74 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



its posterior end has likewise two surfaces which articulate with the axis. As in 

 the case of the alligator, the ventral side of the atlas ring is the basiventral; 

 the sides and top, the basidorsals; parts of the transverse processes are ribs; 

 and the centrum (interventral) is attached to the axis. Draw the atlas from the 

 front. 



The second cervical vertebra, the axis or epistropheus, has a very long and 

 broad neural arch, provided with a neural spine which projects forward over the 

 atlas. Its centrum bears at its anterior end a pointed projection, the odontoid 

 process, which fits into the ring of the atlas, allowing the turning of the head. 

 The odontoid process is in reality the centrum of the atlas. The axis bears 

 laterally the so-called transverse processes, but only the dorsal part of this is the 

 real transverse process, the lower half being a rudimentary rib. This rib, as is 

 usually the case, is united with the vertebra and the transverse process in such 

 a manner as to leave an opening, the vertebrarterial canal. The anterior end 

 of the centrum of the axis has two articulating surfaces for the atlas, its posterior 

 end, one for the next vertebra. Its neural arch has a pair of postzygapophyses. 

 Draw the axis from the side. 



The remaining cervical vertebrae are more or less similar. They have well- 

 developed neural arches and spines, the latter increasing in height toward the 

 thoracic region. All have pre- and postzygapophyses and transverse processes. 

 The latter are in reality in part composed of a rib, resulting as before in the 

 formation of the vertebrarterial canal. Draw a cervical vertebra from the 

 front. 1 



2. The thoracic vertebrae. These vertebrae are recognizable through the 

 fact that they bear long ribs, most of which extend to the ventral side. In the 

 isolated vertebrae the smooth costal facets where the ribs were attached always 

 serve to identify them as thoracic. There are thirteen thoracic vertebrae in 

 the cat, generally twelve in the rabbit, various numbers in other mammals (man 

 has twelve). The majority of the thoracic vertebrae have very tall neural spines, 

 directed caudad, short centra, small pre- and postzygapophyses, and short, stout 

 transverse processes, bearing at their ends a facet for articulation with the 

 upper head of the rib. The centrum bears at its anterior and posterior ends 

 half-facets for the lower head of the rib. The last thoracic vertebrae differ 

 somewhat from the others. They are longer and stouter, with short neural 

 spines, more prominent zygapophyses, and much reduced transverse processes. 

 They bear but one facet for the ribs, a small depression near the anterior 

 end of the centrum. Below the postzygapophysis in these last thoracic verte- 

 brae is a posteriorly directed process, the accessory process, conspicuous in 

 the cat. 



1 Note that the anterior and posterior ends of vertebrae can be recognized as follows: the smooth 

 articulating surfaces or facets of the prezygapophyses face upward or forward while the facets of the 

 postzygapophyses face downward or backward. 



