654 CIRCULATION OF THE BLOOD AND DORSAL VESSEL. 
precedes the contraction of the posterior chamber of the dorsal 
vessel. After the contraction of its anterior chamber, a period 
of rest sets in, the duration of which exceeds the whole period 
of contraction of the pericardium and dorsal vessel. 
Verloren [811] states that if the number of pulsations is 
taken as 60 in a minute, there is a period of active contrac- 
tion, occupying ‘15, and a period of maintained contraction of 
*25". These periods occupy, therefore, *4”, and constitute the 
systole. 
According to the same author, the period of diastole occupies 
-6”, which he divides into a period of active dilatation occupy- 
ing ‘2”,and a period of distension occupying *4”. The latter 
division is, in my opinion, incorrect, and Verloren’s error arose 
from his not having sufficiently separated the movements of 
the dorsal vessel from the flow of blood into the pericardium, 
The distension of the pericardial sinus is apparently due to two 
factors: (1) the flow of the blood into it, resulting from the 
contraction of the ventricular cavities, which drives the blood 
into it through all the venous sinuses. This distension of the 
pericardial space commences with the contraction of the 
dorsal vessel itself, and (2) the contraction of the alar muscles 
of the pericardial septum, which completes the distension of 
the pericardium, and initiates the contraction of the dorsal 
vessel. 
Verloren makes the pause follow the diastole, and precede 
the systole. It can easily be seen in the Blow-fly and in 
Eristalis that the dilatation of the pericardium due to the con- 
traction of the alar muscles immediately precedes the contrac- 
tion of the ventricles of the dorsal vessel, and that the pause 
succeeds both contractions, and does not intervene between 
them. 
The pulsations of the pericardium and dorsal vessel of the 
Blow-fly are visible for a short time through the transparent 
integument of the imago, immediately after its escape from 
the pupa, but, except that they are extremely rapid—about 200 
per minute—it is not possible to observe any details. More 
information can be obtained from the observation of the dorsal 
