FISHES. 287 



in sculling ; for the action of the tail upon the \vatcr is late- 

 ral, like that of an oar, which it resembles in the vertical po- 

 sition of its plane; and the effect is transferred by the resist- 

 ance of the water to the body where the 

 impulse originates. Let us suppose, for 

 example, that the tail is slightly inclined 

 to the right, as shown in Fig. 185. If, in 

 this situation, the muscles on the left side, 

 tending to bring the tail in a right line 

 with the body, are suddenly thrown into 

 action, the resistance of the water, by re- 

 acting against the broad surface of the 

 tail in the direction p r, perpendicular to 

 that surface, will cause the muscular ac- 

 tion to give the whole body an impulse in that direction; and 

 the centre of gravity, c, will move onwards in the direction 

 c B, parallel to p r. This impulse is not destroyed by the far- 

 ther flexion of the tail towards the left side, because the 

 principal force exerted by the muscles has already been ex- 

 pended in the motion from r to m, in hringing it to a straight 

 line with the body ; and the force which carries it on to l is 

 much weaker, and, therefore, occasions a more feeble reac- 

 tion. When the tail has arrived at the position l, indicated 

 by the dotted outline, a similar action of the muscles on the 

 right side will create a resistance and an impulse in the di- 

 rection of K L, and a motion of the whole body in the same 

 direction, c a. These impulses being repeated in quick suc- 

 cession, the fish moves forwards in the diagonal c d, interme- 

 diate between the directions of the two forces. By bending 

 the whole body almost in a circle, and then suddenly straight- 

 ening it, fishes are often able to leap to the top of a high ca- 

 taract, in ascending against the stream of a river. 



Such being the plan upon which progression is to be ef- 

 fected, we find that every part of the mechanism of the fish 

 is calculated to promote its execution. The principal mus- 

 cular strength is bestowed upon the movements of the tail; 

 and the largest assemblage of muscles consists of those which 

 give it the lateral flexions that have been just described. 



