72 PHYSIOLOGY CHAP. 



All causes that reduce the high percentage of water in the 

 composition of the body are able to produce the sensation of 

 thirst. The heat of the atmosphere which increases cutaneous 

 and pulmonary perspiration, and muscular exercise which excites 

 secretion of sweat, accentuate thirst. Hydropic effusions, diarrhoea, 

 diabetic polyuria, haemorrhage, etc. promote the desire to drink 

 and produce poly dips i. Ingestion of highly spiced or salt ^ I 

 foods develops the sensation of thirst by subtracting water from 

 the circulating tissue fluids. 



Adipsia or suppression of the sense of thirst is very rare. It 

 is seen in certain serious fevers, and is a fatal symptom, presaging 

 the final exhaustion of the nervous system. 



The physiological researches directed towards clearing up the 

 "i-iuin of hunger and thirst have not led to aim very satisfactory 

 results. It is a priori evident that the fundamental internal 

 condition of these sensations must consist in the impoverishment 

 of the circulating fluids by loss of water, which produces a corre- 

 sponding impoverishment of the tissues. This can be shown 

 experimentally. If artificially prepared nutrient .substances are 

 introduced into the veins of a. fasting dog, it is possible, according 

 to Schiff, not only to assuage hunger but also to nourish the 

 animal. By means also of intravenous or intraperitoneal trans- 

 fusion of den'brinated blood, hunger can be relieved in dogs, but 

 the starvation deficit cannot be arn-stf-d (Luciani and Bufalini, 

 1882). 



In certain clinical eases in which ingestion of food by the 

 stomach becomes impossible the pangs of hunger may be relieved 

 by nutrient enemata. As regards thirst, Dupuytren caused dogs 

 to run in the sun and then relieved their thirst by intravenous 

 injections of slightly saline water. Schiff repeated this experiment 

 successfully. 



But how is it, since they are determined by a general craving 

 of the whole of the tissues of the body, that the sensations of 

 hunger and thirst are localised in the h'rst place to definite 

 regions of the digestive system? Are these sensations central 

 or peripheral in origin ? Various physiological theories have 

 been propounded in reply to these questions, all of which appear 

 to us to be insufficient or erroneous. Let us see if it is not 

 possible, on the basis of the facts above discussed, to construct a 

 new theory of hunger and thirst better calculated to satisfy the 

 requirements of scientific criticism. 



It is undeniable that hunger and thirst are at the outset true 

 local sensations, and that it is only as they become intensified that 

 they spread and assume the complex characters of general sensa- 

 tions. This fact in no way contradicts the preceding observation 

 that the fundamental quality of hunger and thirst, on which 

 their teleological value as " desires " depends, is more or less 



