Letters 183 



the latter, never from the latter into the former ? For 

 what you say of the impulse of the blood through the 

 arteries does not fully solve the difficulty in the present 

 instance. For if the aorta be tied near the left ventricle 

 of the heart in a living animal, and all the blood 

 removed from the arteries, the veins are still seen full 

 of blood ; so that it neither moves back spontaneously 

 into the arteries, nor can it be repelled into these by 

 any force., whilst even in a dead animal it nevertheless 

 falls of its own accord through the finest pores of the 

 flesh and skin from superior into inferior parts. The 

 passage of the blood into the veins is, indeed, effected 

 by the impulse in question, and not by any dilatation 

 of these in the manner of bellows, by which the blood 

 is drawn towards them ; but there are no anastomoses 

 of the vessels by conjunction (per copulam), in the way 

 you mention, none where two vessels meeting are con- 

 joined by equal mouths. There is only an opening of 

 the artery into the vein, exactly in the same manner as 

 the ureter opens into the bladder (and the biliary duct 

 opens into the jejunum), by which, whilst the flow of 

 urine is perfectly free towards the bladder, all reflux 

 into the smaller conduits is effectually prevented ; the 

 fuller the bladder is, indeed, the more are the sides of 

 the ureters compressed, and the more effectual is all 

 ascent of urine in them prevented. Now, on this 

 hypothesis, it is easy to render a reason for the experi- 

 ment which I have already mentioned. I add further, 

 that I can in nowise admit such anastomoses as are 

 commonly imagined, inasmuch as the arteries being 

 always much smaller than the veins, it is impossible 

 that their sides can mutually conjoin in such a way as 

 will allow of their forming a common meatus ; it seems 

 matter of necessity that things which join in this way 

 should be of equal size. Lastly, these vessels having 

 made a certain circuit, must, at their terminations, 

 encounter one another ; they would not, as it happens, 

 proceed straight to the extremities of the body. And 



