184 REFLEX ACTIONS. [BOOK i. 



almost all the muscles of the body. In other words, the dis- 

 turbance set going in the centre, confined when the stimulus is 

 slight to a small part of the centre, overflows, so to speak, when 

 the stimulus is increased, to other parts of the centre, and thus 

 throws impulses into a larger and larger number of efferent nerves. 



We may add, without going more fully into the subject here, 

 that in most reflex actions a special relation may be observed 

 between the part stimulated and the resulting movement. In the 

 simplest cases of reflex action this relation is merely of such a 

 kind that the muscles thrown into action are those governed by a 

 motor nerve which is the fellow of the sensory nerve, the stimula- 

 tion of which calls forth the movement. In the more complex 

 reflex actions of the brainless frog, and in other cases, the relation 

 is of such a kind that the resulting movement bears an adaptation 

 to the stimulus : the foot is withdrawn from the stimulus, or 

 the movement is calculated to push or wipe away the stimulus. 

 In other words, a certain purpose is evident in the reflex action. 



Thus in all cases, except perhaps the very simplest, the move- 

 ments called forth by a reflex action are exceedingly complex 

 compared with those which result from the direct stimulation of a 

 motor trunk. When the peripheral stump of a divided sciatic 

 nerve is stimulated with the interrupted current, the muscles of 

 the leg are at once thrown into tetanus, continue in the same rigid 

 condition during the passage of the current, and relax immediately 

 on the current being shut off. When the same current is applied 

 for a second only, to the skin of the flank of a brainless frog, the 

 leg is drawn up and the foot rapidly swept over the spot irritated, 

 as if to wipe away the irritation ; but this movement is a complex 

 one, requiring the contraction of particular muscles in a definite 

 sequence, with a carefully adjusted proportion between the amounts 

 of contraction of the individual muscles. And this. complex move- 

 ment, this balanced and arranged series of contractions, may be 

 repeated more than once as the result of a single stimulation of the 

 skin. When a deep breath is caused by a dash of cold water, the 

 same co-ordinated and carefully arranged series of contractions is 

 also seen to result, as part of a reflex action, from a simple stimulus. 

 And many more examples might be given. 



In such cases as these the complexity may be in part due to 

 the fact that the stimulus is applied to terminal sensory organs 

 and not directly to a nerve trunk. As we shall see in speaking of 

 the senses, the impulses which are generated by the application of 

 a stimulus to a sensory organ are more complex than those which 

 result from the direct artificial stimulation of a sensory nerve 

 trunk. Nevertheless, reflex actions of great if not of equal 

 complexity may be induced by stimuli applied directly to a 

 nerve trunk. We are therefore obliged to conclude that in a 

 reflex action, the processes which are originated in the centre by 

 the arrival of even simple impulses along afferent nerves may be 



