Chap. XVIII] THE SKIN 351 



ous forms of kidney disease urea may be present in considerable 

 quantity, the skin supplementing to a certain extent the deficient 

 work of the kidneys. 



Quantity of perspiration. — Under ordinary circumstances, the 

 perspiration that we are continually throwing off evaporates 

 from the surface of the body without our becoming sensible of it, 

 and is called insensible perspiration. When more sweat is poured 

 upon the surface of the body than can be removed at once by 

 evaporation, it appears on the skin in the form of drops, and we 

 then speak of it as sensible perspiration. 



The average amount discharged during twenty-four hours is 

 about one quart (0.946 litre), but it may be increased to such an 

 extent that even more may be discharged in an hour. The secre- 

 tion of sweat is increased by: (1) a dilute condition of the blood, 

 such as results from drinking large quantities of liquids, (2) in- 

 creased temperature or humidity of the atmosphere, (3) exercise, 

 (4) pain, (5) mental excitement or nervousness, (6) dyspnoea, 

 (7) use of diaphoretics, (8) certain diseases such as tuberculosis, 

 acute rheumatism, and malaria, (9) use of electricity to stimulate 

 the secretory nerves. 



The secretion of sweat is decreased by: (1) voiding of a large 

 quantity of urine, (2) cold, (3) diarrhoea, (4) certain drugs, and 

 • (5) certain diseases, such as fevers, diabetes, and some cases of 

 paralysis. 



Activity of the sweat-glands. — The activity of the sweat-glands 

 is supposed to be due to direct stimulation of the nerve endings in 

 the glands, or indirect stimulation of the nerve centres controlling 

 perspiration. An increase in perspiration following an increase in 

 the temperature or humidity of the atmosphere is partly due to 

 stimulation of the nerve endings in the sweat-glands, and partly 

 due to the stimulation from the sensory nerves being carried to 

 the nerve centres, and transmitted along motor nerves to the 

 gland. The activity is also influenced by the vasomotor nerves, 

 which control the size of the blood-vessels in the skin ; an increase 

 in the size of the vessels leads to increased, a constriction of the 

 vessels to diminished, perspiration. That this is not the only 

 factor is proven by the profuse perspiration that often accompanies 

 a pallid skin, or the absence of perspiration that is characteristic 

 of fever. 



