APPENDICES OF THE SKIN. 87 



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, 

 in animals scantily supplied with food. The air-cells of 

 birds give additional buoyancy to their bodies ; and, there- 

 fore, assist them in their flight. 



II. APPENDICES. 



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 zoologist, 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 Helix rufescens^ hispida and spi- 

 nulosa, the Area lactca, and Pectimculus 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 



