THE KIDNEYS AND SKIN. 447 



saccules the substance of the cells becomes charged with oil- 

 drops, the protoplasm disappearing; and finally the whole 

 cell falls to pieces, its detritus constituting the secretion. 

 New cells are, meanwhile, formed to take the place of those 

 destroyed. Usually two glands are connected with each hair- 

 follicle, but there may be three or only one. A pair of seba- 

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

 follicles in Fig. 137. 



The Skin Secretions. The- skin besides forming a pro- 

 tective covering and serving as a sense-organ (Chap. XXXV) 

 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 

 transparent 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 2000 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 increased 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 in- 

 sensible form, drops of sweat only accumulating when the 

 secretion is very profuse, or the surrounding atmosphere so 

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

 perspiration is acid, and in 1000 parts contains 990 of water 

 to 10 of solids. Among the latter are found urea (1.5 in 

 1000), fatty acids, sodium chloride, and other salts. In dis- 

 eased conditions of the kidneys the urea may be greatly 

 increased, the skin supplementing to a certain extent defi- 

 ciencies 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 in- 

 creased 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. 



