REFLEX ACTION 



49 



again. That is one way of picturing the phenomenon. 

 According to the older and more familiar view the impulses 

 which come out are not those which went in, but a new 

 set generated by an energetic metabolic process, a discharge 

 on the part of cells in the brain or the cord. If this is the 

 true conception the afferent impusles serve to "touch off" 

 irritable nervous elements, much as these elements in their 

 turn may touch off muscle-fibers or gland-cells. 



Fig. 6. The principle of reflex action. The subject touches a hot 

 object (H). Afferent nerve-impulses travel the route marked by 

 dots and dashes to the spinal cord (). Efferent impulses return 

 promptly along the route marked by little crosses to the muscle (M), 

 which co-operates with others not shown to withdraw the finger 

 from the stimulating surface. The situation of the co-ordinating 

 center is left undetermined, whether in the brain or the cord. 



The fourth step in the evolution of the reflex is the 

 efferent transmission. This may be said always to be more 

 voluminous than the afferent flow which went before. 

 Impulses go out by many channels, where but few were 

 engaged in bringing them in. A great characteristic of 

 the "central process" is the spreading of the initial stimu- 

 lation, so that there seems to be no proportion between the 



