CHAPTER XL 



THE STRUCTURE AND FUNCTIONS OF THE SKIN. 



The skin serves (1), as an external integument for the protection 

 of the deeper tissues, and (2), as a sensitive organ in the exercise of 

 touch; it is also (3), an important secretory and excretory, and (4), an 

 absorbing organ; while it plays an important part in (5) the regulation 

 of the temperature of the body. 



Structure. The skin consists, principally, of a vascular tissue named 

 the corium, derma, or cutis vera, and an external covering of epithelium 

 termed the cuticle or epidermis. Within and beneath the corium are 

 imbedded several organs with special function, namely, sudoriferous 

 glands, sebaceous glands, and hair follicles ; and on its surface are sen- 

 sitive papillae. The so-called appendages of the skin the hair and 

 nails are modifications of the epidermis. 



A. Epidermis. The epidermis is composed of several strata of cells 

 of various shapes and sizes; it closely resembles in its structure the epi- 

 thelium of the mucous membrane that lines the mouth. The following 

 four layers may be distinguished in a more or less developed form. 1. 

 Stratum corneum (Fig. 231, a), consisting of superposed layers of horny 

 scales. The different thickness of the epidermis in different regions of 

 the body is chiefly due to variations in the thickness of this layer; e. g.< 

 on the horny parts of the palms of the hands and soles of the feet it is 

 of great thickness. The stratum corneum of the buccal epithelium 

 chiefly differs from that of the epidermis in the fact that nuclei are to 

 be distinguished in some of the cells even of its most superficial layers. 



2. Stratum lucidum, a bright homogeneous membrane consisting of 

 squamous cells closely arranged, in some of which a nucleus can be seen. 



3. Stratum granulosum, consisting of one layer of flattened cells 

 which appear fusiform in vertical section: they are distinctly nucleated, 

 and a number of granules extend from the nucleus to the margins of the 

 cell. 



4. Stratum Malpighii or Rete mucosum consists of many strata. 

 The deepest cells, placed immediately above the cutis vera, are columnar 

 with oval nuclei: this layer of columnar cells is succeeded by a number 

 of layers of more or less polyhedral cells with spherical nuclei; the cells 



