102 THE PHYSIOLOGY OF THE COMMON CRAYFISH. 



(fig. 23). The nerve bundle which passes to a muscle 

 breaks up into smaller bundles and, finall3% into separate 

 fibres, each of which ultimately terminates by becoming 

 continuous with the substance of a muscular fibre fig. 19, 

 F.) 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. 



Fig. 2Z.—Astacus fluviatilis. — Three nerve fibres, with the connective 

 tissue in which they are imbedded. (Magnified about 250 dia- 

 meters.) n, 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 u'ritation; 

 and this change is propagated along the nerve, until it 

 reaches the muscle, in w^hich 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. 



If Ave follow the course of the motor nerves in a 



