THE ENDOSKELETON: VERTEBRAL COLUMN AND RIBS 73 



typical cervical vertebrae. They have low neural arches and spines, well- 

 developed zygapophyses, and ribs. The ribs are the masses attached to the 

 sides of the vertebrae and bearing ventrally sharp spines directed posteriorly. 

 The ribs, as in reptiles, articulate with the vertebrae by two heads, forming 

 thereby a vertebrarterial canal. The ribs of the last two cervical vertebrae are 

 longer than the others. The thoracic vertebrae are those whose ribs extend 

 completely to the ventral side. They have high neural spines and well- 

 developed transverse processes; spines, centra, transverse processes, and zyga- 

 pophyses are immovably fused to one another. The last thoracic, the lumbar, 

 the sacral, and the first few caudal vertebrae are fused into one continuous piece, 

 the so-called sacrum or synsacrum, which is separated by a suture from the 

 broad hip bones which extend on either side of it. By examining the ventral 

 side of the synsacrum the individual vertebrae of which it is composed can be 

 distinguished by their separate transverse processes. It should be noted that 

 only certain of these vertebrae are the true sacral vertebrae, corresponding to 

 those of reptiles, the vertebrae in front and behind these having been secondarily 

 fused to them to furnish additional support for the hind limbs. Posterior to 

 the synsacrum are a few free caudal vertebrae ending in an enlarged piece, the 

 pygostyle, which represents several fused vertebrae. Haemal arches are absent 

 from all of the vertebrae. The ribs of birds are divisible into an upper verte- 

 bral rib, composed of bone, and a lower sternal rib, or costal cartilage, partly 

 cartilaginous. The vertebral ribs bear backwardly directed processes, the un- 

 cinate processes, characteristic of birds and serving apparently to lend greater 

 5rmness to the ribs. The ribs of birds are true or intermuscular ribs. 



The vertebrae of birds are, like those of reptiles, gastrocentrous, that is, they consist of a 

 centrum composed of the interventrals and a neural arch composed of the basidorsals. As 

 haemal arches are absent on the vertebrae, it is evident that the basiventral elements are 

 lacking (except presumably in atlas and axis). 



I. VERTEBRAL COLUMN OF MAMMALS 



The vertebral column of mammals is markedly differentiated into the usual 

 five regions: cervical, thoracic, lumbar, sacral, and caudal; and the vertebrae 

 of these regions are so distinct from each other as to be readily identifiable when 

 isolated. Study whole mounted skeletons of the cat or rabbit and isolated 

 vertebrae. 



i. The cervical vertebrae. There are, as in almost all other mammals, seven 

 cervical vertebrae. The first two are differentiated as the atlas and the axis. 

 The atlas is very different from the others; it is ring shaped and has wide wing- 

 like transverse processes, which are perforated by a pair of openings, the verte- 

 brarterial canals. The low, flat neural arch of the atlas is also perforated by a 

 pair of holes for the passage of the spinal nerves. The anterior end of the atlas 

 bears a pair of large, curved articulating surfaces which support the skull, and 



