Chapt. XXIII. 



Tin Swimming of Fish. 



293 



swimming, Borelli, having cut ofi the central (ins of a living fish, pat 



it back again into the pond. It then rolled from Bide to side like a 



drunken man, and could not keep an, upright position. When the 

 fish move with greal velocity the pectoral fins are laid close to the 

 lxuly, in order that they may not retard its motion ; and in rapid 

 motion the tail becomes the great propelling organ. We shall 

 therefore now investigate its mode of action. The first move- 

 ment of a fish from a state of rest is produce. 1 by a flexion of the 

 tail (as seen in Fig. at a) ; during this movement the centre of 

 gravity (e) is drawn slightly backwards. When the tail has arrived 

 at (i it is forcibly extended by its 

 muscles in the direction a i, per- 

 pendicular to its plane; the force 

 "f its action npon the water in a i 

 is translated to the tish in the direc- 

 tion of i a, causing the centre of 

 gravity (c) to move obliquely for- 

 wards, in the direction c h parallel 

 to i a. The tail having reached the 

 central line c d, its power of urging 

 the body forwards not only ceases, 

 but during its flexion on the opposite 

 side in the line a o, it tends to draw 

 the body backward in the direc- 

 tion o e. Having reached the point 

 o it is again rapidly extended in 

 the line o e causing an impulse 

 on the centre of gravity in c b 

 parallel to o e. If the two forces 

 c It and c b acted simultaneously 

 we should obtain the resultant cf; 

 bnt as they do not, the point (c) 

 will not move exactly in the right 

 line c /, but in a curved line 



which lies evenly between d o f and a line drawn parallel to it 

 through h. The fish being in motion while the tail moves from side 

 to side, according to Borelli, it describes an ellipse instead of a 

 circular arc which would be the case if the body were stationary 

 and the tail only moving. The velocity with which fishes move, and 

 the continuance of their movements, are enough to give us an idea of 

 the great strength of their muscles especially when we reflect on the 

 density of the fluid which is opposed to their speed." 



