334 



THE CAT. 



[CHAP. x. 



oc 



deposited at intervals along eacli dorsal lamina, forming cartilaginous 

 neural arches which, however, do not for sometime fully unite together 

 on the dorsal side of the myelon. Cartilages also extend down in 

 each ventral lamina as the cartilaginous predecessors of the ribs, and 

 those of the thorax, by their median fusion in the mid- ventral line, 

 lay the foundation of the sternum. 



The third stage of vertebral development, or the ossification of 

 the spinal column, begins to show itself as three (or four) ossific 

 centres arising in each of the vertebrae. These centres are placed 

 one in each lateral part (at the junction of the transverse process 

 and neural arch), and the other, or third, in the centrum. This 

 last is sometimes double at first. 



Besides these separate centres each vertebra, while immature, has 

 certain epiphyses or temporary separate terminal ossifications. Thus 



the tip of each prominent process 

 (neural spines, transverse and 

 articular processes and the met- 

 apophyses) has its epiphysis, 

 and a thin lamina of bone is 

 developed as an epiphysis on both 

 the anterior and posterior sur- 

 face of each centrum. The atlas 

 ossifies from two lateral ossifica- 

 tions, and one median one ven- 

 trally placed. 



In the axis, in addition to the 

 ordinary ossific centres found 

 in other vertebrae, the odontoid 

 process ossifies from two centres, 

 placed side by side, which soon 

 unite. There is also an epiphy- 

 sis at the apex of the odontoid 

 process, and one between it and 

 the centrum of the axis as well 

 as on the hinder surface of the 

 latter. Thus the odontoid pro- 

 cess ossifies as if it were, as in 

 fact it is, the true centrum of the 

 atlas vertebra. 



The ribs are ossified each from 

 one centre, with an epiphysis for 

 the tuberculum and another for 

 the capitulum. 



In the cervical vertebra more or fewer of the transverse processes 

 ossify, at least occasionally, from a distinct ossific centre in their 

 ventral branch, a circumstance which tends to show (what is in fact 

 the case,) that these double (or perforated) transverse processes are 

 ribs with very short bodies. 



Similarly the sacral vertebrae have each a distinct ossific centre in 

 each of their lateral masses. 



F 



THE 



I 



152. DIAGRAM OF F<ETUS, SHOWING 

 ISCERAL ARCHES AND BUDDING LIMBS. 

 (It represents mainly conditions existing 



between the third and fourth weeks.) 

 n. Prominence produced by incipient cerebrum. 

 l>. Prominence produced by region of third 

 ventricle. 



c. Prominence by region of corpora quadrige- 



mina. 



d. Prominence produced by cerebellum. 



e. Prominence produced by medulla. 

 aL Allantois. 



li. Heart. 



Z. Liver. 



an. Incipient ear. 



oc. Incipient eye. 



na. Incipient nose. 



ns. Naso-frontal process. 



mps. Maxillary process (from first visceral arch) 



laying foundation of upper jaw. 

 1, 2, 3, 4. " Visceral arches ;" between them are 



the "visceral clefts." 



