THE INFLUENCE OF NERVE ON MUSCLE. 101 



throws the tail fin, the two side lol)es of which are 

 spread out, forwards ; while the body is propelled back- 

 Avards by the reaction of the water against the stroke. 

 Then the flexor muscles being relaxed, the extensor 

 muscles come into play ; tlie abdomen is straightened, but 

 less violently and with a far weaker stroke on the water, 

 in consequence of the less strength of the extensors and of 

 the folding up of the lateral plates of the fin, until it 

 comes into the position requisite to give full force to a 

 new downward and forward stroke. The tendency of the 

 extension of the abdomen is to drive the body forward ; 

 but from the comparative weakness and the obliquity of 

 its stroke, its practical effect is little more than to check 

 tlie backward motion conferred upon the bodj' by the 

 flexion of the abdomen. 



Thus, every action of the crayfish, which involves 

 motion, means the contraction of one or more muscles. 

 But what sets muscle contracting ? A muscle freshly 

 removed from the body may be made to contract in 

 various ways, as by mechanical or chemical irritation, or 

 by an electrical shock ; but, under natural conditions, 

 there is onlv one cause of muscular contraction, and that 

 is the activity of a nerve. Every muscle is supplied with 

 one or more nerves. These are delicate threads which, 

 on microscopic examination, prove to be bundles of fine 

 tubular filaments, filled Avith an apparently structureless 

 substance of gelatinous consistency, the iicrre Jihrcs 



