96 PHILOSOPHY OF ZOOLOGY. 
The sheath, of which we have already spoken, likewise 
grows in length with the new parts of the feather. It is 
thickest at first, and falls off in the form of scales; but, 
towards the end of the process, it is thin, and adheres 
closely to the quill, and is that part which requires to be 
removed by artificial means, in the preparation of quills for 
writing. 
Feathers appear to consist of nearly the same constituent 
ingredients as hair. They, perhaps, contain less gelatine. 
They occur only on birds, and characterise the class. 
They furnish the most cbvious marks for the discrimination 
of species ; and have obtained various names, according 
to the parts of the body from which they take their rise. 
These terms will be particularly enumerated when we come 
to treat of birds. 
3. Horns.—Horns take their rise in the same situation 
as hairs or feathers. They may be regarded as hairs ag- 
glutinated, and forming a hollow cone. The fibrous struc- 
ture of horn may be perceived in many animals at the base, 
where it unites with the skin. At this part it receives the 
additions to its growth, the apex of the cone being pushed 
out in proportion as the imcrease takes place at the root, 
and on the inner surface. But horns differ remarkably 
from hair, in having their central cavity filled by a projection 
of bone or other solid substance from the body beneath. 
The different markings of the horns, particularly the 
transverse ridges, are indications of the different layers of 
growth ; and, in many cases, the number of these ridges 
corresponds with the years of life. 
The colour of the horn is, in general, distributed through 
the mass ; sometimes, however, it is collected into bands or 
threads. It seldom experiences much change during the 
life of the animal. 
