ii SENSIBILITY OF THE INTEBNAL OKGANS 73 



diffused over the whole of the living tissue elements. In fact, it 

 is conceivable that the sensory nerves of the upper part of the 

 digestive apparatus are peculiarly sensitive to the general effects 

 of deprivation of food and drink in comparison with all other 

 nerves of common sensibility. They are, so to speak, the advanced 

 guard which transmits to the centres a warning of defective 

 nutriment in the tissues by arousing the characteristic sensations 

 of alimentary desires. An analogous fact may be observed in the 

 cutaneous sensory nerves in regard to pain ; in these the liminal 

 stimulus that causes pain is normally much lower than that of 

 the sensory nerves of the internal organs. They are the sentinels 

 whose duty it is to defend the entire organism against injurious 

 external agents (mechanical, thermal, and chemical), and to arouse 

 appropriate protective reflexes. 



Hunger is therefore specially localised in the stomach for the 

 simple reason that the sensory nerves to the mucous membrane 

 of the latter are the most excitable to deprivation of food. 

 Thirst is specially localised in the pharyngeal and buccal mucous 

 membrane because the sensory nerves to these parts are peculiarly 

 sensitive to lack of water in the circulating fluids of the body. 



What condition of the stomach constitutes the peripheral 

 stimulus of the sensation of hunger ? It is not the state in which 

 the stomach is empty, because all observations made on patients 

 with a gastric fistula, beginning with the famous Canadian subject 

 studied by Beaumont, show that hunger sets in some time after 

 the stomach has been entirely emptied. Nor does the stimulus 

 consist in exaggerated movements of the stomach, for these are 

 much more active during gastric digestion, and cease almost 

 entirely after the stomach has been emptied. Nor can it consist 

 in excess of hydrochloric acid in the stomach, since it is well 

 known that the contents of an empty stomach are slightly acid, 

 or neutral, or sometimes alkaline. The most acceptable hypothesis 

 is that of Beaumont, who attributes the sense of hunger to turgor 

 of the gastric mucous membrane, which increases after the stomach 

 has been emptied, and is due, as Heidenhain showed, to the 

 increased volume of the chief cells of the gastric glands (see 

 Vol. II. Figs. 40, 41, pp. 120, 123). It is possible that the gastric 

 turgescence excites the peripheral endings of the sensory nerves 

 to the mucous membrane ; but it seems to us more probable that 

 the excitation depends on the chemical changes in the epithelial 

 protoplasm. 



On our theory it is easy to account for the fact that the 

 sensations of hunger only last for a couple of days in a prolonged 

 voluntary fast, as was observed on Succi. In fact it is natural to 

 suppose that inanition, which attacks all the tissues, gradually 

 reduces the turgor of the mucous membrane by diminishing the 

 protoplasm of the epithelial cells that act as a" stimulus to hunger. 



