APPENDICES OF THE SKIN. 87 
4 
which the superfluous nourishment is conveyed in the form 
of fat, to be again absorbed according to the wants of the 
body. Hence, the cells are nearly empty and collapsed, 
i animals scantily supplied with food. The air-cells of 
birds give additional buoyancy to their bodies ; and, there- 
fore, assist them in their flight. 
Il. Aprenpices. 
Having examined the structure of the different layers of 
the skin, we now proceed to consider the appendices with 
which it is furnished, and by which it is fitted for a variety 
of purposes in the animal economy. 
1. Hairs.—With the general appearance of hair, every 
one is familiarly acquainted ; but its peculiar structure and 
mode of growth, have seldom been attentively examined 
by the zoclogist, although they furnish important charac- 
ters for the discrimination of species. 
Hairs differ remarkably not only in their structure, but 
likewise in their situation. In some cases, they appear to be 
merely filamentous prolongations of the cuticle, and subject 
to. all its changes. This is obviously the case with the hair 
which covers the bodies of many caterpillars, and which se- 
parates along with the cuticle, when the animal is said to 
cast its skin. Such cuticular hairs are likewise found on 
many shells, such as the Helia rufescens, hispida and spi- 
nulosa, the Arca lactea, and Pectunculus pilosus. 
In true hair, the root is in the form of a bulb, taking its 
rise in the cellular web. Each bulb consists of two parts, 
an external, which is vascular, and from which the hair 
probably derives its nourishment; and an internal, which 
is membranous, and forms a tube or sheath to the hair, 
during its passage through the other layers of the skin. 
From this bulb, and enveloped by this membrane, the hair 
