THE SKIN AND THE SKIN-GLANDS 1163 



signs of proliferation. The cells lying internal to these are much larger, 

 and their protoplasm is transformed into a network in the meshes of which 

 are granules which may show the reaction of fat. Further inwards the 

 protoplasmic network diminishes in amount, while the fatty granules increase 

 in size, so that, in the lumen adjoining the duct, we find only a mass of cell 

 debris and masses of fatty material. It has often been thought that the 

 secretion of sebum depended simply on a fatty degeneration of the cells. 

 The granules, however, when they first appear, stain with acid fuchsin 

 rather than osmic acid, and one must regard the formation of sebum as an 

 act of true secretion in which the secretory granules are gradually trans- 

 formed into the special constituents of the sebum. For it must be noted 

 that the sebum is not a true fat, nor does it correspond in composition with 

 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 

 may be extracted from wool, and is known as wool-fat or lanoline, 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 con- 

 traction will occur whenever cold is suddenly applied to the skin. The con- 

 tracted 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 

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



