TUNNIES AND THEIR RELATIONS 



When these discoidal muscles contract, they pull upon 

 the joining membranes and these, in turn, pull upon 

 the vertebrae. They act directly upon the spinal 

 column or, in other words, upon the shaft which is 

 terminated by the propeller. If the fore part of this 

 shaft, the coherent part, does not move but simply 

 acts as a support, the part behind, on the other hand, 

 bends alternately to right and left according to the 

 muscular contractions of both sides. In this way it 

 conveys to the caudal fin its power of beating the 

 water and so sending the fish forward, and furnishes 

 it with an intense power which it employs in accordance 

 with this constitution. 



Several details in the internal arrangement of this 

 organisation add to its functional capacity. The 

 tunnies, which are large or comparatively large animals, 

 are chopped up into slices in the fish markets and so 

 sold. We see that each of these slices is made up of 

 concentric rings, most of which make four groups, 

 two on the right and two on the left, flanking the 

 spinal column in the middle. These four groups give 

 us cross sections of the discs, one above and one 

 below on each side, and these represent the muscular 

 segments. Their appearance is due to the fact that 

 these segments, instead of extending vertically from 

 above downwards, fit into one another and form folds 

 as a number of wafer cornets, placed one on top of 

 the other, would do. When the fishmonger's knife 

 cuts across the fish, it meets quite a number of them, 

 and they appear in the slice like rings fitting into one 

 another. 



There is a definite purpose in this construction. It 

 increases the power of action. This fitting in of the 

 segments brings together the separate efforts of each 

 and makes them all work together to the same end, 

 assembling them in such a way as to be able to apply 

 them all at once to the hinder part of the trunk, the 

 true tail with its caudal fin. Like the screw in relation 



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