228 



THE CIRCULATION OF THE BLOOD 



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 

 mesentery of the frog, the tails of newts and small fishes, and upon the 

 skin of the finger behind the nail (Hooker) ; 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 variations. 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. 



FIG. 191. Schema Showing the Relation between Blood Pressure, Velocity of Flow, 

 and Vascular Area, in the Arteries, Capillaries, and Veins. Ordinates represent height 

 of pressure and speed of flow. The abscissa, b-c, represents zero pressure and speed. 

 Space between lines a-b and d-e represents arterial system; between d-e and/-g, capillary 

 system, and between f-g and h-i, the venous system. Line A-B equals pressure; line C-D, 

 speed of flow; and line E-F, vascular area. (Modified from Gad.) 



General Variations in Blood-Pressure. The arterial blood-pressure 

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



