THE THEORY OF ANESTHESIA. 313 



tory role which fluctuations in the general responsiveness of an 

 animal play in its normal life. Similar variations of neuro-mus- 

 cular responsiveness occur throughout the animal kingdom. This 

 is well illustrated by sleep, which is an instance of a normal or 

 "physiological" narcosis, characterized by a definite periodicity 

 and by affecting especially certain parts of the central nervous 

 system; the use of opiates illustrates how readily a chemically 

 induced narcosis may pass into the physiological form. From 

 such facts we must conclude that the essential basis of anaesthesia 

 consists not in a purely artificial modification of nervous or other 

 irritability, but in some normal or physiological modification 

 which is capable of being intensified and prolonged by the use of 

 certain physical and chemical agencies; these are the various 

 anaesthetizing agencies, such as the electric current, cold, or nar- 

 cotizing substances. From this point of view, anaesthesia is to be 

 regarded not as an essentially abnormal or artificial phenomenon, 

 but simply as an intensification of a normal physiological condi- 

 tion; and in investigating its essential conditions we are led first 

 to consider the normal inhibitions and depressions shown by all 

 living cells. 



Instances of such normal inhibitions are innumerable. The 

 motor neurones innervating any group of muscles become in- 

 excitable during the activity of the antagonist groups, as Sher- 

 rington has shown; the respiratory nerve cells cease automatic 

 activity with over-oxygenation of the blood ; vasomotor, cardiac, 

 glandular, and muscular activities are subject to various forms- 

 of inhibition, partly nervous and partly chemical in origin. 

 Such inhibitory mechanisms play in normal life a part whose 

 importance is daily more widely recognized by physiologists. 

 Mechanisms of the inverse kind, which exercise sensitizing and 

 reinforcing influence on various functions, are also frequent in 

 organisms. A large part of these normal inhibitions and excita- 

 tions are now known to be due to chemical substances (hormones) 

 present in the blood and derived from ductless glands or other 

 sources of internal secretion. The regulation and integration of 

 bodily activities are thus largely under direct chemical as well as 

 nervous control. Such normal chemical inhibitions are probably 

 of the same nature as artificial inhibitions due to anaesthesia. 



