THE CAPILLARY CIRCULATION. 229 



in contact with it on each side. This form of friction is 

 known in hydro-dynamics as " internal friction" and it is 

 of great importance in the circulation of the blood. In- 

 ternal friction increases very fast as the calibre 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 in- 

 ternal friction than if sent in the same time through four 

 or five smaller tubes, the united transverse sections df which 

 were together equal to that of the single larger ine. In 

 the blood-vessels the increased total area, and consequently 

 slower flow, in the smaller channels partly counteracts this 

 increase of internal friction, bat only to a comparatively 

 slight extent; so that the internal friction, and conse- 

 quently the resistance to the blood -flow, is far greater in the 

 capillaries than in the small arteries, and in the small ar- 

 teries than in the large ones. Practically we may regard 

 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 

 Plow. Since the heart sends blood into the aorta inter- 

 mittently, 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" by ap- 

 plying the finger over the radial artery at the wrist, or 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 

 Tentricular contraction which might be very manifest in 

 the vessels near the heart would become less and less obvi- 

 ous in the more distant vessels. But if this were so, 

 when the blood was collected again from the wide capillary 

 sponge into the great veins near the heart, which together 



