CHEMICAL PROPERTIES AND HISTOLOGY OF HORN.

Horn is a solid, tenacious, fibrous material, and its density in the hoof
varies in different situations. It is softened by alkalies, such as caustic
potash or soda and ammonia, the parts first attacked being the commissures,
then the frog, and afterwards the sole and wall. Strong acids, such as
sulphuric acid and nitric acid, also dissolve it.

The chemical composition of the hoof shows it to be a modification of
albumin, its analysis yielding water, a large percentage of animal matter,
and materials soluble and insoluble in water. The proportions of these, as
existing in the various parts of the hoof, have been given by Professor
Clement as follows:

Wall. Sole. Frog.
Water 16.12 36.0 42.0
Fatty matter 0.95 0.25 0.50
Matters soluble in water 1.04 1.50 1.50
Insoluble salts 0.26 0.25 0.22
Animal matter 81.63 62.0 55.78

Horn appears to be identical with epidermis, hair, wool, feathers, and
whalebone, in yielding 'keratin,' a substance intermediate between albumin
and gelatine, and containing from 60 to 80 per cent. of sulphur.

That horn is combustible everyone who has watched the fitting of a hot shoe
knows. That it is a bad conductor of heat, the absence of bad after-effects
on the foot testifies.

[Illustration: FIG. 31.--PERPENDICULAR SECTION OF HORN OF WALL.]

In a previous page we have described the manner of growth of the horn
tubules, and noted the direction they took in the wall; also, we have
noticed the existence between them of an intertubular horn or cement.

Those who wish to give this subject further study will find an excellent
series of articles by Fleming in the _Veterinarian_ for 1871. We
shall content ourselves here with introducing one or two diagrams and
photo-micrographs, and dealing with the histology very briefly.

Under the microscope the longitudinal striation of the wall is found to be
due to the direction taken by the horn tubules.

Fig. 31 is a magnified perpendicular section of the wall. In it the
parallel dark striae are the horn tubules in longitudinal section. The
lighter striae represent the intertubular material.

Fig. 32 gives us the wall in horizontal section. To the left of this
picture we find the horn tubules cut across, and standing out as so many
concentrically ringed circles. In th