PRESSURE IN THE VENOUS SYSTEM. 121 



peristalsis of the intestines greatly promote the portal venous flow. 

 Respiration not only aspirates and compresses the venous blood into 

 the right heart, but promotes the flow into the left heart from the 

 pulmonary veins. 



It was determined by Braime l that, if the thigh be rolled outwards and 

 backwards, the pressure in the femoral vein under Poupart's ligament falls 5 

 to 10 mm. H 2 below zero, and becomes positive again when the leg is placed 

 in the opposite position. The bones, the muscles, and the fascia, together with 

 the vein, make a suction and force pump, and thus, by this mechanism, walking 

 can aid the flow in the femoral vein. Extension of the veins in their long 

 diameter has been found to increase their capacity, and thus aspiration may 

 be produced therein by alternate extension and flexion of the body or limbs. 



When the arterial supply to any part of the body is entirely cut off, the 

 pressure in the vein which drains the part in question sinks to the pressure of 

 the great vein into which it opens. When, on the other hand, all the venous 

 exits from any part are ligatured, then the pressure rises, peripherally to the 

 seat of ligature, to the arterial pressure. 



If the arteries conveying blood to the part are few and narrow, a few veins 

 are sufficient to secure an adequate escape of blood; conversely, where the 

 arterial supply is great, a moderate obstacle to the venous flow causes marked 

 stasis and transudation. For example, on ligaturing in the rabbit all the large 

 veins of both ears, and dividing the cervical sympathetic nerve on one side, 

 the ear on this side will, within a few hours, display a considerable swelling, 

 while the other will remain unchanged. If, on occlusion of the central end 

 of the femoral vein, the pressure therein is equal to 80 to 100 mm. MgS0 4 

 solution, it will be found, after division of the sciatic nerve, to rise to 280 

 mm. MgS0 4 solution in a few minutes. 2 



Rate of flow in the veins. Turning to the question of the velocity 

 of the venous flow, it is obvious that the average input of the veins per 

 second must equal the average output per second, in order that the 

 circulation may continue. If the veins that enter the heart were of 

 the same sectional area as the arteries that leave it, then the velocity 

 would be the same in these veins as in the arteries. When the venae 

 cavse are filled with blood, their total sectional area is found to be 

 considerably greater than that of the aorta. But, as normally these 

 veins are never filled to their full capacity, it is probable that the 

 velocity of flow in them is approximately equal to that in the aorta. 

 The velocity in the carotid artery and jugular vein, or in the umbilical 

 artery and vein of the sheep's embryo, has been measured with Ludwig's 

 stromuhr, and has been found to be almost the same. 3 



That the progression of the blood in the veins is primarily main- 

 tained by the driving force of the heart, was shown by Magendie. He 

 divided the whole of the thigh, except the femoral artery and vein, from 

 the body. Pressure on the artery immediately stopped the flow in the 

 vein. The velocity, on account of the centripetal diminution of the 

 capacity of the venous system, must increase from point to point, while 

 the pressure must steadily decrease. The oscillations of pressure and 

 velocity are not conducted from the arteries into the veins, because the 

 pulse waves are reflected from the resistance in the arterioles and 



1 Ber. d. k. Sachs. Gesellsch. d. Wisscnsch., math.-phys. CL, Leipzig, 1870, S. 261. 



2 Cohnheim, "Lectures on Pathology," New Syd. Soc. Translation, vol. i. p. 159. 



3 Cyon and Steinman, Melanges UoL Acad. imp. d. sc. de St. Petersbourg, 1871, 

 vol. viii. p. 53; Zunt/ and Cohnstein, Arch. f. d. ges, Phy$iol. t Bonn, 1884, Bd. 

 xxxiv, S. 224, 



