CIRCULATION OF THE BLOOD. 163 



the veins is nowhere very great, but is greatest in the small veins, while 

 in the large veins toward the heart the pressure becomes negative, or, in 

 other words, when a vein is put in connection with a mercurial manometer 

 the mercury* will fall in the area furthest away from the vein and will rise 

 in the area nearest the vein, having a tendency to suck in rather than to 

 push forward. In the veins in the neck this tendency to suck in air is 

 especially marked, and is the cause of death in some operations in that 

 region. The amount of pressure in the brachial vein is said to support 

 9 mm. of mercury, whereas the pressure in the veins of the neck is about 

 equal to a negative pressure of 3 to 8 mm. 



The variations of venous pressure during systole and diastole of the 

 heart are very slight, and a distinct pulse is seldom seen in veins except 

 under very extraordinary circumstances. 



The formidable obstacle to the upward current of the blood in the 

 Teins of the trunk and extremities in the erect posture supposed to be pre- 

 sented by the gravitation of the blood, has no real existence, since the 

 pressure exercised by the column of blood in the arteries, will be always 

 sufficient to support a column of venous blood of the same height as itself: 

 the two columns mutually balancing each other. Indeed, so long as 

 both arteries and veins contain continuous columns of blood, the force of 

 gravitation, whatever be the position of the body, can have no power to 

 move or resist the motion of any part of the blood in any direction. The 

 lowest blood-vessels have, of course, to bear the greatest amount of pres- 

 sure; the pressure on each part being dire'ctly proportionate to the height 

 of the column of blood above it: hence their liability to distension. But 

 this pressure bears equally on both arteries and veins, and cannot either 

 move, or resist the motion of, the fluid they contain, so long as the col- 

 umns of fluid are of equal height in both, and continuous. 





 VELOCITY OF THE CIRCULATION. 



The velocity of the blood-current at any given point in the various 

 divisions of the circulatory system is inversely proportional to their 

 sectional area at that point. If the sectional area of all the branches 

 of a vessel united were always the same as that of the vessel from which 

 they arise, and if the aggregate sectional area of the capillary vessels 

 were equal to that of the aorta, the mean rapidity of the blood's motion 

 in the capillaries would be the same as in the aorta and largest arteries; 

 and if a similar correspondence of capacity existed in the veins and 

 arteries, there would be an equal correspondence in the rapidity of the 

 circulation in them. But the arterial and venous systems may be rep- 

 resented by two truncated cones with their apices directed toward the 

 heart; the area of their united base (the sectional area of the capillaries) 

 being 400 800 times as great as that of the truncated apex representing 



