones. What is the name of the region on either end of a long bone? What is the name of the shaft? How does a young long bone differ from an older one? What is the function of trabeculae? STATION 2: AXIAL SKELETON: VERTEBRAE The following translation will be useful in learning the vocabulary of this section.
coel = cavity pophysis = process amphi = both pleur/o = lateral pro = in front hypo/hyper = upper opistho = behind dispond = twice a/an = without dia = through hetero = other para = beside zygos = yoke
A) Additions to the Centrum (Figure 2) Identify the following on the vertebrae provided:
Neural Arch	Dorsal. What is it protecting?
Neural Spine	Dorsal. What is attached to it? Look at the skeleton of the cat. Do the spines change direction? Why?
Hemal Arch	Ventral, found in the tail of fish. What is it protecting?
Zygapophyses	Form locking devices between successive vertebrae. They are projections of the dorsal region of the neural arch. Examine Prezygapophyses anteriorly, and Postzygapophyses posteriorly. These can be distinguished by the fact that the Prezygapophyses have articulatory surfaces that project anteriorly towards the midline and upward, while those of the Postzygapophyses project posteriorly outward and downward. Zygapophyses do not occur in the fishes which have single-headed ribs. Zygapophyses prevent torsion and are locked in marine mammals. Why do marine mammals need more rigid vertebrae?
Diapophyses and Parapophyses	They are lateral facets, one pair each side of centra in tetrapods, which bear two-headed, ribs. They articulate with the rib heads. Parapophyses are on the centrum and articulate with the capitulum of the rib. Diapophyses are transverse processes, which articulate with the tuberculum of the rib. In what region of the body are they found?
Hypapophyses	These are midventral projections from the centrum, also known as chevron bones found only in the caudal region e.g. cat tai1, opossum. What part of the fish vertebrae are they remnants of?
Pleurapophyses	Lateral projections of the centrum, which represent short, fused, ribs. e.g. some tailed frogs. In what part of the body are they found?
B) Types of Vertebrae Centra (Figure 1) Examine the demonstration material on vertebrae types, and identify:
Centra Type		Where Found	Question	Answer
Amphicoelous		Fish	Where is it concave?
Procoelous		Alligator, Salamander and Frog	Where is it concave?
Opisthocoelous		Lizard	Where is it concave?
Acoelous		Mammal	Is either end concave?
Heterocoelous		Bird cervical vertebrae	How would you describe the ends?

STATION 3: AXIAL SKELETON: REGIONS

Regional Specialization 

Examine the articulated skeletons of Fish, Amphibian, Reptile, Bird and Mammal that are on demonstration. Notice the following.

Fish

Two vertebral regions only, Trunk, and Caudal (tail); trunk vertebrae with ribs, neural arches and spines, caudal vertebrae with neural and hemal arches. In Agnathan (jawless) fish like Ostracoderms and Cyclostomes, the notochord is prominent with small cartilaginous vertebral elements. This condition can also be seen in the sturgeon where the cartilage has been replaced by bone. In sharks and bony fish, the notochord has been reduced to a small thread through the centrum but fills the concavities between vertebrae. Examine the caudal region of the bowfin, Amia and notice that there are two centra per body segment (hypo and pleuro centra). This is the Dispondylous or the Diplospondylous condition. The neural arches, in this caudal region, are borne only on alternate centra. Other fishes display the Diplospondylous condition, but duplicate the neural arches only; still others duplicate both arches and centra.

Amphibia

Note four vertebral regions, cervical, trunk, sacral and caudal. The Anurans (tail-less Amphibia) lack the caudal region. Compare the Frog and Salamander (Necturus) skeletons. Note particularly that both cervical and sacral regions each consist of only one vertebra. Compare ribs in Frog and Necturus. They are fused to the vertebral column in the Frogs (pleurapophyses), and are double-headed articulating ribs in Necturus.

Reptiles

Note five vertebral regions, cervical, thoracic, lumbar, sacral and caudal. Compare the vertebral columns of Alligator, Snake and Turtle. The Alligator has 8 cervical, 11 thoracic, 5 lumbar, 2 sacral (fused) and 40 caudal vertebrae. Moveable, double-headed, ribs are borne on the thoracic vertebrae. Ribs, if present on the lumbar vertebrae are fused. Snakes may have as many as 500 vertebrae. Both thoracic and lumbar regions bear ribs. The Turtle vertebral column has 8 cervical, 10 trunk, 2 sacral and 16 to 30 caudal vertebrae. The first caudal as well as all the sacral and trunk vertebrae are fused with dermal bone to form the carapace. The ribs are expanded and fused to the inner surface of the costal plates of the carapace. The ribs are single headed. Note that in the Reptiles the two anterior cervical vertebrae are specialized. The first cervical vertebra, the Atlas, is ring-like and lacks a centrum, and articulates with the occipital condyles of the skull. The second cervical vertebra, or Axis, has an anteriorly projecting process, the odontoid process, which fits into the cavity of the Atlas, acting as a pivot in turning the head.

Birds

Rigidity of the vertebral column is achieved by the fusion of many vertebrae. The cervical vertebrae number 13 to as many as 25, and have great flexibility, due to the heterocoelous centra. There are 5 thoracic vertebrae, but the last one is fused into the synsacrum, and the first four are fused together. The last thoracic, all the lumbar, the 2 sacral and several caudal vertebra all fuse to form one bone, the Synsacrum. This in turn is fused to the pelvic girdle. There are several free caudal vertebrae then the tail ends in an enlarged Pygostyle, which represents several fused vertebrae. The ribs bear posteriorly projecting Uncinate processes, each being ankylosed to the next posterior rib, the distal region of the ribs are joined to the sternum via sternal processes.

