EXCRETION AND THE EXCRETORY ORGANS 



535 



two and a half millions of them opening on the surface of 

 the Body. 



The sebaceous glands nearly always open into hair-follicles, and 

 are found wherever there are hairs. Each consists of a duct open- 

 ing near the mouth of a hair-follicle and branching at its other end : 

 the final branches lead into globular secreting saccules, which, like 

 the ducts, are lined with epithelium. In the saccules the substance 

 of the cells becomes charged with oil-drops, the protoplasm disap- 

 pearing; and finally the whole cell falls to pieces, its detritus con- 

 stituting 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 sebaceous 

 glands are represented on the sides of each of 

 the hair-follicles in Fig. 142. 



The Skin Secretions. The skin besides form- 

 ing a protective covering and serving as a sense 

 organ (Chap. XIII) also plays an important 

 part in regulating the temperature of the Body, 

 and a less important function as an excretory 

 organ, in carrying off water and traces of other 

 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 



J? . ,; ' J , . ,.. . . ' FIG. 145. A sweat- 



the same individual, in different regions of the gland, a, horny layer 



Body. Its quantity in twenty-four hours is sub- 



ject to great variations, but usually lies between k- The coils of 



700 and 2,000 grams (10,850 and 31,000 grains), embedded in t 



rr,, ,_ a j -IT cutaneous fat, are 



The amount is influenced mainly by the sur- see n below the der- 

 rounding temperature, being greater when this mis * 

 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 sen- 

 sible perspiration. By far the most passes off in the insensi- 

 ble 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 



