452 CIRCULATION OF BLOOD AND LYMPH. 



at the rate of 1 mm. Hg for every 35 mms. of distance. We have 

 such figures as the following: 



Doo. SHEEP. 



Superior vena cava Jugular vein 0.2 mm. Hg. 



(near auricle) = 2.96 mms. Hg. Facial vein 3.0 mms. " 



Superior vena cava Branch of brachial 9.0 ' 



. more distal = 1.38 " " Crural 11.4 " 



External jugular (left) = 0.52 mm. 



Right brachial = 3.90 mms. *' 



Left facial = 5.12 * 



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

 of the veins is nearly nil, and, indeed, owing to the circumstance 

 that the large veins lie in the thoracic cavity, in which the pressure 

 is below that of the atmosphere, the tension of the blood in them 

 may be slightly negative. 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 



Fig. 187. 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. 



capillaries. It is not possible in the case 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 

 capillary pressure 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. * Knowing the necessary weight to 

 produce 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. 



* V. Kries, "Berechte d. Sachs. Gesellschaft d. Wiss. Math.-phys. Classe," 

 1875, p. 148. 



