92 INTERNAL ANATOMY Of INSECTS. 
lamina somewhat nearer to its abdominal than to its 
dorsal edge, and running through the greater part of its 
length suddenly turns and bends its course back towards 
the body, somewhat nearer to the dorsal than to the ab- 
dominal margin of the lamina. The channel thus formed 
in the midst of the granular substance is perfectly trans- 
parent, except where it is occupied by the blood-globules, 
or crossed by the bronchia?. The parietes of the channel 
are not strictly defined, nor formed by any thing like 
the coats of a vessel, the blood circulating through the 
granular Parenchyma ; a circumstance however which is 
not peculiar to this case, but also occurs generally in the 
first states of the circulation, as it presents itself for in- 
stance in the embryo of Fishes, and in the Jigura venosa 
of the incubated egg a . The blood-globules are elongated 
like a grain of wheat, considerably larger than those of 
the human blood, and float in a fluid which is invisible 
because of its transparency, but the existence of which is 
proved by the variations in the position of the globules 
in the current, sometimes following its direction, at others 
crossing it transversely, or more or less obliquely. 
When the animal is vigorous, the current is uninter- 
rupted, although its velocity is accelerated at regular in- 
tervals ; and that not only in the excurrent {arterial), but 
also in the recurrent (venous) part of its course through 
the lamina. When the animal becomes exhausted, or the 
laminae exsiccated, the circulation is interrupted, and in 
the same manner, as under the same circumstances, in 
the larvae of frogs and lizards; the disturbance displaying 
itself not merely by a cessation of the process, but also 
a This seems some confirmation of Dr. Virey's opinion, that in- 
sects in their first slates arc still a kind of fat us. Sec above, 
Vol. III. p. 61-. 
