CHAP, iv.] THE VASCULAR MECHANISM. 163 



corpuscles, these extremely minute passages would occasion a 

 very great amount of friction, and thus present a considerable 

 obstacle or resistance to the flow of blood through them. Still 

 greater must be the friction and resistance occasioned by the 

 actual blood with its red and white corpuscles. The blood, in fact, 

 meets with great difficulties in its passage through the peripheral 

 region, and sometimes, as we shall see, the friction and resistance 

 are so great in the peripheral vessels of this or that area that no 

 blood at all passes through them, and an arrest of the flow takes 

 place in the area. 



The resistance to the flow of blood thus caused by the friction 

 generated in so many minute passages is one of the most important 

 physical facts in the circulation. In the large arteries the friction 

 is small; it increases gradually as they divide, but receives its 

 chief and most important addition in the minute arteries and 

 capillaries : it is relatively greater in the minute arteries than in 

 the capillaries on account of the flow being more rapid in the 

 former, for friction diminishes rapidly with a diminution in the 

 rate of flow. We may speak of it as the ' peripheral friction,' 

 and the resistance which it offers as the ' peripheral resistance.' 

 It need, perhaps, hardly be said that this peripheral resistance 

 not only opposes the flow of blood through the capillaries and 

 minute arteries themselves where it is generated, but, working 

 backwards along the whole arterial system, has to be overcome 

 by the heart at each systole of the ventricle. 



Hydraulic Principles of the Circulation. 



100. In the circulation, then, the following three facts of 

 fundamental importance are met with : 



1. The systole of the ventricle, driving at intervals a certain 

 quantity of blood, with a certain force, into the aorta. 



2. The peripheral resistance just described. 



3. A long stretch of elastic tubing (the arteries), reaching 

 from the ventricle to the region of peripheral resistance. 



From these facts we may explain the main phenomena of the 

 circulation, which we have previously sketched, on purely physical 

 principles, without any appeal to the special properties of living 

 tissues, beyond the provision that the ventricle remains capable 

 of good rhythmical contractions, that the arterial walls retain 

 their elasticity, and that the friction between the blood and the 

 lining of the peripheral vessels remains the same ; we may thus 

 explain the high pressure and pulsatile flow in the arteries, the 

 steady stream through the capillaries, the low pressure and the 

 uniform pulseless flow in the veins, and, finally, the continued flow 

 ,of the blood from the aorta to the mouths of the venae cavse. 



All the above phenomena in fact are the simple results of an 



