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 



