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 fiexor 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 toa 
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 
‘he 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 
isthe 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 with an apparently structureless 
substance of gelatinous consistency, the nerve fibres 
