f-52 CIRCULATION OF THE JiLOOD AND DORSAL VESSEL. 



own ostia, and the contraction of each accelerates the velocity 

 of the moving fluid. 



The figures of the valves given by Verloren suggest that the 

 flow of fluid into the dorsal vessel is due to the velocity of the 

 axial stream, which undoubtedly acts as an injector and draws 

 blood in through the ostia ; but he does not apparently regard 

 this as a factor in the circulation, or, if it occurred to him, he 

 does not mention it. 



Graber [313] attempted an explanation of the diastole, which 

 I am not sure I fully understand ; but he says, after describing 

 the passage of the blood through the narrow space between 



OS. 



Fie. 88. A diagrammatic figure of the ostin. The narrow orifice between the ven- 

 tricles, v", r', is supposed to act as an injector, drawing blood through the ostia, 

 os, in the direction of the arrows. 



the ostia : ' At the same time blood is sucked through the 

 ostia.' 



The dilatation of the dorsal vessel is by no means easy to 

 comprehend, lying as it does in the pericardial space. After 

 the blood has been expelled and the muscular walls relax, there 

 is no very apparent reason why the blood in the pericardial 

 space should pass into its interior. The filling of the dors;d 

 vessel cannot be due to pressure outside it, and it is not easy 

 to discover any effective mechanism by which it can actively 

 dilate. The fine fibres which connect the dorsal vessel with 

 the pericardium no doubt may assist by causing a small 

 quantity of blood to enter through the ostia, especially in the 



