612 Professor G. S. Sherrington [April 19, 



present epoch, for his is the nervous system which, on the whole, is 

 the most developed, much best adapted to dominate the environment. 



To understand a little how the nervous system compasses this end 

 we may turn to examine its performance in some of its simpler 

 governing of the muscles. Its main office is to react to changes in 

 the environment. The animal body is provided with a number of 

 organs specially attuned to react to changes in the environment. 

 These changes, in so far as they excite these organs, are termed stimuli. 

 Thus, it has organs stimulated by the radiant energy of light and 

 heat, others by chemical particles drifting from odorous objects, 

 others mechanically by objects touching the skin, and so on. These 

 organs, specially adapted to environmental stimuli, are called receiptors. 

 Attached to them are nerves. Through these the excitement set up 

 in the receptor by a stimulus spreads to the general nervous system. 

 Arrived there, two kinds of effect ensue from it — one, a change in 

 nerve-cells innervating muscles and glands, the other, a change in 

 consciousness on the basis of sensation. These two effects are separ- 

 able. The former, or " reflex " reaction, is not necessarily accompanied 

 by any manifestation of the latter, though it may be so and very 

 often is so. We will confine ourselves to the former, or purely reflex 

 effect, and to its operation on muscle. 



The endowment with receptor organs is not equally rich in all parts 

 of the body. It is the external surface of the animal which, as we 

 might expect, has them in richest profusion. And the receptors of 

 the external surface are likewise those most developed, specialised and 

 sensitive. This also we might expect ; for it is the external surface 

 that for countless ages has felt the influences of the illimitable out- 

 side world playing on it. Through refinement of the receptors of its 

 outer surface the animal has been rendered sensitive in many cases to 

 stimuli delivered even by the remotest stars. 



It is a feature of receptors generally that they react most to their 

 agent when the intensity of that agent changes, and the more so the 

 more abrupt the change. It is, therefore, changes in the outside world 

 that operate especially as stimuli ; though of course only changes 

 which have relation to the animal in question. If we regard the 

 mutual relation between the animal and the world at any moment as 

 an equilibrium, then we can say that any change in the world which 

 changes that relation disturbs the equilibrium. 



Take the instance of a child asleep. A thousand agencies of the 

 external world are playing upon it. Upon its skin, for instance, there 

 is the pressure of the child's own weight against the receptors, and 

 there is the pressure of the clothes which cover it ; yet it lies restful. 

 Suppose we touch its foot. That is a change in the external world 

 in relation to the child. The familiar fact is that the foot is drawn up 

 out of harm's way as it were. The change has acted upon the child 

 as a stimulus to some receptors of the skin. It may be quite uncon- 

 scious of the touch for its sleep may be deep. Yet the reflex action 



