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CHAPTER XV
THE SKIN AND ITS SECRETIONS
In the structure of the skin of man and vertebrates many different
kinds of substances occur which have already been considered, such as
the constituents of the epidermal formation, the connective and fatty
tissues, the nerves, muscles, etc. Among these the different horn struc-
tures, the hair, nails, etc., whose chief constituent, keratin, has been
spoken of in another chapter (Chapter II), are of special interest.
The cells of the horny structure 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-mem-
branes of many horn-formations are nearly insoluble in caustic alkalies.
Keratin (or the keratins) occurs in the horn structure mixed with other
bodies, from which it is isolated with difficulty. These are detected by
microchemical investigations, and according to Unna l
three different
substances can be detected in the horn substance, designated by him A-,
B- and C-keratin.
The il-keratin, which forms the envelope of the horn and hair cells and the
outer layer of the hair, is the purest keratin. It is not dissolved by fuming nitric
acid at the ordinary temperature and does not give the xanthoproteic reaction,
and its keratin nature is doubtful. The 5-keratin, which occurs as the contents
of the nail cells, gives the xanthoproteic reaction like the C-keratin occurring in
hair, but differs from the C-keratin by being soluble in fuming nitric acid.
Besides these substances, which have been called keratins, the horn
structure also contains other proteins which are soluble in pepsin-
hydrochloric acid. Among these we find residue of nuclei and the
so-called trichohyalin in the hair, which is a substance of unknown
constitution and characterized by great insolubility. From these
statements it is evident that we are here dealing with a mixture of dif-
ferent substances and for this reason it is unnecessary to give the older
elementary analyses of the various epidermoidal structures.
1
Monatsch. f. prakt. Dermat., 44.
837
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