1216 PHYSIOLOGY 



the fat found in other parts of the body. It is true that it contains fatty 

 acids, but these are for the most part in combination, not with glycerin but 

 with higher alcohols, including cholesterol. A somewhat similar material, 

 known as wool-fat or lanoline, may be extracted from wool as well 

 as from the feather-glands of water birds, such as the goose and duck. It 

 must be regarded rather as a wax than a fat. It presents many advantages 

 over ordinary fat as a protective salve for the surface of the body. In the 

 first place, it can take up a large amount, as much as 100 per cent., of water. 

 In the second place, it is not attacked by micro-organisms, so that it does 

 not tend to become rancid or to furnish a nidus for the growth of these 

 organisms on the surface of the body. 



The secretion of sebum is a continuous process, though it is probably 

 quickened in conditions of increased vascularity of the skin. The extrusion 

 of the products of secretion is determined by the presence of unstriated 

 muscle fibres, the arrector pili, which pass from the surface of the cutis 

 obliquely over the outer surface of the sebaceous gland. When these muscle 

 fibres contract, the hair is erected and a certain amount of the sebum 

 squeezed out on to the root of the hair and the surrounding skin. This 

 contraction will occur whenever cold is suddenly applied to the skin. The 

 contracted condition of all the muscles of the hair follicles is shown by the 

 * goose-skin ' produced under such circumstances. There is no evidence 

 that the secretion of sebum is in any way under the control of the central 

 nervous system. 



THE SWEAT GLANDS. Under normal circumstances in temperate 

 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 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 constantly evaporated from the surface of the body as the so-called 

 4 insensible 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 de- 

 termined by evaporation of water from the surface of the cuticle itself, apart 

 altogether from the sweat glands. These are distributed over the whole 

 surface of the skin, and 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 subcutaneous 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 



