VELOCITY AND PRESSURE OF BLOOD-FLOW. 465 



This fact is illustrated in Fig. 189, which gives a graphic repre- 

 sentation of a number of experimental determinations * of systolic 

 and diastolic pressures in the large arteries of the dog. 



If we turn to the other end of the vascular system, the veins, 

 we find that the lowest pressure is in the jugular and that it increases 

 gradually as we go toward the capillary area. According to one 

 observer,! the fall in pressure from periphery toward the heart is 

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

 such figures as the following: 



DOG. 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 " 



Fig. 190. 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. 



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 

 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 



* Dawson, "American Journal of Physiology," 15, 244, 1906. 

 f Burton-Opitz, "American Journal of Physiology," 9, 198, 1903. 

 30 



