Harvey next takes up the consideration of the veins, and, after showing that they permit a flow of the contained blood in only one direction,—viz., that from the extremities toward the heart,—he calls attention to certain experiences which he has had: (1) When a cord is tied lightly around a limb the flow of blood is arrested only in the veins, because these vessels are located near the surface of the skin; but, if the cord is tied more tightly, the flow of blood is also arrested in the arteries, which lie at a relatively great depth. (2) When a vein is tied the resulting distension manifests itself only below (i.e., on the distal side of) the ligature; whereas, when an artery is similarly tied, the distension takes place above (i.e., on the proximal side of) the ligature. It is therefore plain that in the veins the blood flows from the individual parts toward the heart, but that in the arteries the flow is in the reverse direction—i.e., from the heart toward the individual parts. “If one reflects upon the nature of the movement of the blood,” says Flourens, “one will promptly realize how speedy it is. Scarcely has the blood entered the heart before it is hurried into the arteries; and then from these vessels it passes in an instant into the veins, from which, with almost equal speed, it finally travels back to the heart again. It is this never-ending movement from one channel into another, and then eventually back to the starting-point, which constitutes the circulation of the blood.... Modern physiology dates from the discovery of the circulation of the blood. Up to the time of this discovery physiologists followed the ancients; they did not dare to walk alone. Harvey had discovered the most beautiful phenomenon in the animal economy.... From this time forward, instead of swearing by Galen and by Aristotle, one had to swear by Harvey!”
Despite the great care which Harvey took to back up his scheme of the circulation of the blood with unimpeachable proofs of its correctness, he was obliged to pass through the same sort of experience as that to which Vesalius and scores of other pioneers in the field of scientific inquiry had been subjected. Two hostile forces stood constantly ready, during that fruitful period of the Renaissance, to attack with merciless bitterness all those who ventured to add new facts to our stock of knowledge in the domain of medicine. On the one side were the many men of small calibre, men filled with jealousy over the successes gained by co-workers in the same field; and on the other was marshaled the host of those who honestly believed that all medical wisdom ended with Galen. Before his death, however (hardly thirty years later), Harvey had the satisfaction of witnessing the almost unanimous acceptance of his dogma concerning the circulation of the blood. Louis the Fourteenth, King of France at this period, was so appreciative of the importance of Harvey’s discoveries that he appointed Dionis, the distinguished French anatomist, to demonstrate to the students of the Medical School of the Jardin des Plantes at Paris the circulation of the blood and other recent discoveries. Descartes (1596–1650), the celebrated French philosopher, paid an even greater compliment to the high character of the work accomplished by Harvey. His words, as quoted by Flourens, are as follows:—
If I am asked why the supply of venous blood does not become exhausted in flowing thus unceasingly into the heart, and why the arteries—since all the blood that passes through the heart must travel along these vessels—do not become filled to overflowing, I can see no good reason why I should not give to this question the very same answer that William Harvey, an English physician, to whom praise is due for having taught ..., has already given. [Then follows the text of Harvey’s reply.]
Our readers have doubtless noted the fact that, while Harvey, as I have endeavored to show in the preceding account, has clearly established his right to be considered the discoverer of the circulation of the blood in all its most essential features, his scheme fails to furnish any information concerning the composition of the blood and the manner in which it is built up into a life-giving fluid. In the minds of some this may seem to be an omission. A moment’s reflection, however, will satisfy any reasonable person that questions of this nature do not form a legitimate part of the problem which Harvey was engaged in solving, and that they therefore should receive separate consideration. Thus, for example, Harvey’s scheme fails to furnish satisfactory information concerning those portions of the circuit where the blood is obliged to travel through a system of communicating capillary channels, as happens in the lungs and in the tissues generally throughout the body. But Harvey had no means at his command for investigating a question of this nature. Capillary blood-vessels are invisible to the naked eye, and may be studied only with the aid of a microscope; but this instrument was not available until long after the time (1605–1616) when Harvey was engaged in carrying out his investigations into the circulation of the blood.
Other Discoveries Relating to the Vascular System.—To Vesalius is due the credit of having discovered the fact that anastomoses exist between the carotids and the vertebral arteries, thus explaining how a man may continue to live even after both carotids have been severed or ligated. His great rival, Fallopius, described these anastomoses in the most detailed manner, and he noted the further fact that an anastomosis with the basilar artery exists.
By the end of the sixteenth century a certain amount of progress had been made toward a correct knowledge of the lymphatics. Bartholomaeus Eustachius, for example, discovered the existence (in horses) of the thoracic duct, but he supposed it to be a vein. His description of this vessel reads as follows:—
In these animals there is a large vessel which extends downward from the inner aspect of the clavicular vein (= left subclavian vein). At the point where it joins the vein it is closed by means of a semicircular valve. This vessel is of a whitish color and it contains a scanty watery fluid. Not far from its starting-point it divides into two branches which very soon, however, join together again, and then, as a single trunk from which no further branches are given off, it passes down along the left side of the spinal column, penetrates the diaphragm, spreads itself out over the aorta, and ends in a manner unknown to me.
About one hundred years later (1647), Jean Pecquet of Dieppe, France, professor in the Medical School of Montpellier, rediscovered (in a dog) this same duct, with its tributary chyle ducts and also its point of entrance into the left subclavian vein; and, as he had rightly interpreted its nature, anatomists by common agreement accorded him the rights of discoverer.
At a still earlier date (1622) Caspar Aselli of Cremona, Northern Italy, professor in the Medical School of Pavia, discovered the chyle ducts. This discovery was made under the following circumstances, which reveal the fact that good luck sometimes plays an important part in the work of the searcher after truth in the departments of anatomy and physiology:—
Aselli was studying the distribution of the recurrent nerves and the movements of the diaphragm in a well-nourished living dog, when his attention was drawn to the presence of a large number of delicate white threads coursing as it were over the surface of the mesentery. Following the accidental injuring of one of these threads there escaped from the wounded structure quite a large quantity of chyle. Aselli, who instantly appreciated the full significance of what had happened, exclaimed, in the presence of the bystanders, “Eureka!” At the time he supposed that these chyle vessels terminated in the liver and contributed in some manner to the elaboration of the blood (in harmony with Galen’s universally accepted theory of sanguification); but later, after he had carried out a carefully conducted series of experiments, he was able to rectify this erroneous belief. (Haeser.)