314 CIRCULATION OF BLOOD. L.ECT. XVII. 



other mammal, and we shall then see the alternate con- 

 traction and dilatation of the ventricles and auricles. When 

 the right ventricle dilates, the right auricle contracts, and 

 vice versa : the same phenomenon takes place in the other 

 side of the heart ; so that the movements of the homony- 

 mous parts take place simultaneously, and are alternate 

 with those of parts having a different name. At this mo- 

 ment of the contraction of one of these cavities, the blood 

 which it contains is expelled, and the direction which it 

 takes is regulated by the arrangement and action of the 

 valves placed at the orifice of this cavity. At the moment 

 of the contraction of the right auricle the blood is forced 

 towards the auriculo-ventricular orifice, and backward into 

 the vena cava. At this instant the ventricle dilates, the ori- 

 fice opens, and very nearly the whole of the blood rushes 

 into it ; a small quantity only regurgitating into the venae 

 cavae. To these movements succeed the dilatation of the 

 auricle, and the more energetic contraction of this ventricle. 

 The blood which filled the ventricle is expelled from this 

 cavity : two orifices present themselves, that of the pulmo- 

 nary artery, and the auriculo-ventricular orifice. The lat- 

 ter would allow it to return into the auricle, whose con- 

 traction has just ceased, but for the action of the tricuspid 

 valve, which opposes it, and whose structure is well 

 adapted for this purpose. The blood, pressed on by the 

 walls of the contracting ventricle, distends the membrane 

 forming this valve, which yields until it becomes perpen- 

 dicular to the axis of the ventricle, when the orifice becomes 

 almost completely closed. But the fleshy columns, by con- 

 tracting also, prevent the valve from turning over into the 

 auricle, and retain it at the orifice. On the same princi- 

 ple, valves in all our hydraulic machines are constructed. 

 The semi-lunar valves, placed at the orifice of the pulmo- 

 nary artery, on the contrary, yield to the impulse of the 



