CUTANEOUS AND INTERNAL SENSATIONS. 267 



enough to make a distinct and peculiar impression upon our con- 

 sciousness. We designate this feeling as fatigue, but there is no 

 question apparently that this sensation is mediated through the 

 same nerve fibers that ordinarily give us our muscular sensibility. 



Sensations of Hunger and Thirst. Hunger and thirst are 

 typical interior (or common) sensations. We feel them as changes 

 in ourselves. Neither sense has been the direct object of much 

 experimental investigation, and what knowledge we possess is there- 

 fore derived largely from accidental or pathological sources. Hunger 

 in its mild form is designated as appetite. It occurs normally at a 

 certain interval after meals, and is referred or projected more or 

 less accurately to the stomach. It is not known whether this sense 

 is mediated by a special set of sensory fibers distributed to the 

 mucous membrane of the stomach, or whether, perhaps, it 

 may be a quality of the sensory impressions from the muscular 

 coat. The former view seems more probable, especially when it is 

 remembered that loss of appetite or anorexia is so frequently an 

 accompaniment of pathological changes in the membrane of the 

 stomach. The nervous mechanism through which this sense is me- 

 diated is of most essential importance and deserves more careful 

 study at the hands of physiologists and pathologists. Under ordi- 

 nary conditions of life all of 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 this sense. Its 

 striking influence upon the body at large is well illustrated in the case 

 of animals (pigeons, dogs) deprived of their cerebrum. During the 

 period of fasting these animals show all the external signs of hunger 

 and keep in continual, restless movement that seems to imply a con- 

 stantly acting sensory stimulus. We may assume that appetite 

 has its sensory origin, its peripheral nerve endings in the stomach, 

 and that these endings are excited in some unknown way when the 

 stomach is empty. This gastric hunger, as it might be called, 

 disappears, or the appetite is appeased when the stomach is filled. 

 This fact in itself would indicate that the stimulus has a local 

 origin in the stomach, and is not dependent upon any general 

 change in the nutritive condition of the body. The appetite is 

 satisfied by filling the stomach with food long before this food 

 is actually absorbed and distributed to the tissues. The inges- 

 tion of totally indigestible material would probably have temporarily 

 a similar result. The exact nature of the conditions that lead 

 to or cause a stimulation of the sensory nerves of appetite in the 

 stomach remains unexplained. The well-known fact that muscular 

 exercise and low temperatures and particularly a combination of 

 the two cause a marked augmentation of the appetite would suggest 

 that the sensory stimulus is influenced by the extent or character 



