268 THE SPECIAL SENSES. 



of the oxidations in the muscular tissues, and that, therefore, some 

 substance may be formed as the result of these oxidations which 

 affects the sensory nerves of the stomach. The same general sug- 

 gestion is contained in the fact that diabetics exhibit an abnormal 

 appetite in spite of abundant feeding. In these individuals the 

 carbohydrate food escapes oxidation more or less completely, and 

 the metabolism, particularly in the muscles, involves, therefore, 

 to a greater extent, the oxidation of proteid material, a fact which 

 may stand in some relation to the abnormal appetite that is observed. 

 The complexity of the nervous apparatus that controls the the appe- 

 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 " professional 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 



