COMPARATIVE ANATOMY ii 



Examine a fourth or fifth thoracic vertebra. Assuming It to 

 be a typical vertebra, study it In detail; and then compare It 

 with the vertebrae of other regions. 



Centrum. — This is the solid body of the vertebra, ventral 

 to the neural canal. Each centrum articulates with centra of 

 adjacent vertebrae. The smooth ends of the centrum (as on 

 other bones) are the epiphyses. In kittens the epiphyses are 

 easily pulled oiT, but in adult cats they are often so completely 

 fused to the rest of the centrum that they are not readily dis- 

 tinguishable. The centrum is acoelus, meaning that its ends 

 are not hollowed out to form cavities. On the caudal surface, 

 at the dorsal-lateral corner on each side, find a small smooth 

 area bounded by a ridge of bone. These are costal demifacets. 

 Similar costal demifacets, not bounded by bony ridges, are to be 

 found on the cranial end of the centrum. The posterior demi- 

 facet of one vertebra and the anterior demifacet of the succeed- 

 ing vertebra form the surface for the articulation of the head 

 of a rib. 



Neural Arch. — This is the bony arch above the centrum. 

 It is continued dorsally into a long neural spine. The cavity 

 beneath the arch Is the neural canal., which during life contains 

 the spinal cord. Between the arch and the centrum on the 

 posterior end Is a notch, the intervertebral foramen, for the exit 

 of a spinal nerve. From the neural arch on each side extends 

 laterally a short transverse process, which bears on the ventral 

 face of Its distal end a smooth tubercular facet for articulation 

 with the tubercule of a rib. On the cranial border of the neural 

 arch, between the transverse processes, are two slight projec- 

 tions, the prezygapophyses (cranial articular processes), each 

 with a smooth dorsal face, the cranial articular facet. On the 

 caudal border, at the base of the neural spine, are similar 

 postzygapophyses (caudal articular processes), but with the caudal 

 articular facets facing ventrally. What Is the relation of the 

 caudal facets to the cranial facets .^ 



Examine the other thoracic vertebrae In a mounted skeleton, 

 and see how they difi"er in detail from the one studied. 



After studying a thoracic vertebra to serve as a type, com- 

 pare vertebrae from other regions with it. 



Atlas. — This is the first cervical vertebra. The centrum 

 has been appropriated by the second vertebra; its place Is taken 



