THE SKELETAL SYSTEM 



151 



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

 vertebrae. Its lateral wings are modified ribs fused together and 

 articulated with the hip bones. Spinous processes 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 

 strengthened by numerous vertebral ligaments. 



— EPIDERMIS 



I— SCLEROTOME 



NOTOCHORD 



-MYOTOME 

 DERMATOME 



Fig. 141. — 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. 



Development of the Vertebral Column. In man as in other verte- 

 brates the primary axial skeleton is the notochord. Around this the 

 definitive axial skeleton is built; and the notochord disappears, slight 

 traces only being 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 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 

 between the mesoderm and the notochord. Later, by a continuation of 

 the same migration, the neural tube becomes completely surrounded by 

 mesenchyma. 



Before cartilage is secreted in the mesenchyma, the sclerotome median 

 to each myotome becomes differentiated into a denser posterior portion 



