228 THE HUMAN BODY. 



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, 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 form together 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 j*ate of flow is not the same in all parts of it. In 

 the centreTs a very'rapid current carrying along alTthe red 

 corpuscles and known as tne axial stream, while "near the 

 Tv-nll of flip yaffil thfl flnw 1*? mucn slower, as indicateOythe 

 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 layer of the 

 liquid will remain motionless in contact with the tube; 

 the next layer of molecules will move faster, the next 

 faster still; and so on until a very 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 current or axial stream, just as the 

 red corpuscles are in the artery. The white corpuscles, on 

 account of their power of executing independent amoeboid 

 movements and their consequent irregular form, get fre- 

 quently 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 bejhween the blood 



and thfi lininor nf fTia ypageLJ-.Tn-nngfr whlVh ^ flows; Since 



the outermost blood layer in contact with the wall of the 

 vessel is almost motionless. But there is very great fric- 

 tion between the different concentric layers of the liquid, 

 since each of them is moving at a different rate from those 



