122 A MANUAL OF PHYSIOLOGY . 



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

 outwards, and at the same time extended, a manometer con- 

 nected 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 

 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 difference between the mean 

 velocity in the veins and in the corresponding 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. 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. 120, 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 vein in healthy 

 persons, and far more distinctly in certain disorders of the cir- 

 culation. 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 



