28 THE BONES. 



entirely disappears. The insertion eminences also become less salient, 

 and the coccygeal vertebrae are soon reduced to small bony cylinders, 

 narrow in the middle and wider at both extremities, with a convex articular 

 surface at each end (except the last, which has only one articular surface). 

 These small cylinders, the last traces of the vertebral bodies, are each 

 developed from three centres of ossification ; they are very spongy and 

 light. The first coccygeal vertebra is frequently consolidated with the 

 sacrum in aged animals. 



THE SPINE IN GENERAL. 



The vertebral column has now to be considered in its entirety, and 

 examined successively in its superior face, its inferior face, its lateral faces, 

 and its spinal canal. Afterwards its direction and mobility will be 

 noticed. 



Superior face. This presents, on its median line, the series of spinous 

 processes. But little salient in the cervical region, these eminences are 

 much developed in the dorsal and lumbar, where they constitute a long 

 crest, the dorso-lumoar spine, as well as in the sacrum, where they form 

 the sacral spine. They soon disappear in the coccygeal vertebrae. 

 Outwards, and on each side of these processes, is seen a succession of 

 tubercles of insertion, represented in the cervical and lumbar vertebrae 

 by articular processes, and in the dorsal vertebrae by the superior or rugose 

 portion of the transverse processes. These tubercles are disposed in line, 

 and separated from the spinous processes by a channel designated the 

 vertebral groove, which is more or less deep and wide. It is on these, and 

 on the spinous processes, that the extensor muscular fasciculi of the spine 

 receive the greater portion of their fixed or moveable insertions. 



Inferior surface. Wide at the neck, this face becomes narrow in the 

 dorsal region, to be again widened at the lumbo-sacral region, and once 

 more contracted at the coccyx. Crests more or less developed, which divide 

 the vertebral bodies into two lateral portions, right and left, are remarked. 



Lateral surfaces. These offer for study the thirty-six intervertebral 

 foramina, through which the spinal nerves pass. They exhibit besides, 

 on the neck, the transverse processes; in the back, the external facets 

 of these processes, and the intervertebral facets, all destined to sustain 

 the heads of the ribs ; on the loins, the transverse or costiform processes. 

 It may be remarked that the ribs and the transverse processes of the neck 

 and loins furnish points of insertion to the powerful muscles which produce 

 the lateral movements of the spine. In the sacrum, the lateral faces are 

 formed for the articulation of the spine with the ossa innominata. 



Spinal canal. This canal communicates, in front, with the cranial cavity. 

 Very wide in the atlas, for the reception of the odontoid process and to permit 

 the rotatory movements of the head without injury to the spinal cord, this 

 canal suddenly diminishes in the axis. It again dilates at the termination 

 of the cervical region and the commencement of the dorsal ; there the spinal 

 cord presents a greater volume, and the movements of the spine are very 

 extensive. Towards the middle of the back, the spinal canal offers its 

 smallest diameter ; it widens from this part to the lumbo-sacral articula- 

 tion ; after which it contracts rapidly, and disappears altogether near the 

 fourth or fifth coccygeal vertebra. The lumbo-sacral dilatation coincides 

 with the enlargement of the cord in this region, and with the enormous 

 quantity of nerves lying beside it. 



