FINS OF FISHKS. 381 



while the fish darts forwards in its course. The 

 fins which are in paiis (tliat is the pectoral and the 

 ventral fins,) by their alternate flexions and exten- 

 sions, act like oars ; while they are capable, at the 

 same time, of expanding and of closing the rays, 

 like the opening and shutting of a fan, according as 

 their action is required to be effective, or the con- 

 trary. All these auxiliary instruments are chiefly 

 serviceable in modifying the direction, and adjust- 

 ing the variations of force derived from the impulse 

 of the tail. They are employed also in suddenly 

 checking or stopping the motion, or giving it a more 

 rapid acceleration. But still the tail is the most 

 powerful of the instruments for progression, being 

 at once a vigorous oar, an accurate rudder, and a 

 formidable weapon of offence. 



Independently of these external instruments of 

 progression, most fishes are provided with internal 

 means of changing their situation in the water. 

 The structure by which this effect is accomplished 

 is one of the most remarkable instances that is met 

 with of an express contrivance for a specific pur- 

 pose, and of the employment of an agency of a 

 class different from that of the mechanical powers 

 usually resorted to for effecting the same object. 

 We have seen that if the body of a fish were 

 heavier than an equal bulk of water, and if no mus- 

 cular exertions were made, it must necessarily de- 

 scend in that fluid. If, on the contrary, it were 

 specifically lighter, it would as necessarily rise to 

 the surface. But if the animal had the power 

 of altering at pleasure its specific gravity, it 

 would then possess the means of rising or sinking, 

 without calling into action either the fins or the 



