4 io HUMAN PHYSIOLOGY 



Those to the leg and foot come from the upper lumbar region, 

 those for the hand and arm from the fifth to the eighth cervical 

 nerve, and those for the head chiefly from the medulla. 



The centres presiding over these nerves have not been 

 definitely located. But they are capable of (a) reflex stimula- 

 tion, as when pepper is taken into the mouth ; and (6) of 

 direct stimulation by a venous condition of the blood, as in 

 the impaired oxygenation of the blood which so frequently 

 precedes death as the respirations fail. 



But even after the nerves to the sweat glands are cut, the 

 glands can be stimulated by certain drugs e.g. pilocarpin. 

 The action of heat seems also to be chiefly peripheral, setting 

 up an unstable condition of the gland cells so that they re- 

 spond more readily to stimulation. 



3. Chemistry of Sweat. Sweat is a clear, watery fluid, 

 which, when pure, has a neutral or faintly alkaline reaction, 

 but which is generally acid from admixture with the sebaceous 

 secretion. Its specific gravity is low, about 1004, and it 

 contains less than 2 per cent, of solids, of which the chief is 

 NaCl. Of the organic solids, urea is the most important. 

 About 4 or 5 per cent, of the total nitrogen excreted from 

 the body is thrown off by the skin in this form, and, when 

 the action of the kidneys is interfered with, very considerable 

 quantities may be eliminated. The sweat nearly always con- 

 tains epithelial squames from the epidermis, and oil globules 

 derived from the sebum. 



B. Sebaceous Secretion. The sebaceous glands are simple 

 racemose glands which open into the hair follicles, and their 

 function is to supply an oily material to lubricate the hairs. 

 This secretion is produced by the shedding and breaking 

 down of the cells formed in the follicles of the glands. 

 Those lining the basement membrane are composed of proto- 

 plasm and actively divide, but the cells thrown off into the 

 lumen of the follicle disintegrate and become converted into 

 a semi-solid oily mass, which consists of free fatty acids and 

 of neutral fats of glycerine and of cholesterin. These latter 

 are the lanolins, which differ from ordinary fats in being 

 partly soluble in water. Free cholesterin is also present in 

 the sebum. 



C. Milk Secretion 1. Physiology. JBefore pregnancy 



