VENOUS BLOOD PRESSURE AND CAPILLARY PRESSURE 195 



systolic pressure, i.e., the maximal arterial pressure. He gives for the dias- 

 tolic pressure 40 to 45 mm. of mercury below the systolic pressure. Other 

 observers using the same method find a somewhat higher average pressure, 

 see figure 190, which represents a fair type of observation. 



The Venous Blood Pressure and Capillary Pressure. The blood 

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

 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 amount of 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 

 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. 



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. 



General Variations in Blood Pressure. The arterial blood pressure 

 may be made to vary by alterations in either of the chief factors upon which 

 the pressure in the vessels depends, but primarily by the cardiac contrac- 

 tions and the peripheral resistance. Thus, increase of blood pressure may 

 be brought about by either, i, a more frequent or more forcible action of 

 the heart, or, 2, by an increase of the peripheral resistance. On the other 

 hand, diminution of the blood pressure may be produced, either by 0, a 

 diminished force or frequency of the contractions of the heart, or by b, a 

 diminished peripheral resistance. These different factors, however, although 



