84 INTRODUCTION. 



fishes are applied to each other, there is a deep 

 conical cavity, commonly with a small hole in the 

 centre ; and this hole is, in many of the cartila- 

 ginous fishes, so large, that the bodies of their 

 vertebrae represent almost one continuous tube. 

 These cavities are filled, in the living animal, with 

 a soft jelly-like matter, which extends, also, for 

 some little space, beyond their rims, being kept 

 in its place by a tough elastic membrane. The 

 fluid amounts, in some of the larger fishes, to some 

 pints, between every two vertebrae ; and such is the 

 pressure exercised upon it by the membrane by 

 which it is immediately invested, that, if this be 

 suddenly punctured in the skin, as noticed by Sir 

 Everard Home, the liquid is projected with a force 

 sufficient to carry it four or five feet high. Nothing 

 could possibly have been better adapted than this 

 part of the structure of the spine of fishes, to 

 ensure free motion, and to protect the surfaces of 

 the bone from injury when so continually plied. 

 The bodies of each vertebrae, in fact, move, as it 

 were, laterally on each other, by means of so many 

 interposed elastic balls. This motion is almost 

 entirely from side to side ; from the form of each 

 bone, and the presence of the upper and under 

 spinous processes, it must be obvious, that motion 

 in any other direction would be superfluous, while, 

 if it had been permitted, more important uses must 

 have been sacrificed. 



In the motions of all fishes^ the spinal column 

 is an essential assistant, and may be said to be the 



