PHYSIOLOGY OF THE DORSAL VESSEL. 651 



and second chamber. But besides the blood which enters 

 the second chamber in this way, the latter has received 

 blood, during its diastole, through its auriculo-ventricular 

 openings. The second chamber then contracts, and the blood 

 is prevented from passing backward by the inter-veritricular 

 valves between it and the posterior chamber ; it therefore 

 enters the third chamber, which dilates to receive it. The 

 blood thus passes successively from chamber to chamber until 

 it is poured out into the head, from which it returns to the 

 abdomen.' 



The above description of Straus Durckheim is not perhaps 

 precisely accurate, but it appears to me by no means as absurd 

 as Verloren pretends. 



JL. 



Verloren has evidently entirely misunderstood the description 

 given by the earlier writers, and although he has given a more 

 detailed description, it differs from theirs rather in detail than in 

 principle. He incorrectly represents the views of Straus Durck- 

 heim, and attributes absurd statements to him which he after- 

 wards shows are untenable. He admits that the dorsal 

 vessel is constricted at intervals, but denies that the segments 

 contract separately, and he describes the contraction as a 

 wave which commences at the posterior extremity of the 

 vessel and passes rapidly to its anterior extremity. The time 

 occupied, according to Verloren, in the passage of the con- 

 traction wave from the posterior to the anterior extremity 

 is short as compared with the whole period of systole and 

 diastole. So far, I think, Verloren is right, but the wave is 

 certainly delayed at the junctions of the successive chambers, 

 so that, although it travels forward rapidly, each chamber 

 contracts in succession, and the more posterior chambers are 

 fully contracted before those next in front commence contract- 

 ing. Verioren figures the ostia and their valves and describes 

 them with greater accuracy than Straus Durckheim, if Graber's 

 statements are correct, which is, I think, indisputable. 



Verloren also stated that the volume of blood in the dorsal 

 vessel and its velocity increase as it moves forward, since each 

 chamber receives a supply from the pericardium through its 



