1302 PHYSIOLOGY 



of sebum is in any way under the control of the central nervous 

 system. 



THE SWEAT-GLANDS. Under normal circumstances in tem- 

 perate climates the greater part of the water taken in with the food in 

 the course of the day is excreted by the kidneys, a smaller proportion 

 leaving by the lungs and by the surface of the skin. On an average 

 we may say that about 700 c.c. are got rid of through the skin. The 

 excretion of water by the skin is, however, mainly determined by the 

 need for regulating the temperature of the body, so that the amount 

 leaving in this way depends entirely on the heat production of the body 

 or on the external 'temperature, and is very little affected by alterations 

 in the quantity of fluid drunk. A certain amount of water is con- 

 stantly evaporated from the surface of the body as the so-called ' in- 

 sensible perspiration.' If a man's body be enclosed in a vessel through 

 which a current of air is passed, and the temperature of the air gradually 

 raised, it will be noted that the amount of water given off rises slowly 

 up to a certain degree and then rises rapidly. The sudden kink in the 

 curve is due to the setting in of the activity of the sweat-glands, and 

 we are therefore justified in regarding the insensible perspiration as 

 being determined by evaporation of water from the surface of the cuticle 

 itself apart altogether from the sweat-glands. The sweat-glands, 

 which are distributed over the whole surface of the skin, are especially 

 abundant on the palm of the hand and on the sole of the foot. They 

 are composed of single unbranched coiled tubes, which lie in the sub- 

 cutaneous tissue and send their ducts up through the cutis, to open 

 on the surface by corkscrew-like channels which pierce the epidermis. 

 The secreting part of the tube consists of a basement membrane lined 

 by a double layer of cells ; the innermost of these are cubical and 

 represent the secreting cells proper. Between the secreting cells and 

 the basement membrane is a layer of unstriated muscle fibres. The 

 duct of the gland has an epithelium, consisting of two or three layers of 

 cells with a well-marked internal cuticular lining, but there is no 

 muscular layer. 



The sweat formed by these glands is the most dilute of all animal 

 fluids. As collected it generally contains epithelial scales and some 

 admixture of sebum. After filtration it forms a clear colourless fluid 

 of a specific gravity of about 1003. It contains over 99 per cent, of 

 water. Among the solid constituents sodium chloride is the most 

 prominent it may contain from 0-3 to 0-5 per cent, of this salt. It is 

 generally hypotonic as compared with the blood-plasma. It may 

 also contain small traces of proteins. This constituent is especially 

 marked in the horse. It generally contains also a small quan- 

 tity of urea, which may become a prominent constituent in cases of 

 renal disease. The quantity of sweat excreted in the day is very 



