ARTICULATIONS OF THE VERTEBRA 397 



animal, and is confined to the hip and shoulder joints, in which it is far 

 more limited than in the corresponding joints of the human frame. It 

 is displayed when a limb is made to describe a segment of a large cii^cle 

 around the joint which connects it to the body. 



4. Rotation is the movement of a bone on its own axis, and is only seen 

 in the horse in the joint between the two first vertebrae of the neck. 



ARTICULATIONS OF THE VERTEBRAL COLUMN 



The vertebra are connected together by ligaments, fibro-cartilage, and 

 synovial membranes ; the first two serving to retain them in position ; the 

 last to facilitate motion. They correspond, fiz'stly, by their bodies ; secondly, 

 by their spines ; and thirdly, by their oblique and transverse processes. It 

 is necessary to state, that the general details into which this study leads 

 us will apply only to the articulations which unite the six loAver cervical 

 vertebrae, the dorsal and lumbar vertebrae, and the sacrum. 



The bodies connect themselves by their surfaces, which in the cervical 

 region represent, 1st, the anterior, or true head; 2nd, the posterior, or 

 glenoid cavity, which receives the head of the vertebra immediately behind 

 it. In passing from the first dorsal to the sacrum, these tend to efface them- 

 selves, and become plainer ; nevertheless, they preserve throughout the one 

 its convexity, and the other its concavity. Their means of union are — (1) 

 fibro-cartilages, interposed between the articular surfaces ; (2) a common 

 superior vertebral ligament ; (3) a common infeidor vertebral ligament. 



The intervertebral fihro-cartilages are circular or elliptical discs, convex 

 before, concave behind ; firmly fixed to the surfaces of the bones which 

 they separate. The fibro-cartilaginous substance which forms them is 

 composed of an external laminar part, constituting the circumference of an 

 internal soft or pulpy part, which occupies the centre. The laminar part 

 forms more than half the whole mass, and consists of laminae, or plates of 

 fibro-cartilage, and fibrous tissue, alternating one with the other. The 

 central part is a pulpy, elastic material, which is of a yellowish colour, and 

 destitute of the concentric arrangement seen externally. The fibro-cartilages 

 join at their circumference the two common vertebral ligaments, and in the 

 vertebrae of the back help to form the intervertebral cavities destined for 

 the reception of the heads of the ribs. 



The superior vertebral ligament within the spinal canal, and attached to the 

 posterior sui^face of the bodies of the vertebrae, extends from the dentata to 

 the sacrum. In the neck, it spreads across the bodies ; but in the back and 

 loins it is broader opposite the intervertebral cartilage than opj)osite the 

 bodies of the bones. It adheres firmly to the fibro-cartilages and to the 

 contiguous margins of the bodies of the vertebrae ; but it is separated from 

 their middles by a transverse venous plexus. 



The inferior vertebral ligament reaches from the fifth dorsal vertebra to 

 the first bone of the sacrum ; becoming broader and broader as it approaches 

 the sacrum, along its course it connects itself to the inferior spines of the 

 bodies of the vertebrae and to the intervertebral discs. 



The union op the vertebra through their spinal part is effected by a 

 superior spinal ligament and an interspinal ligament. 



