352 EXTERNAL ANATOMY OF INSECTS. 
ranus, Sprengel, Audoin, Chabrier, Carus, and, above 
all, M. Cuvier in his celebrated Lectures on Comparative 
Anatomy, have considerably extended the boundaries 
of our knowledge in this department: and much of what 
I have to say to you in my letters on this subject, will be 
derived from these respectable sources. In the exterior 
anatomy of insects, I flatter myself that I shall be enabled 
to make some material additions to the discoveries of my 
predecessors; though few have occurred to me with re- 
spect to their internal organization. 
In treating of the anatomy of the vertebrate animals, 
it is usual, I believe, to consider, first, the skeleton and 
its integuments, whether of skin or muscle, and their 
accessories; and afterwards the organs of the different 
vital functions and of the senses. But in considering 
the anatomy of Insects, the difference before stated*, ob- 
servable between them and the sub-kingdom just men- 
tioned, as to their structure, renders it advisable to di- 
vide this subject into two parts—the first treating of 
their external anatomy, and the second of their znternal. 
—I shall begin by drawing up for you a Table of the 
Nomenclature of the parts of their external crust; its 
appendages and processes®, external or internal, ac- 
companied by definitions of them; and followed by such 
observations respecting them as the subject may seem to 
require for its more full elucidation. 
Anatomists have divided the human skeleton into three 
* See above, p. 43—. 
> There are certain processes which are a continuation of the in- 
ternal surface of the crust ; and serve, as well as the rest of it, for 
points of attachment to the muscles: these, though completely in- 
ternal, must be considered as parts of the external skeleton. 
