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 environ- 

 ment 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. 540). Th<e 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 hsematoxylin 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 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 connec- 



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