144 THE VASCULAR MECHANISM. 



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 

 backward along the whole arterial system, has to be overcome by the heart 

 at each systole of the ventricle. 



Hydraulic Principles of the Circulation. 



107. 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 circula- 

 tion, 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 contrac- 

 tions, that the arterial walls retain their elasticity, and that the friction be- 

 tween 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 intermit- 

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

 circuit of branching tubes, so arranged that while the individual tubes first 

 diminish in calibre (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 at the heart, and pre- 

 senting at their conjoined bases a conspicuous peripheral resistance, the 

 tubing on one side, the arterial, being eminently elastic, and, on the other, 

 the venous, affording a free and easy passage for the blood. It is the 

 peripheral resistance (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. 



108. Circumstances determining the character of the flow. When fluid 

 is driven by an intermittent force, as by a pump, through a perfectly rigid 

 tube, such as a glass one (or a system of such tubes), there escapes at each 

 stroke of the pump from the distal end of the tube (or system of tubes) just 

 as much fluid as enters it at the proximal end. What happens is very like 

 what would happen if, with a wide glass tube completely filled with billiard 

 balls lying in a row, an additional ball were pushed in at one end : each 

 ball would be pushed on in turn a stage further and the last ball at the 

 further end would tumble out. The escape, moreover, takes place at the 

 same time as the entrance. 



This result remains the same when any resistance to the flow is intro- 

 duced into the tube, as for instance when the end of the tube is narrowed. 

 The force of the pump remaining the same, the introduction of the resist- 

 ance undoubtedly lessens the quantity of fluid issuing at the distal end at 



