THE CIRCULATION OF THE BLOOD AND LYMPH 121 



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 mercury, such as water or a solution of magnesium 

 sulphate, is usually employed, in order 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 ii mm. in the crural vein; in the dog's portal vein 

 about 10 mm. 



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 neighbouring 

 veins, and since the blood is compelled by the valves, it 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. 250). 



3. Aspiration of the Heart. When the heart, after its contraction, 

 suddenly relaxes, the endocardiac pressure becomes negative, 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 

 muscular contraction. When the thigh of a dead body is rotated 

 outwards, and at the same time extended, a manometer connected 

 with the femoral vein shows a negative pressure of 5 to 10 mm. of 

 water. When the opposite movements are made, the pressure 

 becomes positive. 



It follows from the number of casually-acting influences 

 which affect the blood-flow in the veins that it cannot be 

 very regular or constant. We have seen that in the great 

 arteries there is a considerable variation of velocity and of 

 pressure with every beat of the heart ; and although this 

 variation is absent from the veins, since normally the pulse 

 does not penetrate into them, the venous flow is, never- 



