so LECTURE ix. 



or down the back a kind of pulpy cord, analogous' 

 to the spinal marrow, and which ramifies into 

 branches or divisions, which are conducted to the 

 different parts of the body, arid 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 lense* 

 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 cm 

 the internal or opposite part, though affirmed by 

 many, may be doubted; and it is perhaps more 

 probable that each is in reality what opticians 



