i 34 THE APPEARANCE OF JAWS v. 2 



The myotomes consist of blocks of longitudinal muscle-fibres, 

 placed on either side of an incompressible central axis, the notochord 



or vertebral column. The effect of 

 contraction of the muscle-fibres in any 

 myotome is therefore to bend the body. 

 In forward swimming the contraction 

 of each myotome takes place after that 

 in front of it. In this way waves of 

 curvature are passed down the body, 

 alternately on each side. This can be 

 illustrated by a series of photographs 

 of a fish such as the dogfish or eel in 

 which the amplitude of the waves is 

 large (Figs. 90 and 91). 



In other fishes the waves are not so 

 immediately obvious, but serial photo- 

 graphs show that even in such forms as 

 the mackerel and whiting there is a 

 backward movement of waves. The 

 number of waves per minute in steady 

 swimming varies from 54 in the dog- 

 fish to 170 in the mackerel, the 

 corresponding velocities of the waves 

 being 55 and 77 and of the whole fish 

 29 and 42-5 cm /sec. 



Gray has shown how the muscle 

 contractions produce movements of 

 the parts of the body, related to one 

 another in such a way as to transmit 

 a backward momentum to the water. 

 Fig. 92 shows superposed drawings of 

 an eel, made from successive photo- 

 graphs. The region marked XY is 

 moving from right to left and that 

 X 1 Y 1 from left to right and evidently, 

 as Gray puts it, 'all parts of the fish's 

 body which are in transverse motion have their leading surfaces directed 

 backwards and towards the direction of transverse movement, but 

 the angle of inclination is most pronounced when the segment is 

 crossing the axis of longitudinal motion, and at this point the segment 

 of the body is travelling at its maximum speed. Each point of the body 



Fig. 92. Enlarged drawings of suc- 

 cessive photographs of a young eel 

 superimposed on each other so that 

 the tips of the head are on the same 

 transverse axis and the longitudinal 

 axes of motion (ab) are made to 

 coincide. As the wave passes the 

 section XY it first moves to the left 

 and is directed backwards and to 

 the left, whereas X x Y\ moves 

 in the opposite direction. The tip 

 of the tail follows a figure of 8. 

 (From Gray) 



