CHAPTER XIII. 

 THE SKIN AND ITS SECRETIONS. 



IN" the sliructure of the skin of man and vertebrates many 

 different kinds of substances occur which have already been 

 treated of, such as the constituents of the epidermis formation, the 

 connective and fatty tissues, the nerves, muscles, etc. Among 

 these the different horn-formations, the hair, nails, etc., whose 

 chief constituent, keratin, has been spoken of in another chapter 

 (Chap. II), are of special interest. 



The cells of the horny formation show, in proportion to their 

 age, a different resistance to chemical reagents, especially fixed 

 alkalies. The younger the horn-cell the less resistance it has to 

 the action of alkalies; with advancing age the resistance becomes 

 greater, and the cell-membranes of many horn-formations are nearly 

 insoluble in caustic alkalies. Keratin occurs in the horn formation 

 mixed with other bodies, from which it is isolated with difficulty. 

 Among these bodies the mineral constituents in many cases occupy 

 a prominent place because of their quantity. Hair leaves on 

 burning 5-70 p. m. ash which contains in 1000 parts 230 parts 

 alkali sulphates, 140 parts calcium sulphate, 100 parts iron oxide, 

 and 400 parts silicic acid. Dark hair seems generally, but not 

 always, to yield more iron oxide on burning than blond. The 

 nails are rich in calcium phosphate, and the feathers rich in silicic 

 acid. 



The skin of invertebrates has been the subject, in a few cases, 

 of chemical investigation, and in these animals several substances 

 have been found, of which a couple, though little studied, are 

 worth discussing. Among these bodies tumcin, which is found 

 especially in the tunic of the tunicata, and the widely-diffused 



