108 THE PHYSIOLOGY OF THE COMMON CRAYFISH. 
of the musical instrument further, striking a single key 
gives rise, not to a single note, but to a more or less 
elaborate tune; as if the hammer struck not a single 
string, but pressed down the stop of a musical box. 
It is in the ganglia that we must look for the analogue 
of the musical box. A single impulse conveyed by a 
sensory nerve to a ganglion, may give rise to a single 
muscular contraction, but more commonly it originates a 
series of such, combined to a definite end. 
The effect which results from the propagation of an 
impulse along a nerve fibre to a ganglionic centre, whence 
it is, as it were, reflected along another nerve fibre to a 
muscle, is what is termed a reflex action. As it is by no 
means necessary that sensation should be a concomitant 
of the first impulse, it is better to term the nerve fibre 
which carries it afferent rather than sensory; and, as 
other phenomena besides those of molar motion may be 
the ultimate result of the reflex action, it is better to 
term the nerve fibre which transmits the reflected im- 
pulse efferent rather than motor. 
If the nervous commissures between the last thoracic 
and the first abdominal ganglia are cut, or if the thoracic 
ganglia are destroyed, the crayfish is no longer able to 
control the movements of the abdomen. If the forepart 
of the body is irritated, for example, the animal makes 
no effort to escape by swimming backwards. Never- 
theless, the abdomen is not paralysed, for, if it be irri- 
tated, it will flap vigorously. This is a case of pure 
