THE MECHANISM OF CO-ORDINATED MOVEMENTS 343 



pain reflex. We must assume therefore that the mutual inhibition of differ 

 ent reflexes occurs, not in the ' final common path '-^. i n the motor neurons 

 which must always remain open but further back in the arc, probably ne * 

 its afferent side. 



We have reason to believe that the propagation of impulses through the 

 central nervous system involves expenditure of energy, and that the seat of 

 this expenditure may be located in all probability at the synapses. It follows 

 that the result of any particular sensory stimulation will not be absolutely 

 invariable, but that the spread of the nerve process in the nervous system, 



FIG. 172. ' Mark-time ' reflex in spinal dog, inhibited by slight stimulation of 

 the tail (duration of stimulation shown by signal). Note the augmentation of 

 the mark-time reflex following the inhibition (successive spinal induction). 



(SHERRINGTON.) 



and the degree of block presented by the various synapses and determining 

 the potency of any given reaction, will depend on the condition of the 

 various synapses at the time of the stimulation. 



This condition may be altered in various ways. Repeated excitation 

 causes in the synapses, just as in the nerve-endings of the skeletal muscle, a 

 condition of fatigue. Stimulation confined to a single point in the ' scratch 

 area ' of the spinal dog excites a Scratch reflex which rapidly dies away. On 

 shifting the exciting electrodes a little to one side the reflex act begins again, 

 often with greater force than at first, and a very prolonged reaction ca'n be 

 induced by gradually moving the electrodes along the surface of_the skin. A 

 reflex arc therefore rapidly shows signs of fatigue, and the minute change 

 in locus of stimulus which is required to reinduce a practically identical 

 action shows that the seat of fatigue must lie chiefly on the afferent side of 

 the arc ; perhaps in the first synapses through which the impulse has 

 pass. This easy incidence of fatigue tends to cut short any given reaction 

 and to render it easier for other reactions to take its place. 



