212 "V-iTURE OF THE INTERVERTEBRAL SUBSTANCE. 



body, corresponding in its use to the incompressible fluid in 

 the fish. 

 Other animals To ascertdin whether this structure was generally met 

 with in the spines of quadrupctly, Mr. Brodie, at my re- 

 quest, -examined the intervertebral substance in a great many 

 . animals, and found, what, undoubtedly, was very little to be 

 Hog & rabbit, expected, that in the hog and rabbit, in the central part, 

 there is a cavity with a smooth internal surface of the ex- 

 tent of half the diameter of the vertebra, in which is con- 

 tained a thick gelatinous fluid, so that in some quadru|)eds 

 there is an approach towards the intervertebral joint in fish ; 

 but whether this is to answer any essential purpose to these 

 animals, or is only to form an intermediate link in the chain 

 of gradation of structures, which is so uniformly adhered to 

 in the productions of nature, cannot at present be deter- 

 mined. 

 Bullock sheep ^^ ^^^ bullock, sheep, deer, monkey, and man, the struc- 

 deer, monkey, ture corresponds with that of the whale; in the three last, 

 the central substance appears to be the most compact. Be- 

 sides the structures already mentioned, there is in some ani- 

 Allegator. mals one of a very different kind ; in the alligator the ver- 

 tebrae through the whole length of the spine have regular 

 joints between them, the surfaces are covered with articulat- 

 ing cartilages; and there is synovia and a capsular ligament. 

 Snak«. j^ jj^g snake, there is a regular ball and socket joint between 



every two vertebrae; so that the means employed for the 

 motion of the back bone in different 'dnimals, comprehends 

 almost every species of joint with which we are acquainted. 

 Having mentioned a sufficient ntimber of facts to point 

 out the animals, in which the different structures of the in- 

 tervertebral substance are to be found, I have abstained from 

 being more particular in my account; as it would in no re- 

 spect elucidate the principal object of the present commu- 

 nication. 

 Human spine. From the facts and observations which have been stated, 

 it appears that t!ie intervertebral s^ibstance of the human 

 spine does not consist entirely of elastic ligament, dense in 

 its texture at the circumference, and becoming gradually 

 softer towards the centre; but that tSie middle portion is 



composed 



