20 INTERNAL ANATOMY OF INSECTS. 
denied the existence of a drain in insects, regarding it 
merely as the first ganglion of the spine. 
Cuvier and other modern physiologists, from the gan- 
glionic structure of this organ, are of opinion that it is 
not the analogue of the cerebro-spinal system of verte- 
brate animals, but rather of their great sympathetic nerves. 
Indeed, considering solely the external structure of the 
nervous system of insects, a great resemblance strikes us 
between it and these nerves; for besides its general gan- 
glionic structure, there is also in them an upper ganglion 
in the neck, seemingly corresponding with what we have 
named the brain of insects, from which the nervous chord 
dips to the lower part of the neck, where it forms a se- 
cond ganglion, which appears to correspond with what 
we have considered as their second ganglion*. We may 
observe, however, that at least in one respect there is 
even an external resemblance between the brain of in- 
sects and that of vertebrate animals :—it most commonly 
consists, as has been stated, like them, of two lobes, often 
very distinct ; a circumstance which not unfrequently di- 
stinguishes the other ganglions>, and is not borrowed 
from the ganglions of the great sympathetics. With re- 
spect to the internal structure of the ganglions and spinal 
marrow of insects, we know little to build any theory 
upon, except that the internal substance of the former is 
filled with air-vessels; at least so Lyonnet, as has been 
already observed, found in the Cossws, while only the 
tunics of the latter are covered by them,—a circumstance 
which I shall again have occasion to advert to. Taking 
-* Cuv. Anat. Comp. ii. 233—. These are named “ the upper and 
lower cervical ganglions.”’ 
» Lyonnet Anat. ¢. ix. x. Plate XXI. Fic. 1. a. c. 
