SWIMMING IN FISHES. 427 



tension, 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 Esox, 

 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 

 instrumental in effecting progressive motion. 

 The great agent by which the fish is impelled 

 forwards is the tail : the fins, which correspond 

 to the extremities 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, while the fish darts for- 

 wards in its course. The fins that are in pairs 

 (that is the pectoral and the ventral fins), by 

 their alternate flexions and extensions, act like 

 oars ; while they are capable, at the same time, 



