106 THE PIIYSIOLOGV OF THE COMMON CRAYFISH. 



fact, these nerves go either to the integument or to the 

 organs of sense, and they are termed sensory nerves. 



When a muscle is connected by its motor nerve with 

 a ganglion, h-ritation of that ganglion will bring about 

 the contraction of the muscle, as well as if the motor 

 nerve itself were irritated. Not o\i\j so ; but if a sensory 

 nerve, which is in connexion with the ganglion, is irritated, 

 the same effect is produced ; moreover, the sensory nerve 

 itself need not be excited, but the same result will 

 take place, if the organ to which it is distributed is 

 stimulated. Thus the nervous system is fundamentallj^ 

 an apparatus by which two separate, and it may be dis- 

 tant, parts of the body, are brought into relation with 

 one another ; and this relation is of such a nature, that 

 a change of state arising in the one part is followed by 

 the propagation of changes along the sensory nerve to the 

 ganglion, and from the ganglion to the other part ; where, 

 if that part happens to be muscle, it produces contraction. 

 If one end of a rod of wood, twenty feet long, is applied 

 to a sounding-board, the sound of a tuning-fork held 

 against the opposite extremity will be very plainly heard. 

 Nothing can be seen to happen in the wood, and yet 

 its molecules are certainly set vibrating, at the same 

 rate as the tuning-fork vibrates ; and when, after 

 travelling rapidly along the wood, these vibrations 

 affect the sounding-board, they give rise to vibrations 

 of the molecules of the air, which reaching the ear, are 

 converted into an audible note. So in the nerve tract : 



