INTERNAL ANATOMY OF INSECTS. 3 
the Creator. Its pulpy substance is the viszble medium 
by which the governing principle? transmits its com- 
mands to the various organs of the body, and they move 
instantaneously—yet this appears to be but the conduct- 
or of some higher principle, which can be more imme- 
diately acted upon by the mind and by the will. This 
principle, however, whatever it be, whether we call it the 
nervous fluid, or the nervous power, has not been de- 
tected, and is known only by its effects. ‘The system of 
which we are speaking may therefore be deemed the 
foundation and root of the animal, the centre from which 
emanate all its powers and functions. 
Comparative anatomists have considered the nervous 
system of animals as formed upon ¢hree primary types, 
which may be called the molecular, the ganglionic, and 
cerebro-spinal®, ‘The first is where invisible nervous 
molecules are dispersed in a gelatinous body, the exist- 
ence of which has only been ascertained by the nervous 
irritability of such bodies, their fine sense of touch, their 
perceiving the movements of the waters in which they 
reside, and from their perfect sense of the degrees of 
light and heat4. Of this description are the infusory 
animals, the Polyp?, the star-fish and sea-urchins. ‘The 
nervous molecules in these are conjectured to constitute 
so many ganglions, or centers of sensation and vitality ©. 
The second, the ganglionic, is where the nervous system 
2 To ‘Hyepovixov. 
b See Hooper’s Medical Dictionary, under Nervous Fluid, and 
Mr. Sandwith’s useful Introduction to Anatomy and Physiology, 83. 
¢ N. Dict. d Hist. Nat. xvi. 305—. 
4 Cuv. Anat. Comp. ii. 362. Compare MacLeay Hor. Entomolog. 
215—. & N. Dict. d Hist. Nat. ubi supr. 
B2 
