102 THE PHYSIOLOGY OF THE COMMON CRAYFISH. 
(fig. 23). The nerve bundle which passes to a muscle 
breaks up into smaller bundles and, finally, into separate 
fibres, each of which ultimately terminates by becoming 
continuous with the substance of a muscular fibre fig. 19, 
I.) Now the peculiarity of a muscle nerve, or motor 
nerve, as it is called, is that irritation of the nerve fibre at 
any part of its length, however distant from the muscle, 
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Fic. 23.— Astacus fluviatilis—Three nerve fibres, with the connective 
tissue in which they are imbedded. (Magnified about 250 dia- 
meters.) 2, nuclei. 
brings about muscular contraction, just as if the.muscle 
itself were irritated. A change is produced in the mole- 
cular condition of the nerve at the point of irritation; 
and this change is propagated along the nerve, until it 
reaches the muscle, in which it gives rise to that change 
in the arrangement of its molecules, the most obvious 
effect of which is the sudden alteration of form which we 
call muscular contraction. 
Tf we follow the course of the motor nerves in a 
