126 CIRCULATION OF THE BLOOD. 



tricuspid valves; the left auricle in like manner filling the left 

 ventricle, the return of the blood there being opposed by the 

 mitral valves ; and then the ventricles in their turn are propel- 

 ling the blood into either great artery, the reflux in each being 

 prevented by the sigmoid valves in its orifice. Either, conse- 

 quently, the blood must move on incessantly through the 

 lungs, and in like manner within the arteries of the body, or 

 stagnating and pent up, it must rupture the containing vessels, 

 or choke the heart by over distension, as I have shown it to 

 do in the vivisection of a snake, described in my book on the 

 Motion of the Blood. To resolve this doubt I shall relate two 

 experiments among many others, the first of which, indeed, I 

 have already adduced, and which show with singular clearness 

 that the blood flows incessantly and with great force and in 

 ample abundance in the veins towards the heart. The inter- 

 nal jugular vein of a live fallow deer having been exposed, 

 (many of the nobility and his most serene majesty the king, my 

 master, being present,) was divided ; but a few drops of blood 

 were observed to escape from the lower orifice rising up from 

 under the clavicle ; whilst from the superior orifice of the vein 

 and coming down from the head, a round torrent of blood 

 gushed forth. You may observe the same fact any day in 

 practising phlebotomy: if with a finger you compress the vein 

 a little below the orifice, the flow of blood is immediately 

 arrested ; but the pressure being removed, forthwith the flow 

 returns as before. 



From any long vein of the forearm get rid of the blood as 

 much as possible by holding the hand aloft and pressing the 

 blood towards the trunk, you will perceive the vein collapsed 

 and leaving, as it were, in a furrow of the skin ; but now compress 

 the vein with the point of a finger, and you will immediately 

 perceive all that part of it which is towards the hand, to enlarge 

 and to become distended with the blood that is coming from the 

 hand. How comes it when the breath is held and the lungs 

 thereby compressed, a large quantity of air having been taken in, 

 that the vessels of the chest are at the same time obstructed, the 

 blood driven into the face, and the eyes rendered red and suf- 

 fused? Why is it, as Aristotle asks in his problems, that all the 

 actions are more energetically performed when the breath is 

 held than when it is given? In like manner, when the frontal 



