98 T H E C I R C U L A T I O N. 



ent ; especially in the warmer-blooded animals, in which the 

 movements in question are rapid. Nor is this for any other 

 reason than it is in a piece of machinery, in which, though one 

 wheel gives motion to another, yet all the wheels seem to move 

 simultaneously ; or in that mechanical contrivance which is 

 adapted to fire-arms, where the trigger being touched, down 

 comes the flint, strikes against the steel, elicits a spark which, 

 falling among the powder, it is ignited, upon which the flame 

 extends, enters the barrel, causes the explosion, propels the 

 ball, and the mark is attained all of which incidents by reason 

 of the celerity with which they happen, seem to take place in 

 the twinkling of an eye." The ventricles contract much more 

 slowly than the auricles, and in their contraction, probably 

 always thoroughly empty themselves, differing in this respect 

 from the auricles, in which, even after their complete contrac- 

 tion, a small quantity of blood remains. The form and position 

 of the fleshy columns on the internal walls of the ventricle ap- 

 pear, indeed, especially adapted to produce this obliteration of 

 their cavities during their contraction ; and the completeness 

 of the closure may often be observed on making a transverse 

 section of a heart shortly after death, in any case in which the 

 contraction of the rigor mortis is very marked. In such a case, 

 only a central fissure may be discernible to the eye in the place 

 of the cavity of each ventricle. 



At the same time that the walls of the ventricles contract, 

 the fleshy columns," and especially those of them called the 

 musculi papillares, contract also, and assist in bringing the 

 margins of the auriculo-ventricular valves into apposition, so 

 that they close the auriculo-ventricular openings, and prevent 

 the backward passage of the blood into the auricles (p. 100). 

 The whole force of the ventricular contraction is thus directed 

 to the propulsion of the blood through their arterial orifices. 

 During the time which elapses between the end of one con- 

 traction of the ventricles, and the commencement of another, 

 the communication between them and the great arteries the 

 aorta on the left side, the pulmonary artery on the right is 

 closed by the three semilunar valves situated at the orifice of 

 each vessel. But the force with which the current of blood is 

 propelled by the contraction of the ventricle separates these 

 valves from contact with each other, and presses them back 

 against the sides of the artery, making a free passage for the 

 stream of blood. Then, as soon as the ventricular contraction 

 ceases, the elastic walls of the distended artery recoil, and by 

 pressing the blood behind the valves, force them down towards 

 the centre of the vessel, and spread them out so as to close the 



