134 THE CIRCULATION OF THE BLOOD AND LYMPH 



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- act ing 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 con- 

 siderable variation of velocity and of pressure with every discharge 

 of the ventricle, and although this variation is absent from the 

 veins, since normally the pulse, due to the ventricular discharge, 

 does not penetrate into them, the venous flow is, nevertheless, as a 

 matter of fact, more irregular than the arterial. So that if it is 

 difficult to give a useful definition of the term ' velocity of the 

 blood ' in the case of the arteries, it is still more difficult to do so in 

 the case of the veins. Where voluntary movement is prevented, 

 one potent cause of variation in the venous flow is eliminated ; and 

 in curarized animals certain observers have found but little differ- 

 ence between the mean velocity in the veins and in the corre- 

 sponding arteries. Others have found the velocity in the veins 

 considerably less, which is indeed what we should expect from the 

 fact that the average cross-section of the venous system is greater 

 than that of the arterial system. Burton-Opitz, by means of a 

 stromuhr, obtained a mean velocity of 147 mm. per second in the 

 external jugular vein of a 13-kilo dog. 



To sum up, we may conclude that, upon the whole, the blood 

 passes with gradually-diminishing velocity from the left ventricle 

 along the arteries ; it is greatly and somewhat suddenly slowed in the 

 broad and branching capillary bed; but the stream gathers force 

 again as it becomes more and more narrowed in the venous channel, 

 although it never acquires the speed which it has in the aorta. 



Venous Pulse. To complete the account of the circulation in the 

 veins, it may be recalled that, in addition to the venous pulse 

 described on p. 131, which, as an occasional phenomenon, may 

 travel through widened arterioles and capillaries from the arteries 

 into the veins, and therefore in the direction of the blood-stream, 

 a so-called venous pulse, travelling from the heart against the blood- 

 stream and depending on variations of pressure in the right auricle, 

 may be detected in the jugular veins in healthy persons, and more 

 distinctly in certain disorders of the circulation, where indeed it 

 may be evident at a greater distance from the heart for example, 

 over the liver as the so-called liver pulse. In animals a venous 

 pulse of this nature has been demonstrated in the venae cavae, the 

 jugular vein, and with a delicate manometer even in the large veins 

 of the limbs. It moves with a speed of I to 3 metres a second 

 (Morrow). It is most easily observed in the jugular veins in man, 

 because of their proximity to the heart. We have already pointed 

 out the significance of the study of this venous pulse for the analysis 



