654 CIRCULATIOX 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 [311] 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 : (i) 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. 



Yerloren makes the pause follow the diastole, and precede 

 the systole. It can easily be seen in the Blow-fly and in 

 Kristalis 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 

 JUT minute it is not possible to observe any details. More 

 information can be obtained from the observation of the dorsal 



