106 THE PHYSIOLOGY 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, irritation of that ganglion will bring about 
the contraction of the muscle, as well as if the motor 
nerve itself were irritated. Not only 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 fundamentally 
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 us 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: 
