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 (1) 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 
