PHYSIOLOGY OF THE DORSAL VESSEL. 655 
vessel in a decapitated fly. The easiest method of exposing 
the dorsal vessel and pericardium is to cut off the posterior 
half of the abdomen, and then carefully remove the intestine 
and great air-sacs. This brings the pericardium into view, 
and its rhythmic contractions can be observed; very generally 
they are inhibited by the dissection of the parts, but soon 
recommence ; they are very rapid, from 200 to 250 per minute, 
and occasionally the pulsations of the dorsal vessel itself can 
be observed through the pericardium. After a time, some- 
times nearly half an hour, the pulsations of the pericardium 
become slowed down to about 60 in the minute, and then the 
dorsal vessel itself can sometimes be seen contracting, its 
systole immediately following the contraction of the pericardial 
muscles. 
In such a dissection the dorsal vessel is not intact—its 
posterior chambers have been cut away with the posterior part 
of the abdomen. It is much less easy to expose the whole 
length of the vessel, owing to the lateral curling of the 
dorsal portion of the abdominal wall after the removal of the 
ventral parts of the somites. I have been unable to obtain a 
satisfactory view of the dorsal vessel by this means in the 
living insect. 
Stimulation of the pericardium with a needle will inhibit its 
pulsations and those of the dorsal vessel for some minutes, 
after which the pulsations reeommence. At first these follow 
each other very slowly, not more than one a second; they 
gradually increase in frequency as the effect of the inhibitory 
influence passes off, and in a few minutes have regained the 
normal rhythm of about 200 per minute. 
I have frequently observed that when the action of the 
dorsal vessel begins to flag, destruction of the thoracic ganglion 
again accelerates its rhythm, showing that it exerts a constant 
inhibitory influence on the heart. 
Injury to the pericardium causes its pulsations to cease, 
and the dorsal vessel itself then ceases to pulsate; but if the 
dorsal vessel is removed with the pericardium, and the latter 
is entirely separated from it, rhythmic pulsations of the dorsal 
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