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 bv 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 nu 

 impulse along a nerve hbre 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 necessarj^ 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 ai*e cut, or if the thoracic 

 gangUa are destroyed, the crayfish is no longer able to 

 control the movements of the abdomen. If the foi-epart 

 of the bod}' is irritated, for example, the animal makes 

 no efibrt. to escape by swimming backwards. Never- 

 theless, the abdomen is not paralysed, for, if it be irri- 

 tated, it will Hap vigorously. This is a case of pure 



