THE SKIN AND THE SKIN-GLANDS 1301 



effects of the atmosphere, it is kept constantly impregnated with a 

 fatty material known as sebum. This material is formed by the 

 sebaceous glands, which are distributed all over the surface of the skin 

 wherever hair-follicles are to be found, the mouths of the glands 

 opening into the hair-follicles. A sebaceous gland is a pear-shaped 

 body, consisting of a secreting part and a short neck, opening into 

 the follicle. The gland proper is composed of a solid mass of cells. 

 The outermost cells are flattened and generally show signs of pro- 

 liferation. 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 transformed 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 pro- 

 tective 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 to 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 



