VELOCITY AND PRESSURE OF BLOOD-FLOW. 501 



At the heart, therefore, the pressure of the blood upon the walls 

 of the veins is very small, and, indeed, owing to the circumstance 

 that the large veins lie in the thoracic cavity, in which the pres- 

 sure is below that of the atmosphere, the pressure of the blood in 

 them may also be below atmospheric pressure, although doubt- 

 less at this point (vena cava) the pressure within the vein is 

 greater than the pressure on its exterior (intrathoracic pressure). 

 Taking into account the negative intrathoracic pressure (p. 662) 

 it may be estimated that the difference in pressure between the 

 blood in the veins of the neck and that in the superior vena cava 

 is equal to 4 or 5 mms. Hg., and this difference is sufficient to drive 

 the blood into the heart and to fill and distend it rapidly during 

 diastole* To complete the general conception of the pressure 

 relations in the vascular system it is necessary to know the pressure 

 of the blood in the smallest arteries and veins and in the capillaries. 



Fig. 200. Schematic representation of the general relations of blood-pressure (side 

 pressure) in different parts of the vascular system: a, The arteries; c, the capillaries; v, 

 the veins. The mean and diastolic pressures remain nearly constant in the arterial system, 

 as far as they can be measured accurately. The pressures in the veins are represented as 

 uniform at any one point. In the large veins near the heart there are variations of pressure 

 with each respiration and with each heart beat (Venous Pulse, p. 532\ 



It is not possible in the cases of the capillaries, for instance to 

 connect a manometer directly with the vessels, and recourse has 

 been had to a less direct and certain method. The pressure in the 

 capillaries in different regions of the skin has been estimated by 

 determining the pressure necessary to obliterate them that is, to 

 blanch the skin. A glass plate is laid upon the skin or mucous 

 membrane and weights are added until a distinct change in the 

 color of the skin is noted f Knowing the necessary weight to pro- 

 duce this effect and the area submitted to compression, the pressure 

 may be expressed in terms of millimeters of mercury or blood. 



The following example may be used to illustrate this method. Suppose 

 that the glass plate has an area of 4 sq.mms., and that to blaneh the skin under 

 it a weight of 1 gm. is necessary; 1 gm. of water = 1 c.c. or 1000 c. mms. 1 



* Henderson and Barringer, "Amer. Journal of Physiology/' 31, 352, 1913. 

 t V. Kries "Berichte d. Sachs. Gesellschaft d. Wiss. Math.-phys. Classe " 

 1875, p. 138. 



