236 THE HUMAN BODY. 



artery. The white corpuscles, partly because of their less 

 specific gravity, and partly because of their sometimes irregu- 

 lar form, due to amoeboid movements, get frequently pushed 

 out of the axial current, so that many of them are found in 

 the inert layer. 



Internal Friction. It follows from the above-stated facts 

 that there is no noticeable friction between the blood and 

 the lining of the vessel through which it flows: since the 

 outermost blood-layer in contact with the wall of the vessel is 

 changed only by diffusion. There is great 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 hydro- 

 dynamics as "internal friction," and it is of great importance 

 in the circulation of the blood. Internal 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 dis- 

 proportionately 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 internal friction than if sent in the same time through 

 four or five smaller tubes, the united transverse sections ox 

 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 internal friction, but only to a comparatively 

 slight extent; so that the internal friction, and consequently 

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

 laries than in the small arteries, and in the small arteries 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 

 Flow. Since the heart sends blood into the 'aorta intermit- 

 tently, 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 tem- 

 poral artery on the side of the brow. 



The first explanation which suggests itself is that since 



