OF INSECTS. 
151 
to escape, and finds no other passage but that made 
for it in front by the yielding of the valvules which 
separate it from the second cell. Into this, therefore, 
it passes ; but, at the same time, the preceding dilates, 
and the blood contained in the intestinal cavity presses 
against the lateral valvules, which yield and permit it 
thus to enter by the openings which they protected. 
The same process is repeated by the second cell, then 
by the third, %nd so on ; the blood thus traverses 
them all by regular jerks, without any of them being 
ever left completely empty/ 1 * 
This process will be 
better understood from 
an inspection of the an- 
nexed figures, after a 
drawing by Mr. Bower- 
bank, with his accom- 
panying explanation. + 
Fig. 1st, a , a, represents 
two chambers of the 
dorsal vessel in their 
greatest state of col- c 
lapse, when the point 
of the lower valve is 
seen closely compressed 
within the upper one. 
At the commencement 
of the expansion, the 
blood is seen flowing in 
from the lateral aper- 
* Lacord. Intro. II. 72. -j- See Entomol. Mag. I. PI. II. 240. 
Fig.i 
