PHYSIOLOGY OF THE DORSAL VESSEL. 653 



posterior chambers, which are narrow and of small capacity. 

 As soon, however, as the blood begins to move forwards in an 

 axial stream, it is not difficult to see that this stream will draw 

 in blood through the ostia. The more posterior chamber, by 

 its contraction, sets up an axial stream in the chambers in 

 front ; each chamber in turn becomes distended by the con- 

 traction of the chamber behind it and the accompanying in- 

 flow by the ostia, and each chamber contracts as soon as it is 

 fully distended. 



The only difficulty to my mind is to account for the inflow of 

 blood into the posterior chamber, and this is probably brought 

 about by its more intimate connection with the pericardial 

 wall, so that the expansion of the pericardial cavity also fills 

 the posterior heart chamber. The structure of the dorsal 

 vessel in the Blow-fly imago is entirely consistent with the 

 above theory. 



I conclude, therefore, that (i) the pericardial chamber and 

 the posterior part of the dorsal vessel itself are filled by the 

 descent of the pericardial septum, which is due to the contrac- 

 tion of the alar muscles. 



(2) When the pericardial sinus is fully distended, a contrac- 

 tion of the posterior chamber of the dorsal vessel occurs 

 which sets up an axial stream through the whole length of the 

 vessel, but chiefly affects the second chamber. 



(3) This axial stream causes an inflow at the ostia, which 

 increases with the velocity of the axial stream. In this action 

 the dorsal vessel acts like an injector. 



My observations on the dorsal vessel in living insects also 

 show that 



(4) As soon as the second chamber is distended it contracts, 

 and so increases the volume and velocity of the axial stream. 



(5) Each chamber contracts in succession, but only a very 

 short time after the one behind it. 



(6) The period of rest in each chamber is long in comparison 

 with the period of active contraction, so that all the chambers 

 are emptied before the pericardial muscles again contract. 



(7) The contraction of the pericardial muscles immediately 



