272 THE SPECIAL SENSES. 



tite is shown also by many facts from the experiences of life and from 

 the results of laboratory investigations. For example, it is found 

 that large amounts of gelatin in the diet, although at first accepted 

 willingly, soon provoke a feeling of dislike and aversion to this 

 particular foodstuff such as cannot be overcome. An animal will 

 starve rather than use the gelatin, although all of our direct physio- 

 logical evidence would indicate that this substance is an efficient 

 food, playing much the same part as the fats or carbohydrates. 

 A fact of this kind indicates that the sensory apparatus of the appe- 

 tite is influenced in some specific way by the metabolism of this 

 particular material. So also the feeling of satiety and aversion for 

 food that follows overfeeding indicates something more than a sim- 

 ple removal of the sensations of appetite ; it implies an active state, 

 due possibly to the excitation of sensory fibers of a different char- 

 acter. With regard to the -effects of prolonged starvation, the 

 pangs of hunger that are felt at first do not seem to increase in in- 

 tensity to such an extent as to cause actual suffering. The testi- 

 mony of the " prof essional fasters," at least, seems to show that, if 

 water is provided, prolonged deprivation of food is not accompanied 

 by the intense discomfort or suffering popularly associated with 

 the idea of complete starvation. 



The Sense of Thirst. Our sensations of thirst are projected 

 more or less accurately to the pharynx, and the facts that we know 

 would seem to indicate that the sensory nerves of this region have 

 the important function of mediating this sense. The water con- 

 tents of the body are subject to great changes. Through the lungs, 

 the skin, and the kidneys water is lost continually in amounts that 

 vary with the conditions of life. This loss affects the blood directly, 

 but is doubtless made good, so far as this tissue is concerned, by a 

 call upon the great mass of water contained in the storehouse of the 

 tissues. To restore the body tissues to their normal equilibrium 

 in water we ingest large quantities, and the control of this regula- 

 tion is effected through the sense of thirst. We -know little or 

 nothing about the nervous apparatus involved; but it may be 

 assumed that when the water content falls below a certain amount 

 the nerve fibers in the pharyngeal membrane (fibers of the glosso- 

 pharyngeal nerve) are stimulated and give us the sensation of 

 thirst. That we have in this membrane a special end-organ of 

 thirst is indicated, moreover, by the fact that local drying in this 

 region, from dry or salty food, or dry and dusty air, produces a 

 sensation of thirst that may be appeased by moistening the mem- 

 brane with a small amount of water not in itself sufficient to relieve 

 a genuine water need of the body. Our normal thirst sensations 

 might be designated, therefore, as pharyngeal thirst, to indicate 

 the probable origin of the sensory stimuli. Prolonged deprivation 



