THE RECEPTOR SYSTEM 211 



the saliva which is swallowed at frequent intervals, and the sense of 

 thirst thus kept in abeyance. There is a constant loss of water 

 from the Body by means of the various channels of excretion, 

 lungs, sweat glands, etc. When the resultant diminution in the 

 water content of the tissues reaches a certain point the swallowing 

 of saliva no longer prevents stimulation of the thirst receptors, and 

 liquid from outside the Body must be taken if the thirst is to be re- 

 lieved. The liquid need not necessarily be swallowed. Injections 

 directly into the veins are effective in abolishing thirst sensations. 



If, as the result of prolonged deprivation, the water content of 

 the Body is seriously diminished, ordinary thirst gives way to much 

 more pronounced and finally very painful sensations. From these 

 there is no relief with the passage of time as there is in case of 

 hunger. The distress becomes more and more marked leading 

 ultimately, it is said, to mental breakdown. Thirst is believed to 

 be the only sense of which the Body may not be deprived through 

 accident or disease. 



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. 

 Pain spots are more numerous than any of the others; touch spots 

 rank next in number, it being estimated that on the trunk and 

 limbs there are a half million of them; cold spots are only half as 

 numerous as touch spots; warmth spots are fewest of all, their 

 number being estimated at thirty thousand for the entire Body. 



Pain. When the skin is powerfully stimulated by heat, cold or 

 pressure, or is inflamed, we get a sensation which we call pain. 

 This is something quite different from the unpleasantness caused 

 by a dazzling light or a musical discord or a disagreeable odor or 

 taste. We recognize these as being still sight or sound or smell . 



