S4 INTERNAY. ANATOMY OF INSECTS. 
spinal marrow. He likewise describes a large nerve as 
passing through it and becoming recurrent’. 
The fluid which this vessel contains is very abundant ; 
in the animal it appears colourless and transparent like 
water, but when collected in drops it becomes more or 
less yellow, and even orange”. Examined under the 
microscope it appears filled with a prodigious number of 
transparent globules, of incredible minuteness*<. When 
mixed with water, which it does readily, its globules lose 
all their transparency, and coagulate into small clammy 
masses. After evaporation it becomes hard, and cracks 
like gum, as blood does also. This gummy substance is 
so abundant, that the fluid contained in the dorsal vessel 
of the caterpillar of the Cossus yields a mass of it of the 
size of a grey pea‘. 
From the situation of this dorsal vessel, which is pre- 
cisely the same with that of the heart in Arachnida and 
the Branchiopod Crustacea, and from the systole and 
diastole which keep its fluid contents in constant motion, 
who can wonder that the physiologists who first disco- 
vered it, maintained that it was a true heart? And even 
now, our knowledge of this organ is so very circumscribed 
that, till insects have been more widely examined with 
this view, and its real functions are ascertained, it seems 
to savour of temerity to assert, that in no respect it can 
answer the purpose of a heart. Before I advert to those 
arguments that at present may be regarded as proving 
that it is not a heart, I will notice those upon which the 
upholders of the original opinion have founded their 
@ Lyonnet Anat. 413. > Ibid. 426. Cuv. Anat. Comp. iv. 419. 
“ Lyonnet says (426), “ au-dela de trois millions de fois plus petits 
quan grain de sable”! ! * Tbid. 
