CHAPTEE XX. 



THE SKIN AND ITS APPENDAGES. 



IN considering the chemical composition of the skin and its 

 appendages, we shall deal more exclusively with those substances 

 which are more or less peculiar to its epidermal structures. The 

 remaining components have already been studied in detail and 

 require no further consideration at this place. 



The epidermal structures in the case of the vertebrate animals 

 comprise the epithelial lining of the skin, together with its sweat 

 glands, the sebaceous glands and allied glands, the hair, the nails, 

 the hoofs, the feathers, etc. 



On section of the epidermis we discern different layers of cells. 

 The lowest of these, which is known as the Malpighian layer, is 

 composed of the youngest cells, from which all others are derived. 

 These are distinctly protoplasmic in character, but with increasing 

 age they become dry and scaly, and are finally represented by fine 

 lamellae of keratin, which are constantly thrown off and regenerated 

 from below. Keratin, itself, however, is not found in the lower 

 layers of the epidermis, as has been shown by Ernst, but appears 

 only above the so-called stratum granulosum. In the latter we 

 find peculiar granules, which are scattered about the nuclei of the 

 cells, and which Ernst regards as derivatives of the nuclei. They 

 are known as ele'idin granules, and, according to some observers, 

 represent intermediary products in the formation of keratin. Of 

 their chemical nature, however, nothing is known. 



The transition of the soluble albumins of the lower strata of cells 

 into the insoluble keratin also finds its expression in the greater 

 resistance which the upper layers offer to the action of the caustic 

 alkalies. For, while the lower cells are dissolved with comparative 

 ease the upper strata are scarcely affected by the reagent. Pancreatic 

 juice and gastric juice behave in the same manner, and it is thus 

 possible to separate the insoluble keratin from the soluble albumins 

 which may be present at the same time. Like the horny layer of 

 the skin, so also are other epidermal structures, such as nails, 

 hair, horn, feathers, etc., largely composed of keratins, and it is 

 noteworthy that these keratins are especially rich in sulphur. That 

 of human hair, according to Suter, contains 2.52 per cent., of which 

 2.34 per cent, is present in loosely combined form, which, as we 

 have seen, is most likely represented by a cystin group. Horn 



414 



