94 HUMAN PHYSIOLOGY 



know its relationship to cerebral action further than that we 

 have no evidence that consciousness can manifest itself apart 

 from such cerebral activity. 



Fatigue of the Neuro-Muscular Mechanism. 



Continued action of this mechanism leads to fatigue, and 

 this may best be studied by means of some form of ergograph, 

 an instrument which enables the response of a muscle to 

 stimuli to be recorded. If a muscle be reflexly stimulated 

 again and again it finally ceases to react, but if now the out- 

 going nerve is stimulated the muscle contracts at once. This 

 shows that fatigue first manifests itself in the central synapses. 

 If the outgoing nerve be repeatedly stimulated, after a time 

 the muscle no longer responds, but if the muscle be directly 

 stimulated it contracts; but since the electrical changes 

 which accompany conduction in a nerve still go on it is 

 obvious that the nerve still acts. It is therefore the nerve 

 ending in the muscle which is fatigued. Fatigue is due 

 to the accumulation of the products of the activity of muscle, 

 and it may be induced in a normal dog by injecting the 

 blood from a dog which has been fatigued. 



It is most important to keep clearly in mind the meaning 

 of stimulus, reaction, and sensation, (a) Stimulus is the 

 change in the surroundings which produces (6) the Reaction, 

 the modification in the action of some part of the body. 

 (c) Sensation is the change in the consciousness which may 

 accompany the application of a stimulus and the reaction. 



