"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] After more than twenty years of the comment and criticism, called forth by his treatise of 1628, he said to Riolanus:-- "As to whether the moving blood be attracted, or impelled, or move itself by virtue of its own intrinsic nature, enough has been said in my little book on the motion of the heart and blood."[258] Yet about two years after the Exercises to Riolanus, Harvey, in writing a private letter, judged it necessary to accentuate, as follows, his denial that forces of attraction really play the part in physiology which the ancients had conceded to them. Speaking of the impulsion of the blood through the arteries, he says:--