NATURE OF THE INTERVERTEBRAL SUBSTANCE. 211 



vity is not in all exactly the same; in the skate it is ve- 

 ry similar to that in the squali, but in the common eel it is 

 more oblong, the loni^itudinal diameter beiuo- about one 

 third longer than th.e transverse one. 



It it evidently contrived for producing the quick vibra- 

 tory lateral motion, which is peculiar to the back bones of 

 fish while swimming, and enables them to continue that 

 motion for a great length of time, with a small degree of 

 muscular action. 



In the sturgeon, there are some curious peculiarities in Peculiarity in 

 the structure erf the spine. Externally there is the common *^^ ^l''"^ °^ 

 appearance of regular vertebrjae, but these prove to be only * ' 



cartilaginous rings, the edges of which are nearly in contact, 

 and are united together by elastic ligaments, forming a tube 

 the whole length of the spine; this is lined throughout its 

 internal surface with a firm compact elastic substance, about 

 the thickness of a cartilaginous tube ; within this is a soft 

 flexible substance in a small degree elastic; in the centre 

 there is a chain of cavities in the form of lozenges, contain- 

 ing a fluid, and communicating with one another by very 

 small apertures bearing a slight similarity to ^he interverte- 

 bral cavities of the spine in other fish. 



As all the different parts of which this spine is composed 

 are more or less elastic, except the central fluid, it must have 

 great flexibility adapting it to tlie motions of this particular , 



fish. The structure of the spine in the lamprey eel resem- prey. 

 bles that of the sturgeon. 



The intervertebral joint, which is common to fish, is not spineof the 

 met with in any of the whale tribe, whose motion through whale similar 

 the water is principally eff'ected by means of their horizon- j" ' ,e|is° ^"^' 

 tal tail ; in them the substance employed to unite the verte- 

 bree together is the same as in quadrupeds in general, and 

 from the size of the vertebrse it is on a larger scale, and ren- 

 dered more conspicuous. 



The external portion is very firm and compact, is ranered . , . 



., ., J., .. ° A gelatinous 



in concentric circles with transverse fibres uniting the layers substance ia- 



together, it becomes softer towards the middle, and in the ^^^-^'^ '^f '•''^ 



centre there is a pliant soft substance without elasticity, but 



admitting of extension more like a jelly than an organized 



P 2 body. 



