COMPARATIVE ANATOMY 9 



species, the divisions of the vertebral column are not very 

 marked. In the shark there are only two regions, body and 

 caudal. 



Body Vertebrae. — Use a prepared specimen consisting of 

 several vertebrae. The vertebrae of the shark are compara- 

 tively simple in structure, composed largely of cartilage with some 

 deposition of lime, but have no real bone formation. The more 

 or less cylindrical basal portion of each vertebra is the centrum 

 or body. Above this is the neural arch forming a roof over the 

 neural canal, which serves for the passage of the spinal cord. 

 The centra are amphicoelus; that is, they are concave on each 

 end. At the bottom of each concavity is a small canal which 

 leads through the centrum to the concavity on the other end 

 of the vertebra. The continuous space thus formed Is occupied 

 by the remains of the notochord. On the ventral side of each 

 centrum transverse processes extend laterally. Attached to 

 each of these, during life, is a slender rib. The ribs are usually 

 absent on the separate preparations of vertebrae, but may be 

 seen on the mounted skeletons. The neural arches over the 

 central canal are composed of alternating neural processes and 

 interneural plates. These are roughly triangular In shape. 

 Each neural process has Its base resting on the centrum; the 

 Interneural plates fill In the spaces between successive neural 

 processes. The interneural plates of each side unite dorsally; 

 the neural processes may or may not. A foramen for the exit 

 of the ventral root of a spinal nerve lies In each neural process; 

 a foramen for the exit of the dorsal root In each Interneural 

 plate. 



Caudal Vertebrae. — The centra, neural processes and 

 interneural plates are similar to those of body vertebrae. A 

 haemal arch, which protects the large blood vessels of the tail, 

 lies ventral to the centrum. There are no transverse processes 

 and no ribs. 



Vertebral Column, Sternum and Ribs of Necturus 



Study the specimens mounted in glass jars. The vertebral 

 column consists of three regions: cervical (one vertebra), body, 

 and caudal. There is little differentiation of the vertebrae. 

 In the caudal region haemal arches are present. A single 



