312 RALPH S. LILLIE. 



both cases, but in the one case the change produced is reversible, 

 in the other irreversible. The degree of reversibility is however 

 itself subject to variation. In many colloidal systems changes 

 which are reversible in their earlier stages may become irrever- 

 sible later; and the fact that anaesthesia, especially if profound, 

 cannot be prolonged indefinitely without danger to^ life, may 

 find its explanation here. 



In any theoretical discussion of anaesthesia it is important to 

 recognize from the first that normal or physiological conditions 

 of reversible inhibition or suspended activity are in no sense 

 unusual among organisms. In both animals and plants irrita- 

 bility and automatic activity are fluctuating properties, with a 

 wide range of strictly physiological variation. Thus in higher 

 animals we have conditions ranging from the profound narcosis 

 of sleep a state due apparently to the accumulation of fatigue- 

 products to one of complete mental and physical alertness or 

 wide-awakeness. Generally speaking, responsiveness is largely 

 a matter of metabolic condition; and most vital activities are 

 subject to inhibition or enhancement according to the physio- 

 logical requirements. Variability of this kind is in fact a neces- 

 sary condition of adaptation to the changing conditions of life. 

 Thus the activities of animals as a class are influenced to a 

 marked degree by variations in the food-requirements. In 

 general they become sluggish and irresponsive when well fed, 

 and show heightened activity when deprived of food. In other 

 words, both the automatic motor activity and the responsiveness 

 to the stimuli of food-substances the physiological condition 

 expressed in consciousness as hunger are increased when the 

 supply of energy-yielding material is depleted and vice versa. 

 For example, the fresh water Hydra shows restless swaying move- 

 ments when hungry; these movements increase the area swept by 

 the tentacles, which respond promptly to the contact of small 

 organisms or food-particles by capturing and conveying to the 

 mouth. 1 When well fed the creature is quiescent, and the ten- 

 tacles are indifferent to such contact; they are, as it were, in an 

 anaesthetized condition ; this state passes off as the organic demand 

 for food reasserts itself. Such an instance illustrates the regula- 



1 Cf. S. J. Holmes, "The Beginnings of Intelligence," Science, N. S., 1911, Vol. 

 33- P- 473- 



