THE SKIN, AND ITS DERIVATIVES 257 



The skin excretes by means of glands which may be com- 

 posed of single cells or many cells. Examples of the latter 

 are to be found in the mammary, sebaceous, and sweat-glands 

 of the mammals. These glands arise in the epidermis and 

 project inwards into the underlying dermis. In some animals 

 the glands may be modified into poison-glands ; and in deep- 

 sea fish they may produce a luminous secretion. 



The epidermis may be modified into a variety of structures 

 such as horny scales (corneoscutes) which are present in reptiles, 

 birds (chiefly on the feet) and mammals (all over the body 

 of the Pangolin, at the base of the tail of the rat). The 

 epidermis also gives rise to feathers which are characteristic 

 of birds (see p. 224) ; hairs which are characteristic of mammals 

 (see p. 234) ; the termination of the digits which may take 

 the form of claws, nails, or hoofs ; and the horny covering of 

 the beak in tortoises and birds. The " horns " of cattle are 

 formed of a layer of epidermal horn overlying a central dermal 

 bony core. The horn of the rhinoceros is made of fused hair. 

 Special epidermal structures on the edge of the mouth of 

 Petromyzon, frog tadpoles, and Ornithorhynchus, give rise to 

 the so-called " horny teeth," which have nothing to do with 

 true teeth. Lastly, the epidermis produces the cap of enamel 

 which forms a covering to the dentine of denticles and true 

 teeth. 



Hairs and feathers are commonly moulted at intervals and 

 replaced. 



The dermis forms the leathery layer of the skin. It con- 

 tains blood-vessels which serve to supply the cells of the 

 epidermis as well as those of the dermis, and especially the 

 papillae at the bases of hairs and feathers, and the glands. In 

 amphibia this dermal circulation also serves respiratory pur- 

 poses, and in the mammals it forms part of the mechanism for 

 regulating the heat of the body. In amphibia the dermis is 

 separated from the underlying muscles by lymph-spaces, but 

 in higher forms the skin is firmly attached to the muscles by 

 connective tissue. In higher forms, special muscles arise in 

 connexion with the dermis. Some of them are attached 

 to scales, feathers, or hair-follicles, which they move. It is 



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