140 LOCOMOTION. [CHAP. vi. 



most exposed, since it is the part of the head which would strike 

 upon the ground when a man falls backwards."* 



Of the Spine. The spinal column, in man, is a vertical, elastic 

 pillar, expanded inferiorly where it rests upon the sacrum. It is 

 composed of a series of light, and spongy bones, between each pair 

 of which (except the first) a compressible and elastic disc of fibro- 

 cartilage is placed. It has a three-fold office in the human subject : 

 first, it is the great bond of connexion between all the parts of the 

 skeleton ; secondly, it forms a canal for the lodgement and protec- 

 tion of the spinal cord; and, thirdly, it is a column of support 

 for the head. For these purposes, the spinal column requires consi- 

 derable strength, as the central pillar of the trunk : it needs mobi- 

 lity, to adapt itself to the various attitudes and movements of the 

 body ; and elasticity, to guard the tender organ contained within it, 

 as well as the brain, from concussion. 



The strength of the spinal column is abundantly provided for in 

 the powerful ligament which binds the bodies of the vertebrae toge- 

 ther in front (anterior common ligament}, and in the strong and 

 elastic intervertebral discs, which at once connect and separate 

 them. The degree of motion which may take place between 

 any two vertebrae is regulated partly by the thickness of the 

 intervertebral disc, and partly by the disposition of the joints of 

 the articular processes. When the latter are vertical in their direc- 

 tion, the vertebras are so locked in, that their movements are very 

 much impeded ; but when they approach the horizontal direction, 

 as in the neck, the range of motion is greater. The mobility of the 

 spine may be compared to that of a chain ; between any two links 

 of which there is but little motion, while the whole chain is abund- 

 antly pliant. This restriction of motion between each pair of ver- 

 tebrae enhances the strength of the column, and affords complete 

 protection to the spinal cord, which would speedily suffer, did any 

 vertebra pass beyond its prescribed limits. 



In the flexuous form of the spinal column, and in the connexion 

 of the vertebral laminae by broad bands of yellow elastic ligament, 

 we see further provision for its elasticity, in addition to that 

 afforded by the discs of fibro- cartilage which lie between the bodies 

 of the vertebrae. The concavity in the region of the back is doubt- 

 less intended to give full scope to the play of the important organs 

 within the thorax ; and the cervical and lumbar curves necessarily 

 result from this, in order that the relation of the whole column 

 the line of gravitation of the body may be duly preserved. T] 



* Sir C. BeU. 



