122 A MANUAL OF PHYSIOLOGY 



theless, 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* 



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 must be added that in addition to the venous pulse described 

 on p. 119, which travels through widened arterioles and capillaries 

 from the arteries into the veins, 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 seen in the jugular vein in 

 some healthy persons, but more frequently and more distinctly in 

 cases of incompetence of the tricuspid valve. 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). The changes of pressure in the great veins due to the 

 respiratory movements (p. 250) are also sometimes spoken of as a 

 venous pulse, but they are produced in an entirely different way. 



The Circulation-time. Hering was the first who attempted to 

 measure the time required by the blood, or by a blood-corpuscle, to 

 complete the circuit of the vascular system. He injected a solution 

 of potassium ferrocyanide into a vein (generally the jugular), and 

 collected blood at intervals from the corresponding vein of the oppo- 

 site side. After the blood had clotted, he tested for the ferrocyanide 

 by addition of ferric chloride to the serum. The first of the samples 

 that gave the Prussian blue reaction corresponded to the time when 

 the injected salt had just completed the circulation. This method 

 was improved by Vierordt, who arranged a number of cups on a 

 revolving disc below the vein from which the blood was to be 

 taken. In these cups samples of the blood were received, and 

 the rate of rotation of the disc being known, it was possible to 



