652. CIRCULATION OF THE BLOOD 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 [818] 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 
Fic. 88.—A diagrammatic figure of the ostia. The narrow orifice between the ven- 
tricles, 2”, a’, 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 dorsal 
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 
