366 ESSENTIALS OF PHYSIOLOGY. 



skin, and to a smaller extent in warming the expired air and the 

 excreta. On the average, 77 to 80 per cent, of the heat loss takes 

 place through the skin, 17 to 20 per cent, is lost from the lungs, and 

 3 per cent, in the excreta. The total daily loss varies greatly with the 

 conditions under which the individual is living, being as a rule from 

 2500 to 3500 calories. 



The skin consists of two layers, a superficial layer, the epidermis, 

 consisting of stratified squamous epithelium, and a deeper layer, the 

 dermis, formed of fibrous tissue. The dermis rests upon connective 

 tissue of a looser texture, which is called the subcutaneous tissue and 

 contains a variable amount of fat. 



The epidermis consists of two principal layers, a deeper layer of cells 

 called the rete mucosum, and a superficial layer known as the stratum 

 corneum or horny layer. The cells of the rete mucosum are mostly 

 irregular in shape and are connected by protoplasmic bridges, between 

 which are tiny channels along which lymph flows for the nourishment 

 of the cells. 



In the horny layer a transformation has occurred whereby the 

 protoplasm has been converted into keratin-, at the same time the 

 cells have become flattened and scaly and possess no visible nuclei. 

 Between the stratum mucosum and the horny layer can be seen two 

 narrow layers, stratum granulosum and stratum lucidum, in which the 

 cells are undergoing transformation. The surface cells of the epidermis 

 are continually being shed, and are replaced by the multiplication and 

 subsequent alteration of the cells of the rete mucosum. 



The dermis consists of dense fibrous tissue which presents papillae 

 or projections on its surface. The epidermis is moulded on these 

 papillae, and where they are arranged in rows, as on the palmar surface 

 of the hand and fingers, the epidermis shows corresponding ridges. 

 Blood-vessels run in the dermis and form capillary loops in the papillae. 

 Lying near the junction of the dermis and subcutaneous tissue over 

 the whole surface of the body are the sweat glands, consisting of coiled 

 tubes, the ducts of which run through the dermis and open into cork- 

 screw-shaped channels in the epidermis leading to the surface. 



The skin is protected and kept supple by sebum, which is a fatty 

 material secreted by the sebaceous glands. These glands are found 

 wherever hairs are present, and their ducts open into the upper part 

 of the hair follicles. Each gland is composed of a solid mass of cells, 

 in the central part of which the cells are loaded with fat and the proto- 

 plasm has largely disappeared. The fatty material in sebum is not a 

 true fat, but consists chiefly of fatty acids combined with cholesterol. 



The secretion of sebum is always taking place, the semi-liquid 



