210 NATURE OP THE INTERVERTEBRAL SUBSTANCE. 



ral principle upon which all intervertebral joints are form- 

 ed, which has been hitherto but imperfectly understood. 

 With thib view, I will first describe the principle upon which 

 this particular joint is formed, and then show the resem- 

 blance that it bears to those of other animals, in which the 

 parts are not so readily distinguished from one another, and 

 consequently their precise use has not been accurately as- 

 certained. 

 Use of the '^^^ Huid contained in the cavity, being incompressible, 



fluid. preserves u proper interval between the vertebrae, to allow of 



the play of the lateral elastic ligaments, and forms a ball 

 round which the concave surfaces of the vertebrae are mov- 

 ed, and readily adapts itself to every change, which takes 

 place in the form of the cavity. 

 Elasticity of The elasticity of the ligaments, by its constant action ren- 

 the ligaments, (Jers the joint always firm, independent of any other sup- 

 port, and keeps the ends, of the vertebrae opposed to each 

 other, so that the whole spine is preserved in a straight line, 

 unless it is acted on by muscles or some other power. 

 When a muscular force is applied to one side of the spine, 

 it stretches the elastic ligament on the opposite side of the 

 joint, and as soon as that force ceases to act, the joint re- 

 a substitutefor turns to the former state. This is one of the most beauti- 

 ful instances in nature of elasticity being employed as a 

 substitute for muscular action. 



The extent of the motion in each particular joint is un- 

 doubtedly small, but this is compensated by their number, 

 and the elasticity of the vertebrae themselves. 

 Other fish Fish in general have their vertebrae formed with similar 



have a similar ^^Q^cavities to those of the squalus maximus; these, when 

 examined after death, contain a solid jelly, but in the living 

 fish it is found in a fluid state. This fact was acertained in 

 the skate, the smallness of the quantity of fluid in any one 

 joint, and the readiness with which it coagulates after death, 

 prevented it from being before observed: the fluid in the 

 skate is found by Mr. Brande to have the same properties, 

 sa far as the small quantity that can be collected admits of 

 examination, with that in the squalus maximus. 

 Form *f the Although this structure of the intervertebral joint ap- 



pears to be common to fish in general; the form of the ca- 



vitv 



muscular ac- 

 tion. 



cavity difFei 