Mammals

Note the five vertebral regions - cervical, thoracic, lumbar, sacral and caudal. In the cat there are 7 cervical, 13 thoracic, 7 lumbar, 3 sacral and 4 to 26 caudal vertebrae. Only the thoracic vertebrae bear ribs. The ribs are double headed. The three sacral vertebrae fuse to form one bone, the sacrum, with which the pelvic girdle articulates. Between the vertebra are intervertebral cartilages (disks), which are composed of fibers and notochord remnants.

Examine the vertebrae on demonstration and in the cat skeleton. Note the differences between vertebrae from different body regions.

How can you distinguish the atlas?

How can you distinguish the axis?

How can you distinguish a cervical vertebra?

How can you distinguish a thoracic vertebra?

How can you distinguish a lumbar vertebra?

How can you distinguish sacral vertebrae?

How can you distinguish caudal vertebrae?

STATION 4: APPENDICULAR SKELETON

Sternum

Examine the sternum in the different classes, notice that in Amphibia like Salamanders it is of cartilage, in Frogs cartilage and bone, and attached to the pectoral girdle only. It acts as a skid and is not attached to the short ribs. Note that in the Lizard and Alligator the sternum is still cartilaginous and attached to the ribs. In Turtles it is absent but replaced by the plastron. In the Birds, in the flying forms, the sternum is one large body element, articulating with many ribs. It has a strong central keel, or carina for insertion of flight muscles.

Examine the sternum on the human skeleton, and notice the rib attachment via the costal cartilages, and the fact that only in mammals is the sternum of many segmented bones.

Pectoral Girdle

The pectoral girdle consists of both dermal and endochondral (replacement) bone. The cartilage replacement bones are:

1. Coracoid
2. Scapula
3. Suprascapula

The dermal investing bones are:

1. Clavicle
2. Cleithrum
3. Supracleithrum
4. Post-temporal

Cartilage elements alone occur in the Chondrichthyes (cartilaginous fishes). See the demonstration material of the shark skeleton and pectoral girdles. Note the coracoid bar, lateral scapular processes and the iliac processes at their tips. Examine the Teleost (bony fish) pectoral girdle, and note the addition of the dermal elements; the post-temporal bone may abut the posterior of the skull, as in the demonstration. The opercular plates conceal much of the girdle. Only the scapula is ossified in Necturus, and this is correlated with weak limb movement. Examine the demonstration material of Frog pectoral girdle, using the numbered sheet to identify the bones; note the loss of many dermal bones including the cleithrum, associated with movement of the humeral muscles on to the dorsal part of the girdle. The ventral coracoid processes are separate and sometimes overlap. Reptiles have the dermal elements of interclavicles, clavicle and occasionally parts of the cleithrum. Replacement bones are the scapula and a portion of the coracoid (procoracoid). In the Birds, clavicles are retained, and unite to form the furcula (wishbone). Posterior to it runs the paracoracoid bone and a dorsal scapula lies on top of the rib cage. Note loss of the coracoid in Mammals. Mammals have only two bones, the scapula (replacement) and the clavicle (dermal). In running Mammals (cursors) the clavicle may be reduced or absent. See the small splinter of bone in the cat skeleton that is all that remains of the clavicle. Compare this with the prominent clavicle of the Bat and the Opossum. The scapula persists through all the groups, and in the mammals it is a broad flat plate, divided by a scapular spine. It provides the main area for muscle insertion. The small coracoid process is a remnant of the coracoid bone. The scapula articulates with the humerus at the glenoid fossa.

Sketch and label the pectoral girdle of humans.

Pelvic Girdle (Figure 59, pg. 111)

The pelvic girdle has no dermal elements. It is composed of cartilage replacement bones only. These are the:

1. ILIUM - wing

2. ISCHIUM - back

3. PUBIS - front

Note the simple cartilage girdle in the Shark with a single puboischiac bar and small iliac process near each fin. In the bony fishes it is still simple, but ossified into a single Ischio-pubis, but here, as in the shark, it is still unattached to the vertebral column. Examine the tetrapod pelvic girdles to see the three main ossification areas. In lower forms the ventral area, the puboischiac plate is expanded for limb muscle attachment, but in more advanced forms the ilium becomes the largest element. Note the symphysis that occurs in most forms, but not in the bird. The latter lacks the ventral meeting of elements, and this is associated with the production of a large egg. To maintain strength, the three pelvic bones fuse together as the innominate bone and it is fused with the vertebral synsacrum.

Sketch and label the pelvic girdle in humans.

Limbs

Forelimbs, as early as the amphibians, have a single bone, the humerus in the upper limb and two forearm bones, the larger, anterior radius and posterior ulna, which often articulates with the radius. There are many wrist bones; primarily the carpals and the digits are composed of proximal metacarpals and distal phalanges.

Hind limbs consist of a thigh containing the femur bone and the shank with an anterior, larger, tibia and a posterior fibula. The ankle has many bones; primarily tarsals and the digits have proximal metatarsals and distal phalanges.

Examine the cat limb for the typical pentadactyl pattern. Compare plantigrade (Man) with digitigrade (Cat) and with unguligrade (Horse). Note the position of the wrist and ankle is quite different in these groups.

How many digits bear weight in plantigrades?

How many digits bear weight in digitigrades?

How do digitigrades differ from plantigrades?

How many digits bear weight in unguligrades?

There are two unguligrade types, the one with only digit III remaining (mesaxonic), as found in the horse (Perissodactyl, perisso = odd), the other with digits III and IV remaining to bear the body weight (paraxonic) and found in the pig, cow and deer (Artiodactyl).

Examine the adaptations for flight and swimming that are on demonstration.

Updated by Sandra Millen, September 2003.

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