THE CIRCULATION OF THE BLOOD AND LYMPH 121 



its commencement. In the veins only a small remnant of resistance 

 remains to b3 overcome, and the lateral pressure must sink again 

 rather suddenly about the end of the capillary tract. Fig. 50 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 manometer containing some lighter liquid than mer- 

 cury, 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 u mm. in the 

 crural vein. Opitz obtained the following pressures in dogs (of 

 about 15 kilos) : left facial vein, 5*1 ; right external jugular, 

 -O'ii ; central end of superior vena cava, 2- 8 ; femoral vein, 

 5-4 ; renal vein, 10-9 ; portal vein, 8*9 mm. of mercury. 



The venous pressure being so low, or, in other words, the 

 potential energy which the systole of the heart imparts to the 

 blood being so greatly exhausted before it reaches the veins, 

 other influences begin here appreciably to affect the blood- 

 stream : 



1. Contraction of the Muscles. This compresses the neighbour- 

 ing veins, and since the blood is compelled by the valves, if it 

 moves at all, to move towards the heart, the venous circulation 

 is in this way helped. 



2. Aspiration of the Thorax. In inspiration the intrathoracic 

 pressure, and therefore the pressure in the great thoracic veins, 

 is diminished, and blood is drawn from the more peripheral parts 

 of the venous system into the right heart (p. 210). 



3. Aspiration of the Heart. When the heart, after its contrac- 

 tion, suddenly relaxes, the endocardiac pressure becomes nega- 

 tive, and blood is sucked into it, just as when the indiarubber 

 ball of a syringe is compressed and then allowed to expand. 

 But we cannot attribute any great importance to this ; and, 

 of course, it is only the relaxation of the right ventricle which 

 could directly affect the venous circulation. 



4. Every change of position of the limbs, as in walking, aids 

 the venous circulation (Braune), and this independently of the 



