THE INFLUENCE OF NERVE ON MUSCLE. 101 



throws the tail fin, the two side lobes of which are 

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

 wards by the reaction of the water against the stroke. 

 Then the flexor muscles being relaxed, the extensor 

 muscles come into play; the 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 

 the backward motion conferred upon the body 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 only 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 wliicli, 



on microscopic examination, prove to be bundles of fine 



tubular filaments, filled with an apparently structureless 



substance of gelatinous consistency, the nerve fibres 

 6 



