146 



nate motions, which become fainter as the part gets cold. If you attempt 

 to check the diastole of the heart, the organ resists the hand which com- 

 presses it, and its cavities appear endowed with a power which Galen 

 termed pulsive; in virtue of which they dilate to receive the blood, and 

 not because they receive it. In that respect, the heart differs essentially 

 from the arteries, whose dilatation is occasioned by the presence of the 

 blood, whatever some physiologists may have said to the contrary. I 

 have repeated, but unsuccessfully, the famous experiment by which it is 

 attempted to be proved, that these vessels have the power of moving in- 

 dependently of the presence of the blood. An artery tied ar.d emptied 

 of blood, contracts between the two ligatures, and is no longer seen to 

 move in alternate contractions. 



LIII. The heart manifestly shortens itself, and the base approaches 

 towards the apex, during the systole or contraction of the ventricles. If 

 it became elongated, as some anatomists have thought, the tricuspid and 

 miral valves would ie incapable of fulfilling the functions to which they 

 are destined, since the columnae earner, whose tendons are inserted in 

 the edges of these valves, would keep them applied to the parietes of the 

 ventricles. The pulsations which are felt, in the interval between the 

 cartilages of the fifth and sixth true ribs, are occasioned by the apex of 

 the heart which strikes against the parietes of the chest. In the expla- 

 nation of this phenomenon, it is not necessary to admit the elongation of 

 the heart during its systole; it is sufficient to consider, that the base of 

 the heart, in which the auricles are situated, rests against the vertebral 

 column; that these two cavities, by dilating at the same time, and by their 

 inability to move the vertebrae, before which they are situated, displace 

 the heart, and thrust it downwards and forwards. This motion depends, 

 likewise, on the effort which the blood sent into the aorta makes, to 

 bring to a straight line, the curvature of that artery, which re-acts and 

 carries downwards and forwards the whole mass of the heart, as it were, 

 suspended to it. 



The quantity of blood which each contraction of the ventricles sends 

 into the aorta and pulmonary artery, most probably, does not exceed two 

 ounces in each of these vessels. The force with which the heart acts on 

 the blood which it sends into them, is but imperfectly known, however 

 numerous the calculations by which it has been endeavoured to solve 

 this physiological problem. In fact, from Keil, who estimates at a few 

 ounces only, the force of the heart, to Borelli, who makes it amount to 

 one hundred and eighty thousand pounds, we have the calculation of Mi- 

 chelot, Jurine, Robinson, Morgan, Hales, Sauvages, Cheselden, Sec.; but 

 as Vicq-d'Azyr observes, not one of these calculations is without some 

 error, either anatomical or arithmetical: hence we may conclude with 

 Haller, that the force of the heart is great, but that it is, perhaps, im- 

 possible to estimate it with mathematical precision. If we open the 

 chest of a living animal, and make a puncture in his heart, and introduce 

 a finger into the wound, a considerable pressure is felt, during the con- 

 traction of the ventricles*. 



* The difficulty of determining the exact degree of power exerted by the heart, is 

 strikingly illustrated by the total disagreement in the estimates of different writers. 

 Before we engage in an} calculations respecting the matter, the following data should 

 be clearly established. 



1. The quantity of blood expelled from each ventricle at every contraction* 



