CHAP, in ] REFLEX ACTIONS. Ill 



set going in the central nerve-cells, confined when the stimulus is 

 slight to a few nerve-cells and to a few nerve-fibres, overflows, so to 

 speak, when the stimulus is increased, on to a number of adjoining 

 and (we must conclude) connected cells, and thus throws impulses 

 into a larger and larger number of efferent nerves. 



Certain relations may be observed between the sentient spot 

 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 stimulation 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 with- 

 drawn 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, com- 

 pared 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, part of the complexity may be 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 stimulation of a sensory nerve-trunk. Never- 

 theless, 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 central nerve-cells by the arrival of even 

 simple impulses along afferent nerves may be highly complex; and 

 that it is the constitution and condition of the nerve-cells which 



