SWIMMING BLADDER OF FISHES. 297 



of fishes and the feet of quadrupeds, there is none in the 

 manner in which they are instrumental in clFccting pro- 

 gressive motion. Tlie great agent h}^ which the fish is im- 

 pelled forwards is the tail: the fins, wliich correspond to the 

 extremities of land animals, are useful chiefly for the pur- 

 poses of turning, stopping, or inclining the hody, 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 hody, (the mesial plane,) prevent the rolling of 

 the body, wdiile the fish darts forwards 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, of expanding 

 and of closing the rays, like the opening and shutting of a 

 fan, according as their action is required to be eflective, or 

 the contrary. All these auxiliary instruments arc chiefly 

 serviceable in modifying the direction, and adjusting the 

 variations of force derived from the impulse of the tail. 

 :^They are employed, also, in suddenly checking or stopping 

 the motion, and giving it a more rapid acceleration. But 

 still the tail is the most powerful of the instruments for pro- 

 gression, being at once a vigorous oar, an accurate rudder, 

 and a formidable weapon of ofience. 



Independently of these external instruments of progres- 

 sion, most fishes are provided with internal means of changing 

 their situation in the water. The structure by which this 

 efiect is accomplished is one of the most remarkable in- 

 stances that is met with of an express contrivance for a spe- 

 cific purpose, 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 muscular 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. Were 

 the animal to acquire the power of altering, at pleasure, its 

 specific gravity, it would then possess the means of rising 

 Vol. I. 38 



