That the passage of the blood along the arteries is from the heart towards the extremities, is well illustrated by the effect of ligatures placed upon them ; for in this case we find that the part of the artery between the ligature and the heart is distended, while the more remote part becomes partially emptied of blood. The same circumstance is ex emplified in wounds of the arteries, where the upper part of the vessel is seen to send out jets of blood, correspond ing with the action of the heart, while only a slight dis charge issues from the lower part of the divided vessel. The reverse of all this happens from ligatures, or wounds of the vein's : here the distention, or the stream of blood, occurs in the part of the vein more remote from the heart, while the part contiguous to the heart becomes relaxed and emptied.
The curious operation called transfusion, proves the course of the blood to be, as we have described it, from the arteries into the veins. In this process, which seems to have been first practised by Lower, the artery of one ani mal is connected by a tube with the vein of another, when we find that the first animal is emptied of blood, while the second becomes preternaturally distended with it. By per mitting the blood of the second animal to escape through an opening from its veins, the greatest part of the blood in the body may, in this way, be changed; and at a time when diseases were thought to depend upon some morbid qua lities residing in the fluids, the operation of transfusion was held up as a most important method of restoring the health; and, repugnant as it now appears to our feelings, the blood of lambs or calves was actually transmitted into the vessels of persons labouring under different diseases, or debilitated by old age.
Besides these proofs of the direction of the blood through the sanguiferous system, the observations that were made by the microscope may be mentioned among the points that were insisted upon by Harvey and his contemporaries. Alalpighi taught us the method of rendering the circula tion visible to the eye, by applying this instrument to the web of a frog's foot, or other transparent membrane in the cold-blooded animals, when we have the interesting ap pearance presented to us, of the arteries projecting the blood in successive waves, which is returned in a uniform stream by the veins. But although the two kinds of ves sels may be distinctly perceived, as well as the difference in the motion of their contents, we are not able to trace their connexion with each other, nor the relation which they bear to the other parts of the sanguiferous system.
The regularity with which these actions succeed each other was a circumstance that excited great surprise among the earlier physiologists, and gave rise to many specula tions. Bellini, who was a strenuous advocate for the ma thematical doctrines, conceived that the auricles and ven tricles were antagonists to oach other. Baglivi, a very in genious physiologist, whose premature death was a real loss to science, accounted for the regularity, of the action by an alternation of pressure on the brain and nerves, in consequence of the alternate distention and relaxation of the ventricles, and an opinion something similar to this was maintained by Boerhaave. But these, and many other refined theories, were all discarded by Il aller, who reduced the action of the heart to the simple effect of a stimulus, acting at successive intervals, on a contractile organ, the mechanism of which is so adjusted that each part, when re laxed, receives the stimulating power from the neighbour ing part.
A calculation has been made by Blumenbach of the length of the time which the whole mass of blood requires to pass through the entire circulation, a point which ad mits of being reduced to numbers with tolerable accuracy. Supposing the whole mass of blood to be thirty-three lbs. that the heart contracts seventy-five times in a minute, and expels two ounces at each contraction, the blood will com plete the circuit in two minutes and thirty-six seconds, and will be carried through the vessels nearly twenty-three times in an hour.
To describe the minute anatomy of the heart and blood vessels would be foreign to the object of this article; but there is one fact, which we may notice as a remarkable, although not an unexampled instance, of the bigotted at tachment of many of the moderns to Galen. Vesalius, the great restorer of anatomy, perceived that the descrip tion of the vessels connected with the heart, which was left by Galen, did not quite correspond with what he found in the human subject, while it exactly resembled the struc ture of this part in apes and monkeys ; hence he naturally concluded that Galen had employed these animals in his dissections. A learned French anatomist, Du Bois, a warm advocate for the ancients, and a violent antagonist of Ve salius, in his zeal to support the honour of his master, se riously maintained that the human form had undergone a change in its structure since the age of Galen, and that formerly the vessels were distributed,as he described them.