ACTION OF THE HEART. 121 



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 appear, 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 con- 

 tract, the fleshy columns, and especially those of them 

 called the musculi papittares, 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. 123). The whole force of the ven- 

 tricular contraction is thus directed to the propulsion of 

 the blood through their arterial orifices. During the time 

 which elapses between the end of one contraction of the 

 ventricles, and the commencement of another, the com- 

 munication 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 



