90 LECfURfi iX. 
or down the back a kind of pulpy cord, analogous 
-io the spinal marrow, and which ramifies into 
branches or divisions, which are conducted to the 
different parts of the body, and form the nervous 
: system in insects. 
The Eyes in insects differ in the different tribes, 
; but by far the greater part of insects are furnished 
with eyes apparently two in number, and situated 
on each side the head. The outward surface or 
coat of these eyes is composed of a prodigious 
number of minute hexagonal convexities, like so 
many convex lenses or glasses, but the exact man- 
ner in which vision is performed by these organs 
is not perhaps exactly ascertained. Some have 
supposed each of the hexagonal divisions or lenses 
to operate as a real and separate eye, and that the 
optic nerves are expanded into a separate retina 
or coat at the bottom of each. The head of a 
common Dragon-Fly or Libellula is furnished with 
no less than twenty-five thousand of these dimi- 
nutive lenses. That they are really convex exter- 
nally is certain, but that they are also convex on 
the internal or opposite part, though affirmed by 
many, may be doubted 3 and it is perhaps more 
probable that each is in reality what opticianf? 
