21 6 THE CIRCULATION OF THE BLOOD 



while in the large veins near the heart the pressure may become negative, or 

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

 eter the mercury may fall in the arm farthest away from the vein and will 

 rise in the arm nearest the vein, the action being that of suction rather than 

 pressure. In the large veins of the neck the tendency to suck in air is es- 

 pecially marked, and is the cause of death in some accidents or operations in 

 that region. The amounfof pressure in the brachial vein is said to support 

 9 mm. of mercury, whereas the pressure in the veins of the neck may fall to 

 a negative pressure of from 3 to 8 mm. 



The variations of venous pressure during systole and diastole of the 

 heart are very slight, and a distinct pulse is never seen in veins except under 



FIG. 190. Tracing taken with Erlanger's Sphygmomanometer. The figures indicate 

 pressure in millimeters of mercury. Systolic pressure 160; diastolic pressure, 120. (New 

 figure by Hill.) 



extraordinary circumstances. In certain forms of cardiac valvular insuffi- 

 ciency there may be considerable regurgitation of the blood with a strong 

 venous pulse. 



Careful observations upon the web of the frog's foot, the tongue and mesen- 

 tery of the frog, the tails of newts and small fishes, and upon the skin of the 

 finger behind the nail (von Kries) ; as well as estimations of the amount of 

 pressure required to empty the vessels of blood under various conditions, 

 all indicate that the capillary blood pressure is subject to very great varia- 

 tions. Apparently the variations follow the variations of pressure in the 

 arteries, though the measurements of the capillary pressure of the skin 

 in man indicate that it is occasionally markedly influenced by the venous 

 pressure variations (Hough). In the skin in man it is from 30 to 50 mm. 

 mercury. 



The pulse in the arterioles, capillaries, and venules becomes more and 

 more evident as the extravascular pressure is increased. The pressure in 

 the web of the frog's foot has been found to be equal to about 14 to 20 mm. of 

 mercury; in other capillary regions the pressure is found to be equal to from 

 one-fifth to one-half of the ordinary arterial pressure. 



