132 THE CIRCULATION OF THE BLOOD AND LYMPH 



is complicated in the vascular system by the branching of the channel 

 and the variation in the total cross-section, yet theory and experiment 

 agree that in the larger arteries the lateral pressure diminishes but 

 slowly from the heart to the periphery, the resistance being small com- 

 pared with the resistance of the whole circuit. In the capillary region 

 the vascular resistance abruptly increases ; the velocity (and therefore 

 the kinetic energy) abruptly diminishes, and the lateral pressure falls 

 much more steeply between the beginning and the end of this region 

 than between the heart and its commencement. In the veins only a 

 small remnant of resistance remains to be overcome, and the lateral 

 pressure must sink again rather suddenly about the end of the capillary 

 tract. Fig. 59 shows by a rough diagram the manner in which the 

 pressure, velocity and cross-section probably change from part to part 

 of the vascular system. 



The Circulation in the Veins. The slope of pressure, as we have 

 just explained, must fall rather suddenly near the beginning and 

 near the end of the capillary tract. It continues falling as we pass 

 along the veins, till the heart is again reached. In the right heart, 

 and in the thoracic portions of the great veins which enter it, the 

 pressure may be negative that is, less than the atmospheric 

 pressure. And since nowhere in the venous system is the pressure 

 more than a small fraction of that in the arteries, its measurement 

 in the veins is correspondingly difficult, because any obstruction 

 to the normal flow is apt to artificially raise the pressure. A man- 

 ometer containing some lighter liquid than mercury, such as water 

 or a solution of sodium citrate or magnesium sulphate, is usually 

 employed, so that the difference of level may be as great as possible. 

 In the sheep the pressure was found to be 3 mm. of mercury in the 

 brachial, and about n mm. in the crural vein. Burton-Opitz 

 obtained the following pressures in dogs (of about 15 kilos) : left 

 facial vein, 5-1 ; right external jugular, - o-n ; central end of superior 

 vena cava, 2*8; femoral vein, 5-4; renal vein, 10-9; portal vein, 

 8-9 mm. of mercury. 



Estimation of Venous Pressure in Man. The venous pressure in man 

 has been estimated by several observers with more or less satisfactory 

 results. The best-known method is that of v. Recklinghausen. A 

 circular rubber bag, with a central opening, is laid over the course of 

 a vein, so that the vein can be observed through the opening, as in 

 Fig. 60. The bag is smeared with glycerin. A glass plate is laid over 

 the opening and held firmly, so that the vein and the surrounding skin 

 are in a closed chamber. The bag is provided with a side-tube, which 

 connects it with a pump and a water manometer. By means of the 

 pump air is forced into the bag till the vein is just seen to collapse. 

 The pressure indicated on the manometer at this moment is taken as 

 the pressure in the vein. 



By means of a modification in this method, Eyster and Hooker have 

 found that the pressure in the small veins of the arm or hand generally 

 varies between 3 and 10 cm. of water. In conditions of congestion of 

 the venous system the pressure may rise to 20 cm. of water (say 15 mm. 

 of mercury) or more. 



In making this measurement it is necessary to take account of the 



