THE CIRCULATION OF THE BLOOD 357 



tube there is a certain amount of friction between the moving 

 liquid and the walls of the tube. There is also friction between 

 the different concentric layers of the liquid, since each of them is 

 moving at a different rate from that in contact with it on each 

 side. This form of friction is known in hydrodynamics as " in- 

 ternal friction/' and it is of great importance in the circulation 

 of the blood. The friction increases very fast as the caliber of the 

 tube through which the liquid flows diminishes: so that with the 

 same rate of flow it is disproportionately much greater in a small 

 tube than in a larger one. Hence a given quantity of liquid forced 

 in a minute through one large tube would experience much less 

 resistance from friction than if sent in the same time through 

 four or five smaller tubes, the united transverse sections of which 

 were together equal to that of the single larger one. In the blood- 

 vessels the increased total area, and consequently slower flow, in 

 the smaller channels partly counteracts this increase of friction, 

 but only to a comparatively slight extent; so that the friction, 

 and consequently the resistance to the blood-flow, is far greater 

 in the capillaries and arterioles than in the small arteries, and in 

 the small arteries than in the large ones. Practically we may re- 

 gard the arteries as tubes ending in a sponge: the united areas of 

 all the channels in the latter might be considerably larger than 

 that of the supplying tubes, but the friction to be overcome in 

 the flow through them would be much greater. 



The Conversion of the Intermittent into a Continuous Flow. 

 Since the heart sends blood into the aorta intermittently, we 

 have still to inquire how it is that the flow in the capillaries is 

 continuous. In the larger arteries it is not, since we can feel 

 them dilating as the "pulse," on applying the finger over the 

 radial artery at the wrist, or over the temporal artery on the side 

 of the brow. 



The first explanation which suggests itself is that since the 

 capacity of the blood-vessels increases from the heart to the 

 capillaries, an acceleration of the flow during the ventricular 

 contraction which might be very manifest in the vessels near the 

 heart would become less and less obvious in the more distant 

 vessels. But if this were so, then when the blood was collected 

 again from the wide capillary sponge into the great veins near 

 the heart, which together are but little bigger than the aorta, we 



