Harvey rightly denied that the arteries possessed any pulsific power of their own, and maintained that their pulse is owing solely to the sudden distension of their walls by the blood thrown into them at each contraction of the ventricles. But the remission which succeeds the pulse was regarded by him as caused simply by collapse of the walls of the arteries due to elastic reaction. Knowing nothing of the muscular coat of the arteries, he was unaware of the fact that the elastic reaction of the arteries, after their distension, is aided by the tonic contractility of their walls; the two forces, physical and vital, acting in concert with each other—the former converting the intermittent flow from the heart into an even stream in the capillaries and veins; the latter, through the vaso-motor system, regulating the flow of blood to particular parts in order to meet changing requirements.
It is somewhat surprising to find that such an accurate observer as Harvey should have failed to recognize the significance and importance of the system of lacteal vessels. But such was the case. Eustachius, in the sixteenth century, had discovered the thoracic duct in the horse, although he seems to have thought that it was peculiar to that animal. Aselli, while dissecting the body of a dog in 1622, accidentally discovered the lacteals, and thought at first that they were nerves; but upon puncturing one of them, and seeing the milky fluid which escaped, found them to be vessels. He, however, failed to trace them to the thoracic duct, and believed them to terminate in the liver. Pecquet of Dieppe followed them from the intestines to the mesenteric glands, and from these into a common sac or reservoir, which he designated receptaculum chyli, and thence to their entry by a single slender conduit into the venous system at the junction of the jugular and subclavian veins. The existence of the lacteals had not entirely escaped Harvey, however. He had himself noticed them in the course of his dissections before Aselli's book was published, but "for various reasons" could not bring himself to believe that they contained chyle. The smallness of the thoracic duct seemed to him a difficulty, and as it was a demonstrated fact that the gastric veins were largely absorptive, the lacteals appeared to him superfluous. He is not "obstinately wedded to his own opinion," and does not doubt "but that many things, now hidden in the well of Democritus, will by-and-by be drawn up into day by the ceaseless industry of a coming age."
Late in the author's life, as we have seen, the work on the "Generation of Animals" appeared; but neither physiological nor microscopical science was sufficiently advanced to admit of the production of an enduring work on a subject necessarily so abstruse as that of generation. It was impossible, however, for so shrewd and able an investigator as Harvey to work at a subject even as difficult as this without leaving the impress of his original genius. He first announced the general truth, "Omne animal ex ovo," and clearly proved that the essential part of the egg, that in which the reproductive processes begin, was not the chalazæ, but the cicatricula. This Fabricius had looked upon as a blemish, a scar left by a broken peduncle. Harvey described this little cicatricula as expanding under the influence of incubation into a wider structure, which he called the eye of the egg, and at the same time separating into a clear and transparent part, in which later on, according to him, there appeared, as the first rudiment of the embryo, the heart, or punctum saliens, together with the blood-vessels. He was clearly of opinion that the embryo arose by successive formation of parts out of the homogeneous and nearly liquid mass. This was the doctrine of epigenesis, which, notwithstanding its temporary overthrow by the erroneous theory of evolution,[20] is, with modifications, the doctrine now held.
Of Harvey's scholarship and culture we are not left in ignorance. Bishop Pearson, writing about seven years after the doctor's death, and Aubrey[21] have told us of his appreciation of the works of Aristotle, and in his own writings he refers more frequently to the Stagirite than to any other individual. Sir William Temple[22] has also put it on record that the famous Dr. Harvey was a great admirer of Virgil, whose works were frequently in his hands. His store of individual knowledge must have been great; and he seems never to have flagged in his anxiety to learn more. He made himself master of Oughtred's "Clavis Mathematica" in his old age, according to Aubrey, who found him "perusing it and working problems not long before he dyed."
Nor should it be forgotten that this illustrious physiologist and scholar was also the first English comparative anatomist. Of his knowledge of the lower animals he makes frequent use, and he says (in his work on the heart), "Had anatomists only been as conversant with the dissection of the lower animals as they are with that of the human body, many matters that have hitherto kept them in a perplexity of doubt, would, in my opinion, have met them freed from every kind of difficulty." Aubrey says that Harvey often told him "that of all the losses he sustained, no grief was so crucifying to him as the loss of his papers (containing notes of his dissections of the frog, toad, and other animals), which, together with his goods in his lodgings at Whitehall, were plundered at the beginning of the rebellion."
FOOTNOTES:
[20] According to the theory of evolution, the egg contained from the first an excessively minute, but complete animal, and the changes which took place during incubation consisted not in a formation of parts, but in a growth, i.e. in an expansion of the already existing embryo (see p. [40]).
[21] See p. lxxxii. of "Life," by Dr. Willis.
[22] "Miscellanies:" Part II. on Poetry, p. 314.