THE RECEPTOR SYSTEM 175 



ditions, they may also be, and more powerfully, excited by poverty 

 of the blood in foods and water; such deficiency probably acts, in 

 the case of thirst, at least, by bringing about stimulation of many 

 afferent nerves which are normally not concerned in the sense of 

 thirst. This stimulation may become so powerful as to upset the 

 higher brain structures. Loss of reason is said to be the inevitable 

 result of too prolonged deprivation of water. 



That hunger is not altogether determined by the degree of empti- 

 ness of the stomach is shown by the experience of nearly all adults, 

 who are less hungry in the morning, after a fast of such length that 

 the stomach is practically certain to be empty, than at noon or 

 night, when the interval after the preceding meal is so short as to 

 make it often unlikely that the stomach has emptied itself. We 

 must recognize, however, that the so-called hunger of well nour- 

 ished human beings is rather appetite than true hunger, and de- 

 pends more on habit than on real bodily demands. So that true 

 physiological hunger cannot be explained in terms of it. 



Fatigue is the least definitely localized of the senses. It is felt 

 as a state of the whole Body, rather than as arising from any par- 

 ticular region. It is not definitely proven that fatigue is a sense so 

 far as that term implies the possession of receptors with their in- 

 dividual afferent nerves. Many physiologists think that it arises 

 from the presence in the blood of " fatigue products," chemical 

 substances given off from active tissues, and that these may pro- 

 duce the feeling of fatigue by direct chemical stimulation of the 

 central nervous system itself, or in some other way not involving 

 a particular set of fatigue receptors. 



The Cutaneous Senses. These occur over the entire body, not 

 uniformly distributed but scattered in fine dots over the surface. 

 This punctiform arrangement can be demonstrated by exploring 

 the skin with fine needles. Such a procedure shows that the dif- 

 ferent cutaneous senses occur in distinct spots which do not over- 

 lap, but which in most parts of the Body are so intermingled as to 

 leave no area of any size devoid of any one of the senses. Sensory 

 spots are much more numerous and more closely packed together 

 in such regions as the hands and face which are liable to come in 

 contact with foreign bodies, than they are in the better protected 

 surfaces of the trunk and limbs. Four sorts of cutaneous sense 

 spots are recognized; those of pain, touch, warmth, and cold. 



