The heart, from its structure and action, may justly be considered as a living or self-moving double forcing-pump, which is continually filled at one part and emptied at another. During one-third of the time of a complete action of the heart, the blood in tbe arteries is impelled onwards by the direct impulse of the ventricles at their systole. During the other two-thirds of the time, while the ventricle is inactive, the communication between its cavity and the great arteries is stopped by the closure of the semilunar valves, and the blood must, herefore, at this time be propelled by the elastic and other forces of the arteries them selves. But the heart continues to receive blood from the veins during a longer time than it gives out any of that fluid, for the auricles offer a resistance to the entrance of blood du ring only a space of less than a quarter of the time employed in a complete action of the heart, and the blood is continually impelled into the auricles as well as the ventricles du ring the whole time that these cavities are not contracted, although more blood enters the auricles immediately after their relaxation, and more. is propelled into the ventricles just be fore their contraction than in the rest of the time.
During the systole of the ventricles, while the stream of blood issues from their cavities into the first adjoining parts of the large arteries, the folds of the semilunar valves are laid close to the inner side of these vessels. As soon as the contractile force of the ventricles ceases, the free edges of the semilunar valves are brought towards the middle of the vessel, and applied firmly against one another so as to close the ventriculo-arterial orifices : this is effected by the pressure of the column of blood acted upon by the elastic coats of the arteries, assisted perhaps by the elasticity of the borders of the valves themselves and by the change of position consequent on dilatation of the ventricles.
During the systole of the ventricles, the auriculo-ventricular or tricuspid and mitral valves are closed, so as to prevent in a great measure regurgitation of the blood from the ventricles into the auricles. When the ven tricles are in the relaxed state, the valves are opened by the stream of blood flowing from the auricles. The circumstance that the free margins of the mitral and tricuspid valves are bound down to the inner walls of the ventricles by the tendinous cords at tached to the fleshy pillars, and that, by the contraction of these pillars, the free margins of the valves must be pulled further down into the ventricle than in the relaxed state, has occasioned to some a difficulty in under standing their action, and led them to suppose that the columnze carnem must necessarily be relaxed at the time of the ventricular systole, and that by contracting while the ventricle is its diastole, the fleshy pillars contribute to open the valves. The direct observation of the contraction of the column carne in the heart of an animal taken from the body, and an attentive observation of the structure of these valves, from which it appears that the tendinous cords passing to opposite flaps of the valves frequently come from the same columnze carne or point of attachment in the ventricular paries, sufficiently prove that these fleshy pillars actually contract at the same mo ment as the rest of the parietes of the ven tricles, and that their contraction, besides drawing the free margins of the valves down wards into the ventricles, must also tend to make them approach one another more nearly; and we are therefore. entitled to form the con
clusion, that, while the tendons serve to fix the valves, the action of the columnw carnem is to draw these down so as to allow the blood to pass behind them, and to press them to gether and close them in the same manner as the semilunar valves of the aorta and pulmo nary artery are shut.
The apparently greater facility of the en trance of blood into the heart at one time than at another, has given rise to the opinion enter tained by some physiologists that the dilatation of the heart is, like' the contraction, accom panied with the production of a new force, which draws the blood from the veins towards the heart. Some who regard muscular elon gation as a source of new power have gone so far as to suppose that this force is even greater than that accompanying contraction, but it is manifest that such a view is opposed by every thing we know of muscular action, which leads to the belief that the shortening of muscular fibre ought alone to be considered as an active, and the subsequent elongation as entirely a passive change. Others suppose the ventricles of the heart to dilate in consequence of elas ticity, in the same manlier as a bag of caout chouc does after being compressed with some degree of force. Attempts have even been made to measure the extent of the force pro duced during the dilatation of the ventricles, by endeavouring to ascertain the weight which is displaced by this motion of the heart. We would not wish to be understood to deny the possibility of the heart's exerting some slight force in this way during its dilatation, but it appears very clear that a measurement of the kind referred to must be so difficult as to be almost useless; indeed, it is very probable that some have mistaken the contraction for the dilatation, and we shall afterwards find that the power of suction, exerted by the heart on the blood, as measured by the force with which the veins are emptied, is very small indeed. It is clear that the blood driven on from behind by a propelling power, or flowing through parts which are pressed upon by neighbouring organs, must enter the heart more easily during the relaxation of the pa rietes of the ventricle than at any other period during the heart's action, so as to give rise to an appearance of suction, but direct expe riments make it sufficiently obvious that the force of impulsion from behind is almost the sole cause of the entrance of the blood from the trunks of the great veins into the cavities of the heart.