WORKING OF THE HEART AND BLOOD - VESSELS. 235 



The reason of the slower flow of the capillaries is that 

 their united area is considerably greater than that of the 

 arteries supplying them, so that the same quantity of blood 

 flowing through them in a given time has a wider channel 

 to flow in and moves more slowly. The area of the veins is 

 smaller than that of the 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 auri- 

 cles are greater than those of the arteries leaving the ventri- 

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

 given time as leaves it, the rate of the current in the pul- 

 monary veins and the verise cava3 is less than in the pulmonary 

 artery and aorta. We may represent the vascular system as 

 a double cone, widening 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 

 centre is a very rapid current carrying along all the red cor- 

 puscles and known as the axial stream, while near 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 for- 

 cibly driven through a fine tube which it wets, water for in- 

 stance through a glass tube, the outermost layer of the liquid 

 will remain motionless in contact with the tube; the next 

 layer of molecules will move a little, the next faster still; 

 and so on until a rapid current is found in the centre. If 

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

 water, these will all be carried on in the central faster cur- 

 rent or axial stream, just as the red corpuscles are in the 



