145 



cal 



are in a state of relaxation, and the blood would be forced back into 

 the ventricle, if the sigmoid valves, by rising suddenly, did not prevent 

 it. Supported on a kind of floor formed by three valves, which lie across 

 the calibre of the vessel, the blood pervades the tissue of the lungs, and 

 flows along the divisions of the pulmonary vessels; from the arteries it 

 passes into the veins, and these, four in number, convey it into the left 

 auricle. This auricle, stimulated by the presence of the blood, contracts 

 in the same manner as the right, part of the blood flows back into the 

 lungs, but the greatest part enters the last ventricle, which sends it along 

 the aorta, to every part of the body, whence it returns to the heart by the 

 veins. The return of the blood into the left auricle, is prevented by the 

 mitral valve which is similar to the t-ricuspid, except that its loose edge is 

 divided only into two divisions. As soon as the blood has reached the 

 aorta, this vessel contracts, its sigmoid valves fall, and the blood is sent 

 to every part of the body which is v supplied by some of the innumerable 

 branches of that great artery. 



In a natural state, the circulation is not carried on as has been just 

 stated; and, we have supposed this successive action of the four cavities 

 of the heart, only to render more intelligibler'the mechanism of the circu- 

 lation in that organ. If we Jay bare the heart in a living animal, we ob- 

 serve, that the two auricles contract at the same time, that the contrac- 

 tion of the ventricle is likewise simultaneous, so that while the auricles 

 are contracting, to expel the blood which fills them, the ventricles are 

 dilating to receive it. This successive contraction of the auricles and 

 ventricles is readily explained, by the alternate application of the stimu- 

 lus which determines the action of these cavities. The blood which the 

 veins bring into the auricles, does not excite their contraction, till a suf- 

 ficient quantity has been collected. While this accumulation is taking 

 place, they yield, and the resistence which is felt on touching them, dur- 

 ing their diastole, depends, almost entirely, on the presence of the blood 

 which separates and supports their parietes. The same applies to the 

 ventricles; they cannot contract, until a sufficient quantity of blood is 

 collected within them; that there remains some blood in these cavities, 

 (for they are never completely emptied,) is no objection to the theory; 

 since this small quantity is not sufficient to bring on contraction of the 

 heart, and is not worth taking into account. 



If I am asked, why the four cavities of the heart do not all contract at 

 once, I answer, that it is easier to assign the final than the proximate 

 cause. If the contraction of these cavities had been simultaneous, in- 

 stead of being successive, it is evident that the. auricles could not have 

 emptied themselves into the ventricles. The alternate action is more- 

 over absolutely necessary, as the heart, as well as the other organs, 

 is unable to keep up a perpetual action; the principle of its motion, 

 which is soon exhausted, being incapable of restoring itself, except dur- 

 ing rest. But, as was observed at the beginning of this work, in speak- 

 ing of the vital power and functions, the alternations of action and repose 

 in organs which, like the heart, perform functions essential to life, must 

 be extremely short in their duration, and at very close intervals. 



The cavities of the heart, however, are not entirely passive during 

 dilatation, and the action of that organ does not wholly depend on the 

 excitement of the blood on its parietes, since the heart after it has been 

 torn from the body of a living animal, palpitates, its cavities contract 

 and dilate, though quite emptied of blood, and appear agitated by alter- 



