380 THE MECHANICAL FUNCTIONS. 



the upper and lower portions of the trunk ; their 

 limits being strongly marked by a line running 

 longitudinally the whole length of the body on each 

 side. The inclination of their fibres is somewhat 

 different in each. The advantage in point of velo- 

 city of action which results from this obliquity has 

 already been pointed out. 



Those fins which are in pairs are capable of four 

 motions ; namely, those of flexion and extension, 

 and also those of expanding and closing the rays, 

 for each of which motions appropriate muscles are 

 provided ; and indeed each ray is furnished with a 

 distinct muscular apparatus for its separate motion ; 

 and these smaller muscles regulate with great nicety 

 all the movements of the fins, expanding or closing 

 them like a fan, according as their action is to be 

 strengthened or relaxed. This feathering of the 

 fin, as it may be called, takes place in most fishes, 

 and is particularly observable in the tail of the 

 JEsox, or pike tribe. Each ray of these fins, indeed, 

 is furnished with a distinct muscular apparatus, for 

 its separate motion. 



Whatever analogy may exist in the structure 

 of the fins of fishes and the feet of quadrupeds, 

 there is none in the manner in which they are in- 

 strumental in effecting progressive motion. The 

 great agent by which the fish is impelled forwards 

 is the tail : the fins, which correspond to the extre- 

 mities of land animals, are useful chiefly for the 

 purposes of turning, stopping, or inclining the body, 

 and for retaining it in its proper position. The 

 single fins, or those which are situated in a vertical 

 plane, passing through the axis of the body (the 

 mesial plane), prevent the rolling of the body, 



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