But, although the attempts that were made to solve this problem were in themselves of little value, some of them may be considered as deserving of our attention, both in consequence of the celebrity which they at one time ac quired, and also because they afford us a good specimen of the nature of the reasoning which was employed by the most eminent physiologists of the age. Borelli, proceeding upon the gratuitous hypothesis which he had formed, that the power of the muscles is in proportion to their weight, es timated that the force of the heart was equal to the enor mous sum of 180,0001bs.
Keill perceived the extravagance of Borelli's estimate, and attempted to arrive at the truth by a more complicat ed process. He stated, that the force of the heart produces two effects ; it propels a quantity of blood from its car., ties, and communicates motion to the contents of the arteries. Ile first endeavoured to estimate the quantity of blood thrown out of the heart at each of its contractions ; and, by taking the diameter of the aorta, he could then cal culate the velocity with which it passes along this vessel. He found the quantity of blood to be about two ounces, the area of the great vessel to be about three-quarters of a square inch, while the actual contraction of the ventricle was conceived to occupy the 200th part of a minute. Hence it follows, that the blood sent into the aorta would compose a cylinder of eight inches in length, and be dri ven along with a velocity of 156 feet in a minute. But, in producing this velocity, the heart has to overcome all the resistances which the blood meets with in its passage along the the vessels ; and the next step was to determine their amount. For this purpose he opened a living animal, and laid bare the iliac artery, together with the corres ponding vein. He laid down the position, that all the blood which is transmitted by the artery must return by the vein in the same time, but with a diminished force; the arterial blood having the force which is necessary, to overcome all the resistances, and the venous blood having only that degree of force left which remains after all the resistances have been overcome. He then opened the artery, and received all the blood which passed through it in a given time; and he next received the blood which flowed through the vein during the same time, when he found that these quantities were to each other in the pro portion of to 3. From this he drew the conclusion, that the actual force of the heart is to the force given to the blood in the artery as 74- to :3, and consequently, that the real quantity of force exercised upon the blood would be sufficient to propel it 390 feet in a minute, had it no ob stacle to overcome. From this datum he estimates the force necessary to move a given column of blood with a known velocity in a given time ; and this he determines to be in man 52 ounces—a quantity about half a million of times less than the calculation of Borelli.
We may admit a certain share of ingenuity in the ex periment and reasoning of Keill ; but they are obviously incorrect in many particulars. In the first place, there is a great resistance offered to the blood at its first entrance into the aorta, which must have been overcome before it arrives at the artery on which he made his experiment. In the second place, the quantity of blood which flows from a divided vessel is no measure of what passes through it at other times, because blood will be sent from all the neighbouring parts to a point where the resistance is di minished. In the third place, it is by no means correct to suppose, that exactly what passes along an artery is returned by what we call the corresponding vein ; besides, that, in Keill's experiment, a greater quantity than ordi nary would pass off by the vein from the anastomoses which the veins have with each other. However, it is unnecessary to dwell longer upon this experiment, as the remarks which we have made are sufficient to show that it does not afford even an approximation to the truth.
The only other calculation of this kind which we shall notice, is that of Hales. He attempted to estimate the relative force of the arteries and veins, by inserting tubes into vessels near the heart, and observing the comparative height to which the blood was impelled into them. Although he found it to vary in different experi ments, yet it was always greater in the arteries than in the veins, upon the average as about 10 to 1. In order to ob tain the absolute force of the heart, Hales inserted tubes into the aorta, soon after it leaves the left ventricle ; and he found the column of blood projected into the tube to be of such a height, that, by comparing it with the cavity from which it proceeded, and by taking into account the time and the area of the vessel, the force of the heart would be about 50Ibs.
We may conceive that the estimate of Hales is nearer the truth than that of any of his predecessors; but still there are many points in which it is defective, even re garding the heart merely as a hydraulic machine; and when we consider contractility as a variable power, de pending upon a number of causes connected with life, which it is impossible to appreciate, we shall be convinc ed of the futility of all such calculations. Hales, however, was a man of a truly philosophical cast of mind, who was far from being blindly devoted to any set of opinions ; and whose experiments, considering the period in which he lived, exhibit a powerful understanding. But the genius of the age was so decidedly turned to the employment of mathematical reasoning in every department of philoso phy, that he was led to apply it to many parts, both of the animal and vegetable economy, which depend altogether upon different principles.