APPENDICES OF THE SKIN. 97 
It is permanent, or does not experience those periodical 
renovations which we have stated to take place with hair and 
feathers. ‘The deciduous horns of the stag are different in 
their nature from true horns, and will be afterwards taken 
notice of. 
The term Horn is usually restricted to the coverings of 
the projections of the frontal bones of oxen, sheep, and si- 
milar quadrupeds ; but various appendices of the skin, com- 
posed of the same materials, and equally permanent, al- 
though seated on other parts of the body, may, with pro- 
priety, be included under the same appellation. Among 
these may be enumerated, 
a. Beaks.—The substance which covers the external 
surface of the maxillary bones of birds, is composed of 
horn. It varies greatly in texture and markings, according 
to the species. It differs from the horns of quadrupeds 
chiefly in this circumstance, that the colouring matter is not 
so completely fixed, but is probably preserved in vessels 
which are still in communication with the body. At least 
it is well known, that the colour of the bill varies with the 
season, and experiences considerable changes after death. 
This, however, is most remarkable in those birds, the horny 
covering of whose bills is thin, so that the colour may de- 
pend on the fluids below. 
Horny beaks are found on birds and many reptiles ; and, 
in the inferior classes, in all those whose jaws are destined 
to cut hard substances. 
b. Hoofs.—Hoofs resemble horns in their manner of 
growth, and in containing a central support, formed by the 
termination of the extreme bones of the feet. They grow 
from the inner surface and base, and are thus fitted to sup- 
ply the place of those parts which are worn away by being 
exposed to friction against hard bodies. Hoofs are peculiar 
to certain herbivorous quadrupeds. 
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