THE SKELETAL SYSTEM 



209 



The sacnim is a spade -shaped bone formed by the fusion of five 

 vertebrae. Its lateral wings are modified ribs fused together and also 

 articulated with the hip bones. Spinous proc- 

 esses are much reduced. Between the costal 

 processes four pairs of sacral foramina provide 

 exit for nerves and blood vessels. The sacral 

 canal is the continuation of the vertebral canal. 



The coccyx consists of four fused centra 

 which lack neural arches and processes. 

 Frequently the first of these vertebrae fuses 

 with the sacrum, and only the last three form 

 the coccyx. 



Successive vertebrae are connected to form 

 a continuous column by intervertebral discs of 

 fibrous cartilage. Interconnexions are further vertebrate embryo at begin- 



.... ning of scleroblast migration. 



Strengthened by numerous vertebral ligaments, showing grooves (g) between 

 Development of the Vertebral Column. In notochord and spinal cord 



. . . , and blood vessels, a, artery; 



man as m other vertebrates the primary axiai 



Fig. 165. — Diagram 



tn, myotome; tns, 

 Around this the ing mesenchyme; 



migrat- 

 , noto- 

 chord; s, spinal cord; v, 

 vein. (From Kingsley's 



skeleton is the notochord 



definitive axial skeleton is built; and the 



notochord disappears, slight traces only being "Comparative Anatomy of 



left as nuclei pulposi of the intervertebral 



cartilages. The processes involved in this replacement are complicated, 



beginning with the appearance of mesenchyma cells around the notochord 



__^^^^^^^^^^^s§«:ss2Sii:^^^^^S5EES — EPIDERMIS 



I— SCLEROTOME 



NOTOCHORD 



— MYOTOME 

 — DERMATOME 



Fig. 166. — A horizontal section of an elasmobranch embryo, showing the differentia- 

 tion of the mesoderm (epimere) into sclerotome, myotome and dermatome. The 

 sclerotome surrounding the notochord gives rise to the centrum of the vertebra. 



and the neural tube. In this mesenchymal matrix, cartilage develops 

 only to be destroyed in its turn and replaced by bony vertebrae. 



The mesenchyma from which the vertebrae arise is produced by 

 proliferation of the sclerotome, the cells of which migrate into the space 



