488 A TEXTBOOK OF PHYSIOLOGY 



a spiral passage through the epidermis, and so reach the surface of 

 the skin. 



The perspiration, or sweat, is a watery fluid (99 per cent, is water), 

 generally neutral or faintly alkaline in reaction. The 1 per cent, 

 of solids is chiefly sodium chloride and fatty bodies, as is seen from 

 the following table : 



Water ' 98-88 



Solids 1-12 



Salts 0-57 



Sodium chloride 0-22-0-33 



Alkaline sulphates, phosphates, lactates, and 



potassium chloride . . . . . . . . 0-18 



Fats, fatty acids and cholesterol . . . . 0-41 



Epithelium 0-17 



Urea 0-08 



Usually the excretion of urea by the skin is negligible, but during 

 a day's march on a very hot day as much as 12 per cent, of the total 

 nitrogen output may be excreted in the sweat. Sometimes a pink sweat 

 is secreted in the' axillae, coloured, it is said, by products of putrefaction 

 absorbed from the large intestine. 



The amount of sweat varies largely with the temperature of the 

 surroundings and the heating of the body by muscular work e.g.? 

 600 grammes on an ordinary, and 3,000 grammes on a warm day 

 in which considerable exercise is taken. 



The function of the sweat is to moisten the skin and to cool the body 

 by evaporation. As the evaporation of 1 gramme of water requires 

 540 calories, we see the cooling efficiency of the sweating mechanism. 

 A man without sweat glands was only able to work in the hot 

 sun by wetting his shirt frequently, and so artificial!}- making good 

 the absence of sweat. In a hot chamber his body temperature quickly 

 became febrile. The importance of the sweat as an excretion of 

 material is little or nil. 



Sweating is under the control of the central nervous system; the 

 centre is stated to be situated in the floor of the fourth ventricle, and 

 is provoked by a rise of temperature of the blood which circulates 

 through it. Such a rise is generally produced by muscular work. 

 Experimentally it can be shown that the warming of the carotid 

 blood induces sweating. It may also be stimulated by increased con- 

 centration of C0 2 in the blood, as in asphyxia, and by certain druga 

 known as diaphoretics e.g., morphine. 



The centre is also affected by afferent nervous impulses. Stimula- 

 tion of the central tnd of the sciatic nerve causes sweating reflexly. 

 Such sweating is associated with a vaso-constriction in the limb, 

 showing that the vaso-dilatation of the skin, which generally 

 accompanies sweating, is not a necessary, although a favourable, 

 condition. 



The efferent channels for the sweat nerves run in the sympathetic 

 system, arising from the thoracic region of the spinal cord (see p. 70). 

 Communication is established with the sympathetic ganglia by the 

 white rami communicantes of the various nerves. Leaving the ganglia,. 



