DISSECTION OF THE BACK AND THORAX. 139 



is continued backwards beneath the sacrum, but it is not traceable as 

 a distinct ligament farther forwards than the 6th dorsal vertebra. 



The Superior Common Ligament lies on the floor of the spinal canal, 

 and must be exposed by the removal of the neural arches. It is a 

 riband-like structure adherent to the vertebral bodies, and to the inter- 

 articular ligament of the ribs or to the upper edge of the intervertebral 

 disc. The edges of the ligament are scalloped, the ligament being 

 widest where it passes over the intervertebral discs, and narrowest at 

 the middle of the vertebral bodies. It is continued into the sacral and 

 coccygeal regions. 



The Intervertebral Substance. — Between every two adjacent vertebral 

 bodies there is interposed a disc of fibro-cartilage. This will be best 

 seen on making a vertical mesial section of two centra. The disc is 

 thinner in the back than in the loins or neck. The disc between the 

 last lumbar body and the sacrum is especially thick. In the dorsal 

 region they concur in forming the cavity for the head of a rib. The 

 disc is not of uniform texture throughout. The peripheral part of each 

 is composed of alternating layers of fibrous tissue and fibro-cartilage. 

 In each layer the fibres pass in an oblique direction between the two 

 bones, and in successive layers the fibres are alternately oblique in 

 opposite directions. The central portion of the disc is pulpy, soft, 

 and elastic; and is interesting as being a persistent portion of the 

 foetal chorda dorsalis. The peripheral part of the disc constitutes an 

 extremely resistant bond of union between the two vertebrae, while 

 the central pulpy portion permits rotation of the one bone on the other. 



Movements of the dorso-lumbar part of the spinal column. These are 

 flexion, extension, lateral inclination, and rotation. Flexion and extension 

 are opposite movements taking place in a vertical plane. In flexion the 

 downward concavity of the column is increased, in extension it is 

 diminish'ed. These movements are much more restricted here than in 

 the neck owing to the thinness of the intervertebral disc and the large 

 size of the spinous processes. They have a greater range in the loins 

 than in the back. Lateral bending is also much less free than in the 

 neck, being impeded by the thinness of the intervertebral substance, 

 and by the ribs and lumbar transverse processes. Its greatest range of 

 movement is in the anterior part of the lumbar region. Rotation is the 

 twisting or turning of a vertebra round ~a longitudinal axis passing 

 through its body. It is not permitted in the lumbar region, owing to 

 the form of the articular processes ; and even in the back it. is scarcely 

 appreciable. 



the spinal cord. 



Directions — To expose the spinal cord of the horse in the whole of its 

 extent is a tedious and difficult operation. -Moreover, where the dissec- 



