364 THE MECHANICAL FUNCTIONS. 



The body of a fish is nearly of the same specific 

 gravity as the water it inhabits ; and the effect 

 of gravity is therefore ahnost wholly counterba- 

 lanced by the buoyant force of that tiuid ; for the 

 weight of a mass of water, equal in bulk to the 

 body itself, is the exact measure of this buoyant 

 force. If this weight were precisely the same as 

 that of the fish, the animal would be able to remain 

 suspended in any part of the fluid without the 

 necessity of employing any voluntary motion or 

 exertion for that purpose ; but as the body of a 

 fish is generally a little heavier than the fluid 

 medium, especially if it be fresh water, it is neces- 

 sary for the animal to give its body some degree 

 of motion, in order to prevent its sinking. 



In land quadrupeds, the limbs have to perform 

 the double office of supporting the body, and of 

 effecting at the same time its locomotion ; but as 

 nearly the whole of the weight of a fish is already 

 sustained by the element in which it is immersed, 

 its instruments of motion may be employed exclu- 

 sively for progression ; and the powerful hydrostatic 

 pressure, which supports the body on all sides, 

 supersedes the necessity of that cohesive rigidity 

 of frame, which is essential to the safety of terres- 

 trial animals. Hence we find that in one whole 

 tribe of fishes, the skeleton is composed merely of 

 cartilage ; and, in all, it exhibits much less of the 

 osseous character than in the higher classes. The 

 framework of the skeleton, even of osseous fishes, 

 has not the compactness possessed by that of 

 quadrupeds or reptiles : the pieces which compose 

 it are joined together less firmly ; many of them. 



