CHAPTER XVIII 

 THE SKIN AND THE SKIN-GLANDS 



IN all classes of animals the skin performs two functions. In the first place, 

 it serves to protect the more delicate underlying parts from injury and 

 from penetration or invasion by foreign organisms. In the second place, 

 it serves as a sense-organ, and is richly supplied with nerves, by means oi 

 which the activities of the body as a whole may be brought into relation 

 with the changes going on in the environment and affecting the external 

 surface of the body. In warm-blooded animals the skin plays an important 

 part in the regulation of the body temperature, since the loss of heat to 

 the body must occur almost entirely through its surface. In the present 

 chapter we have to deal only with the first and third of these functions. 



The development of the skin as an organ of protection shows wide 

 modification in various classes of animals. Thus it may become the seat of 

 formation of horny plates, as in the alligator ; of poisonous glands, as in 

 the toad ; or of mucous glands, as in many varieties of fishes. In warm- 

 blooded animals the development of hair from the deeper layers of the 

 epidermis serves to diminish the loss of heat. Since the hair-follicles are 

 richly supplied with nerve fibres, the hairs act also as organs of sensation. 

 In man, where the hairs are rudimentary, except in certain localities, 

 practically only this tactile function is retained. The external layer of the 

 skin in man consists of a tough horny layer formed by the keratinisation of 

 the external layers of cells of the epidermis. The skin is composed of two 

 parts, the epidermis and the cutis (Fig. 542). The epidermis is a stratified 

 squamous epithelium. The deeper layers form the rete mucosum, being soft 

 and protoplasmic, while the superficial layers forming the cuticle are hard 

 and horny. The most superficial layer of the rete mucosum is formed 

 of flattened cells filled with granules of a material staining deeply with hsema- 

 toxylin and eosin, known as eleidin. This layer is called the stratum granu- 

 losum. Immediately superficial to this layer is another in which the cells 

 are indistinct. The cells are clear in section and form what is known as the 

 stratum lucidum. These two layers evidently form the intermediate stages 

 in the transformation of the cells of the rete mucosum into the horny scales 

 which make up the superficial cuticle. The cutis or corium is composed of 

 dense connective tissue, which becomes more open in texture in its deeper 

 part, where it merges into the subcutaneous connective tissue. The most 

 superficial layer of the corium is prolonged into minute papillae over which 



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