Now let us turn to some delineations of the movement of the blood made by Harvey himself. I have found no evidence that he knew the venous flow to be promoted by the aspiration of the chest; but he knew well the effect of the muscular movements of the body upon that flow. Of course he had a perfect grasp of the fundamental truth that the main cause of the venous return is the forcible emptying of the ventricles into the arteries. He says to Riolanus:—
"Among these things should be noted the force and violence and rapid vehemence which we perceive by touch and sight in the heart and greater arteries; and the systole and diastole of the pulse in the larger and warmer animals I do not affirm to be the same in all the vessels which contain blood, nor in all blood-containing animals; but to be such and so ample in all that as a result thereof a streaming and an accelerated course of the blood through the small arteries, the porosities of the parts, and the branches of all the veins are necessarily brought about; and as a result thereof a circulation....[252] In the case of the arteries, over and above the shock, pulse, or vibration of the blood (which is not equally perceptible in all), a continual flow and movement thence take place until the blood returns to the point whence it started first, namely, the right auricle."[253]
The Title-page of William Harvey's Exercitatio Anatomica de Motu Cordis et Sanguinis in Animalibus, Frankfort, 1628.
With the calm quantitative account which a reader of Hales' "Statical Essays" will find given by that clergyman of his epoch-making physical experiments upon the blood-pressure, it is interesting to compare the following vivid qualitative recital of inferences made from surgical observations by his great predecessor. Harvey says:—
"Moreover, whoever shall have seen and thought upon the amount of difficulty and exertion with which the blood is stanched by compression, ligatures, or various appliances, when it leaps impetuously out of a petty artery, even the smallest, which has been cut or torn in two; and shall have seen or thought upon the amount of force with which the blood, as though thrown out from a syringe, flings off and drives before it the whole of the appliances, or traverses them—that man will hardly believe it probable, I think, that any of the blood can pass backward against so great an impulse and influx of the entering blood, unless from a point whence it is driven back with equal force."[254]
Harvey rightly discountenanced the ancient idea of direct anastomoses between the mouths of veins and the mouths of arteries, as opposed to fine and multiplied communications. In some situations, however, he admitted that ampler communications exist comparable to such anastomoses; and it throws light upon his state of mind as to the movement of the blood that, despite his recognition of the very forcible exit of the blood from the arteries, he suggested in his old age that in the cases aforesaid regurgitation from vein to artery is guarded against by a valvular arrangement, the terminal part of the artery traversing the wall of the vein obliquely, as the ureter traverses the wall of the bladder and as the biliary duct traverses the wall of the duodenum.[255] We should not forget that in his day the capillary vessels, the existence of the corpuscles, and the chemistry of the blood were still unknown; so that the passage into the veins of the mysterious hot vital liquid through the "porosities" of the parts might naturally present itself to his mind in a way very strange to us. He tells us this:—
"The blood does not take its course through the looser texture of flesh and parenchyma in the same way as through the more compact consistency of tendinous parts. Indeed, the thinner and purer and more spirituous part passes through more quickly; the thicker, more earthy, ill-composed[256] part tarries longer and is rejected."[257]