778 MOTORIAL AND VISCERAL SENSATIONS [CH. LIV. 



well known it is intensified by muscular exertion, especially if the 

 air is cool. It has been suggested that the oxidation processes which 

 occur in the muscles produce some substance or substances which 

 excite the sensory nerve-terminals in the stomach. In diabetes, 

 where oxidation runs an unusual course, carbohydrates escaping 

 oxidation to a large extent, intense appetite may be present in spite 

 of abundant feeding. 



Hunger is due to pronounced motor activity of the stomach ; this 

 excites the sensory nerve-terminals there (Hertz) ; these movements, 

 and therefore the sensation of hunger, can be appeased by filling the 

 stomach even with indigestible or non-nutritious material. Carlson 

 has recently shown that the movements are reflexly inhibited when 

 food enters the mouth and is masticated ; the nerves of taste act as 

 the afferent channel for the reflex ; hence the feeling of hunger passes 

 off long before absorption of food begins. These observations confirm 

 the view that its origin is a local condition set up in the stomach by 

 its condition of emptiness, and that it is not immediately due to any 

 general change in the nutrition of the body as a whole. We must, 

 however, recognise that the gastric sense is a complex one, as is 

 illustrated by the aversion for food felt during monotonous diets or 

 after over-feeding, or in the case of certain articles of diet, but 

 the explanation of these and similar phenomena we do not know. 



Thirst is a sensation referred to the pharyngeal region rather 

 than to the stomach, and appears, like hunger, to be a protective 

 signal, locally excited to warn the living organism of the necessity 

 for regularity in the intake of nutriment. Although its intensity 

 increases with the loss of water from the body, it occurs normally 

 long before there is any serious upset of the normal relationship of 

 the water percentage of the organs and tissues, and may be artifici- 

 ally produced by drying of the throat ; it is appeased immediately by 

 the administration of fluid, and although fluids reach the absorbing 

 surface of the duodenum sooner than was formerly supposed to be 

 the case (see p. 555), it is unquestionable that the relief of thirst is 

 mainly the result of moistening the local surface, the impulses from 

 which excite the sensation. Very frequently thirst can be relieved 

 by letting the water touch the pharyngeal mucous membrane without 

 its being swallowed. Thirst which is due to prolonged deprivation 

 of water is not a mere local sensation, but is no doubt produced by 

 loss of water in the tissues, generally, exciting widespread sensory 

 terminations therein ; the bodily and mental anguish experienced are 

 then of an intense character. 



The independence of the two sensations hunger and thirst is well 

 illustrated in many diseases, where a loss of appetite occurs without 

 any corresponding loss of desire for fluid. 





