fPENTI 



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 NEUKO-MUSCULAK MECHANISM 99 



in past generations and inherited from the parents. In young 

 fowls, as soon as they are hatched, the acts of walking and 

 of pecking are at once performed, and in many families 

 particular gestures or expressions follow certain modes of 

 stimulation in many different individuals without the con- 

 sciousness of the person being involved. They are inherited 

 cerebral reflexes. (2) Paths may also have been developed 

 in the individual as the result of previous activities of the 

 nervous mechanism. For, if a given action has once followed 

 a given stimulus, it always tends to follow it again. This, 

 in fact, is the basis of all rational education alike in man and in 

 the horse and dog to open up paths in the nervous system by 

 which the most suitable response may be made to any given 

 stimulus, and to prevent the formation of paths by which 

 inappropriate reaction may be produced. 



3. FATIGUE OF THE NEURO-MUSCULAR MECHANISM 



Continued action of the neuro-muscular 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 " voluntarily " or 

 reflexly stimulated again and again it finally ceases to react. 

 But if now the outgoing nerve is stimulated the muscle con- 

 tracts 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. The muscle 

 therefore is not fatigued. 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 fatigues after the central synapses. 



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. 



In studying the neuro-muscular mechanism it is most import- 

 ant to keep clearly in mind the meaning of the terms stimuliis, 

 reaction, and sensation, (a) Stimulus is the change in the sur- 

 roundings which produces (6) the Eeaction, the modification in 

 the action of some part of the body, (c) Sensation is the 



