128 THE FLOW IN THE VEINS. [BOOK i. 



this direction is very incomplete ; but in all probability the pressure 

 in a vein varies within much wider limits than does the pressure in 

 the corresponding artery. 



In the small veins the velocity of the current, measured in the 

 same way as in the case of the arteries, is very slight. It increases 

 'in the larger veins, corresponding to the diminution of the area of 

 'the bed' ; it is about 200 mm. per sec. in the jugular vein of the 

 dog. 



Thus the flow in the veins presents strong contrasts with that 

 in the arteries. In the arteries, even in the smallest branches, 

 there is a considerable mean pressure. In the veins, even in the 

 small veins where it is largest, the mean pressure is very slight. 

 In other words, there is always a difference of pressure tending to 

 make the blood flow continuously from the arteries into the veins. 

 \^^A pulse is present in the arteries, but, with certain exceptions, 

 absent in the veins. The velocity of the stream of blood in the 

 tjj arteries is considerable ; in the small veins it is much less, but it 

 increases in the larger trunks; for in both arteries and veins it 

 corresponds with the area of the bed, diminishing in the former 

 from the heart to the capillaries, and increasing in the latter from 

 the capillaries to the heart. 



Hydraulic Principles of the Circulation. 



All the above phenomena are the simple results of an intermit- 

 tent force (like that of the systole of the ventricle) working in a 

 closed circuit of branching elastic tubes, so arranged that while the 

 individual tubes first diminish (from the heart to the capillaries) 

 and then increase (from the capillaries to the heart), the area of 

 the bed first increases and then diminishes, the tubes together 

 thus forming two cones placed base to base at the capillaries, with 

 their apices converging to the heart. To this it must be added 

 that the friction in the small arteries and capillaries, at the junction 

 of the bases of the cones, offers a very great resistance to the flow 

 of the blood through them. It is this peripheral resistance (in the 

 minute arteries and capillaries, for the resistance offered by the 

 friction in the larger vessels may, when compared with this, be 

 practically neglected), reacting through the elastic walls of the 

 arteries upon the intermittent force of the heart, which gives the 

 circulation of the blood its peculiar features. 



Circumstances determining the character of the flow. When 

 fluid is driven by an intermittent force, as by a pump, through a 

 perfectly rigid tube (or system of tubes), there escapes at each 

 stroke of the pump from the distal end of the system just as much 

 fluid as enters it at the proximal end. The escape moreover takes 

 place at the same time as the entrance, since the time taken up by 



