FATIGUE, THIRST, AND HUNGER 371 



neural, and vascular, have time to develop by growth and repair to 

 their new requirements. In neural fatigue the changes are probably 

 largely metabolic and vascular, while in nerve-exhaustion the shrinking 

 of the nerve-cells (as Hodge has shown) is obvious under the microscope. 

 This is probably of far-reaching import. All degrees of it are perceptible, 

 some arising even from what might be fairly termed normal fatigue 

 (see Fig. 26, page 58). 



The general sensations bring out the difference between a massive or 

 voluminous sensation of low intensity and a special sensation which 

 originates in the functioning of a single sort of end-organ. Compare 

 the feelings caused by a day's long climb with those arising in an un- 

 trained finger fatigued on one of the ancient Mosso ergographs so com- 

 mon in the laboratories. The latter experience is mostly a localized ache, 

 the former a wide-spread general sensation of low intensity, while neither 

 is like the pain caused by the overstimulation of a few pain-organs by 

 part of the red-hot coal of a parlor-match. Widespread stimulation 

 to their limit of intensity of cutaneous sense-organs is dangerous to life 

 from sheer neural shock. It appears to be the tissue-protoplasm itself 

 which gives the feeling of fatigue. To attempt therefore to trace out 

 nerves or nerve-centers of these sensations is worse than useless, because 

 from one point of view misleading. For fatigue is in the protoplasm 

 of the muscles mostly, although that of the nervous system and doubtless 

 of the glandular tissues have their share. (For the metabolic changes 

 occurring in muscular action, see below, page 382.) 



Thirst is also a general sensation (one originating all over the body) 

 dependent on a decrease in the fluidity of the body-protoplasm and of 

 the circulating lymph-plasma. By an arrangement whose nature is 

 not understood, this universal need is referred to one place, the throat 

 and mouth. The principle here as elsewhere is that the protective 

 sensation is felt in the physiologically right place. In this case the sensa- 

 tion requires that the water, needed all over the organism, shall be 

 placed in the entrance to the alimentary canal, for from this organ alone 

 it may be promptly absorbed into the circulation. The condition is 

 general and not local, for it may be promptly relieved by injecting water 

 into the stomach without its touching the mucosa of the tongue and 

 pharynx, as, for example, through a gastric fistula made for feeding the 

 patient in cases of cancer of the esophagus. Again, on taking the 

 required water by the mouth the condition is not relieved as the liquid 

 passes over the tongue, etc., but only after the twenty seconds or so 

 required for absorption from the duodenum into the blood to begin. 

 The sensation may have perhaps a local origin besides the normal 

 one in the general body-protoplasm. Opium, for instance, checks 

 secretion in the alimentary mucosa, as do many other substances, while 

 salt dries the membranes by changing their osmotic relations to the blood 

 and lymph within them. Mild inflammation of the gastric and intestinal 

 membranes reflexly produces sometimes the sensation of thirst, as many 

 persons are apt to know the morning after a too hearty dinner. 



