3o6 



ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



— originally dorsal in position, spread downwards in a ventral direction, 

 interposing themselves between the more ventral part of the coelome 

 and the surface of the body, so that the whole of the side of the body 

 becomes muscularized down to the mid-ventral Une. In a fish the some- 

 what ^-shaped myotomes are easily seen if the skin is stripped off a 

 portion of the body : they form the flakes into which a cooked piece of 

 fish readily divides. 



In this arrangement of the main muscles in the form of segmental 

 blocks of longitudinal fibres we have one of the most fundamental 

 characteristics of the phylum Vertebrata. In the fishes throughout life 

 the main part of the muscular system remains in the form of typical 

 myotomes. In other types of vertebrate — e.g. in a man or a bird, where 

 the myotomes become during development broken 

 up into pieces, displaced in position and obscured 

 in various ways — they are still apparent in per- 

 fectly typical condition during early stages of 

 development. 



This fundamental arrangement of the verte- 

 brate muscular system is .also of great physio- 

 logical interest, for such an arrangement is clearly 

 intended for the production of movement like 

 that of an Eel when it swims : it indicates to 

 us that the primitive vertebrate swam by waves 

 of lateral flexure passing backwards along the 

 body. It is obvious that if the individual fibres 

 extended the whole length of the body, contrac- 

 tion of those on one side would simply bend 

 the body in a single curve towards that side 

 (Fig. 126, A). The division of the longitudinal muscles into short 

 segments renders it possible on the other hand for restricted portions 

 of the body to be bent to one side or the other irrespective of what may 

 be happening in other portions. Further, by the muscle segments being 

 caused to contract in order from the head-end backwards, a succession 

 of waves of curvature (Fig. 126, B) may be passed back along the body, 

 the mechanical effect of which will be to push the body forwards. This 

 is what happens when an Eel swims and the universal presence of these 

 segmental blocks of muscle in early stages in the development of all 

 vertebrates affords strong evidence that the type of movement described 

 was the primitive mode of movement in the common ancestors of existing 

 vertebrates. 



The fins, both paired and unpaired, are provided with special muscular 



A B 



Fig. 126. 

 Lateral curvature of the 

 body : (A) as it would, 

 take place if the muscle 

 fibres extended from end to 

 end of the body ; (B) as it 

 takes place with the longi- 

 tudinal muscles subdivided 

 into successive segments. 



