THE CIRCULATION OF THE BLOOD 319 



capillaries but greater than that of the arteries, and hence the 

 rate of movement in them is also intermediate. Almost always 

 when an artery divides, the area of its branches is greater than 

 that of the main trunk, and so the arterial current becomes 

 slower and slower from the heart onwards. In the veins, on the 

 other hand, the area of a trunk formed by the union of two or 

 more branches is less than that of the branches together, and the 

 flow becomes quicker and quicker towards the heart. But even 

 at the heart the united cross-sections of the veins entering the 

 auricles are greater than those of the arteries leaving the ventricles, 

 so that, since as much blood returns to the heart in a given time 

 as leaves it, the rate of the current in the pulmonary veins and 

 the venae cavae is less than in the pulmonary artery and aorta. 

 We may represent the vascular system as a double cone, widen- 

 ing from -the ventricles to the capillaries and narrowing from the 

 latter to the auricles. Just as water forced in at a narrow end of 

 this would flow quickest there and slowest at the widest part, so 

 the blood flows quickest in the aorta and slowest in the capillaries, 

 which taken together form a much wider channel. 



The Axial Current and the Inert Layer. If a small artery in 

 the frog's web be closely examined it will be seen that the rate of 

 flow is not the same in all parts of it. In the center is a very 

 rapid current carrying along all the red corpuscles and known as 

 the axial stream, while hear the wall of the vessel the flow is much 

 slower, as indicated by the rate at which the pale blood-corpuscles 

 are carried along in it. This is a purely physical phenomenon. 

 If any liquid be forcibly driven through a fine tube which it wets, 

 water for instance through a glass tube, the outermost layers of 

 the liquid will remain nearly motionless in contact with the tube; 

 the next layers of molecules will move a little, the next faster 

 still; and so on until a rapid current is found in the center. If 

 solid bodies, as powdered sealing-wax, be suspended in the water, 

 these will all be carried on in the central faster current or axial 

 stream, just as the red corpuscles are in the artery. The white 

 corpuscles, partly because of their less specific gravity, and partly 

 because of their sometimes irregular form, due to amoeboid move- 

 ments, get frequently pushed out of the axial current, so that 

 many of them are found in the inert layer. 



The Resistance to the Blood-Flow. As liquid flows through a 



