KIDNEY AND SKIN AS EXCRETORY ORGANS. 867 



pelvic nerves are constantly in tonic action, one tending to relax 

 and the other to increase the tonicity of the bladder musculature, 

 forming thus balanced antagonists. An interesting feature of the 

 reflex is the part taken by the internal sphincter. During the 

 filling of the bladder this sphincter is in tone, at the time of empty- 

 ing its tone presumably is inhibited as a part of the reflex act. 

 Experiments indicate that stimulation of the pudic nerve causes 

 contraction of the sphincter, while stimulation of the pelvic nerve 

 causes relaxation. It is possible, therefore, that in reflex mic- 

 turition the pelvic nerves carry the motor impulses that cause 

 contraction of the bladder musculature and, at the same time, 

 through other fibers, inhibitory impulses that relax the sphincter. 

 The mechanism is, however, complex and needs further investiga- 

 tion.* 



Excretory Functions of the Skin. The physiological activi- 

 ties of the skin are varied. It forms, in the first place, a sensory 

 surface covering the body, and interposed, as it were, between the 

 external world and the inner mechanism. Nerve fibers of pressure, 

 temperature, and pain are distributed over its surface, and by means 

 of these fibers reflexes of various kinds are effected which keep the 

 body adapted to changes in its environment. The physiology of 

 the skin from this standpoint is discussed in the section on special 

 senses. Again, the skin plays a part of immense value to the body 

 in regulating the body temperature. This regulation, which is 

 effected by variations in the blood supply or the sweat secretion, 

 is described at appropriate places in the sections on Nutrition and 

 Circulation. In the female, during the period of lactation, the mam- 

 mary glands, which must be reckoned among the organs of the 

 skin, form an important secretion, the milk; the physiology of this 

 gland is referred to in the section on Reproduction. In this section 

 we are concerned with the physiology of the skin from a different 

 standpoint, namely, as an excretory organ. The excretions of 

 the skin are formed in the sweat-glands and the sebaceous glands. 



Sweat. The sweat or perspiration is a secretion of the sweat 

 glands. These latter structures are found over the entire cutaneous 

 surface except in the deeper portions of the external auditory meatus, 

 the prepuce, and the glans penis. They are particularly abundant 

 upon the palms of the hands and the soles of the feet. Krause 

 estimates that their total number for the whole cutaneous surface 

 is about two millions. In man they are formed on the type of 

 simple tubular glands; the terminal portion contains the secretory 

 cells, and at this part the tube is usually coiled to make a more or 



* Consult Barrington, "Quarterly Journal of Exper. Physiology," 8, 33, 

 1914; Elliot, "Journal of Physiology," 35, 367, 1907; Stewart, "American 

 Journal of Physiology," 2, 182, 1899. 



