158 HANDBOOK OF PHYSIOLOGY. 



again from these to the capillaries, and thence along the veins to the right 

 auricle. The blood-pressure in the veins is nowhere very great, but is 

 greatest in the small veins, while in the large veins towards the heart the 

 pressure becomes negative, or, in other words, when a vein is put in con- 

 nection with a mercurial manometer, the mercury will fall in the arm 

 farthest away from the vein, and will rise in the arm nearest the vein, 

 which has a tendency to suck in rather than to push forward. In the 

 large veins of the neck this tendency to suck in air is especially marked, 

 and is the cause of death in some surgical 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 

 veins of the trunk and extremities in the erect posture supposed to be 

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

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

 ways 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 direc- 

 tion. The lowest blood-vessels have, of course, to bear the greatest 

 amount of pressure; the pressure on each part being directly propor- 

 tionate to the height of the column of blood above it: hence their liability 

 to distention. 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 columns of fluid are of equal height in both, and 

 continuous. 



Velocity of the Blood Current. 



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

 divisions of the circulatory system is inversely proportional to their sec- 

 tional 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 maybe represented by two 

 truncated cones with their apices directed towards the heart; the area of 



