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 of 

 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 from 

 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 dimmish 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. 555). 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 hsemotoxylin and eosin, known as eleidin. 

 This layer is called the stratum granulosum. 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 constitute the inter- 

 mediate 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 con- 

 nective 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 the epidermis is moulded. These papillae 

 contain for the most part capillary vessels; a few contain touch corpuscles, special 

 organs of tactile sensation. The Mood vessels of the skin form a close capillary network 

 immediately at the surface of the cutis, sending up loops into the papillae. All parts 

 of the skin, except the palms of the hands and the soles of the feet, are beset with hair 

 follicles. The hair follicles are small pits which extend downwards into the deeper 

 part of the corium, being downgrowths of the rete mucosum. The Hair grows from 

 a small papilla of cells at the bottom of the follicle, the part of the hair lying within the 

 follicle being known as the hair root. The hair itself consists of long tapering, horny 

 cells, the nuclei of which are still visible, though the cell substance has been almost 

 entirely converted into keratin. 



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