292 THE SPECIAL SENSES. 



mental state connected with stimulation of the nerves of taste or 

 odor and dependent upon past associations of an agreeable char- 

 acter. Carlson* also separates sharply the two sensations. 

 The sensory apparatus for hunger lies in the walls of the stomach, 

 probably in the muscular coats, and is stimulated by the contrac- 

 tions of the musculature. The sensations aroused are more or less 

 disagreeable. Appetite, on the contrary, is an entirely pleasant 

 sensation aroused in part through the sensory nerves of taste 

 and smell, but also by sensory fibers in the gastric mucosa. This 

 gastric component of the appetite-sense explains the increased 

 appetite observed sometimes at the beginning of a meal after 

 partaking of some palatable food. Under ordinary conditions of 

 life the regulation of the amount and quality of the food necessary 

 to the proper nutrition of the body and the maintenance of body 

 equilibrium is effected through the sense of hunger or of hunger and 

 appetite. Its striking influence upon the body at large is well illus- 

 trated in the case of animals (pigeons, dogs) deprived of their cere- 

 brum. During the period of fasting these animals show all the ex- 

 ternal signs of hunger, and keep in continual, restless movement 

 that seems to imply a constantly acting sensory stimulus. The 

 complexity of the nervous apparatus that controls the appetite is 

 shown 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 will- 

 ingly, soon provoke a feeling of dislike and aversion to this particu- 

 lar foodstuff which cannot be overcome. An animal will starve 

 rather than use the gelatin, although all of our direct physiological 

 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 appetite is 

 influenced in some specific way by the metabolism of this particu- 

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

 food that follows overfeeding indicates something more than a 

 simple removal of the sensations of hunger; it implies an active 

 state, due possibly to the excitation of sensory fibers of a different 

 character. 



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 

 * Carlson and Braafladt, "American Journal of Physiology," 36, 153, 1914. 



