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 recommence. 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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