480 THE HUMAN BODY 



of sebaceous glands are represented on the sides of each of the hair- 

 follicles in Fig. 143. 



The Skin Secretions. The skin besides forming a protective 

 covering and serving as a sense-organ (Chap. XIII) also plays an 

 important part in regulating the temperature of the Body, and, as 

 an excretory organ, in carrying off certain waste products. 



The sweat poured out by the sudoriparous glands is a trans- 

 parent colorless liquid, with a peculiar odor, varying in different 

 races and, in the same individual, in different regions of the Body. 

 Its quantity in twenty-four hours is subject to great variations, 

 but usually lies between 700 and 2,000 grams (10,850 and 31,000 

 grains). The amount is influenced mainly by the surrounding 

 temperature, being greater when this is high; but it is also in- 

 creased by other things tending to raise the temperature of the 

 Body, as muscular exercise. The sweat may or may not evaporate 

 as fast as it is secreted ; in the former case it is known as insensible, 

 in the latter as sensible perspiration. By far the most passes off in 

 the insensible form, drops of sweat only accumulating when the se- 

 cretion is very profuse, or the surrounding atmosphere so humid 

 that it does not readily take up more moisture. The perspiration 

 is acid, and in 1,000 parts contains 990 of water to 10 of solids. 

 Among the latter are found urea (1.5 in 1,000), fatty acids, sodium 

 chlorid, and other salts. In diseased conditions of the kidneys the 

 urea may be greatly increased, the skin supplementing to a certain 

 extent deficiencies of those organs. 



The Nervous and Circulatory Factors in the Sweat Secretion. 

 It used to be believed that an increased flow of blood through the 

 skin would suffice of itself to cause increased perspiration; but 

 against this view are the facts that, in terror for example, there 

 may be profuse sweating with a cold pallid skin ; and that in many 

 febrile states the skin may be hot and its vessels full of blood, and 

 yet there may be no sweating. 



Direct experiment shows that the secretory activity of the 

 sweat-glands is under immediate control of nerve-fibers, and is 

 only indirectly dependent on the blood-supply in their neighbor- 

 hood. Stimulating the sciatic nerve of the freshly amputated 

 leg of a cat will cause the balls of its feet to sweat, although there 

 is no blood flowing through the limb. On the other hand, if the 

 sciatic nerve be cut so as to paralyze it, in, a living animal, the 



