VELOCITY AND PRESSURE OF BLOOD-FLOW. 499 



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. 658) 

 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. 

 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 blanch the skin under 

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

 Therefore to blanch this area would require a column of water contain- 

 ing 1000 c.mms. with a cross-area of 4 sq.mms. The height of this column 

 would therefore be equal to is> ^ or 250 mms. of water, that is, 18.5 mms. 

 Hg. 



The results obtained by this method are not very constant and 

 can only be considered as approximate. It would appear, how- 

 ever, that the pressure lies somewhere between 20 and 40 mms. 

 of mercury. Thus, upon the gums of a rabbit von Kries found a 

 capillary pressure of 33 mms. Hg. 



By means of a more adjustable instrument von Reckling- 

 hausenj estimates that in man the pressure within the capil- 

 laries of the finger-tips or, to be more accurate, within the small 

 arteries supplying these capillaries, is equal to 55 mms. Hg. 

 (See p. 507.) 



The general relations of the pressures in arteries, veins, and 

 capillaries may be expressed in a curve such as is shown in Fig. 

 200. 



It should be added that in this curve and in all the figures 

 so far quoted in regard to the actual pressure within the different 



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

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

 1875, p. 148. 



J Von Recklinghausen, "Archiv f. exp. Path. u. Pharmak.," 55, 375, 1907. 



