412 THE MECHANICAL FUNCTIONS. 



carpio, or carp. The muscular flesh of fishes is 

 likewise softer than that of the higher classes ; 

 and the cellular substance more attenuated and 

 more gelatinous ; so that the membranes which 

 it forms are of a looser and more pulpy texture. 

 Progressive motion in fishes is effected by the 

 simplest means, the principal instrument em- 

 ployed for this purpose being the tail ; for the 

 fins, as we shall presently find, are merely auxi- 

 liary organs, serving chiefly to balance the body 

 while it receives its propulsion from the tail. A 

 fish moves in the water upon the same principle 

 as a boat is impelled in sculling ; for the action 

 of the tail upon the water is lateral, like that 

 of an oar, which it resembles in the vertical 

 position of its plane ; and the effect is transferred 

 i) by the resistance of the water to 



the body where the impulse ori- 

 ginates. Let us suppose, for 

 example, that the tail is slightly 

 inclined to the right, as shown 

 in Fig 185. If, in this situa- 

 tion, the muscles on the left side, 

 tending to bring the tail in a 

 right line with the body, are sud- 

 denly thrown into action, the resistance of the 

 water, by reacting against the broad surface of 

 the tail in the direction p R, perpendicular to that 

 surface, will cause the muscular action to give 

 the whole body an impulse in that direction ; and 



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