TUNNIES AND THEIR RELATIONS 



is a beating scull and not a turning screw, but this 

 caudal scull is nevertheless attached to an internal 

 shaft set in motion by the considerable mass of 

 muscle which composes the greater part of the bulky 

 trunk. If, like the propeller of a steamship, it seems 

 small in comparison with the body, on the other 

 hand the muscular mass which is designed to impel 

 it takes up a space whose great size is an indication of 

 its capacity. 



Nature has even improved upon the science of the 

 shipbuilder. In the construction of the various parts 

 of this mechanism it has introduced an arrangement by 

 sections or compartments which ensures a degree of 

 suppleness while it secures rigidity. The shaft which 

 works the caudal fin is none other than the spinal 

 column, constructed of bony vertebrae which are solid, 

 succeeding one another, connected by intervertebral 

 ligaments which form little cushions. In the medial 

 and anterior regions of the trunk, the vertebrae, thicker 

 there than in other parts, ensure to the shaft they 

 form a firmness like that of the ridge-pole of a roof, 

 but in the posterior, caudal zone, where they are 

 not so thick, the shaft is flexible, elastic, able to 

 bend to right and left, and so to give the appropriate 

 movement to the scull. 



Even this arrangement, remarkable though it is, is 

 surpassed in perfection by that of the musculature. 

 The latter, designed after the same manner as the 

 spinal column, is divided into a series of segments on 

 either side of it. These segments are in pairs and 

 are like disks placed one on top of the other from the 

 front of the body to the back. They are attached to 

 one another by transverse membranes which form a 

 sort of joining wall. Each of these is attached to the 

 centre of a vertebra so that the osseous system and 

 the muscular system, arranged in corresponding seg- 

 ments, work together perfectly. Now we are in a 

 position to understand how this living machine works. 

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