126 NUTRITIONAL PHYSIOLOGY 



the whole, is relatively high. The aorta is passed at a 

 speed of at least a foot in a second. The veins also are 

 traversed at a high velocity, though the figures are some- 

 what lower than for the arteries. The movement in the 

 capillaries is in sharp contrast to that in both arteries and 

 veins, being exceedingly slow, perhaps ? V mc h m a second. 

 It appears that a corpuscle may take as long to go through 

 a capillary link which would scarcely span the breadth of a 

 pin-head as to travel from the heart to the brain. 



When it is remembered that the actual service of the 

 blood to the tissues is rendered in the capillaries (since all 

 other vessels have walls too thick to permit free diffusion), 

 the value of the slow passage is obvious. At the same time, 

 one recognizes the desirability of the rapid transit to and 

 from this department of the system. The heart itself and 

 all the main vessels may be thought of as accessory to the 

 capillaries. The explanation of the slowing of the stream 

 in the small channels is entirely simple, yet often mis- 

 apprehended. Whenever an artery forks to form two 

 branches, these are individually of smaller cross-section 

 than the parent stem, but their combined cross-section is 

 greater. The result is the same that is seen when a river 

 widens or deepens the current slackens. If the river 

 broadens into a lake the current may become impercep- 

 tible, yet we know that the water is still setting forward 

 toward the outlet. 



Are we then to believe that the capillary system is many 

 times wider than the aorta or the great veins? There is no 

 escape from this conclusion: their number is so vast that, 

 despite their infinitesimal size as single conveyors of the 

 blood, collectively they form the broadest division of the 

 entire path. If they fail to suggest a lake, an analogy may 

 be found in the tangled swamp in which a stream loses 

 itself, breaking into many sluggish arms, from which at 

 last the waters converge to resume a rapid course over a 

 narrow bed. The acceleration noticed as the veins are fol- 

 lowed toward the heart is merely a sign that as their num- 

 ber grows less their combined cross-section also contracts. 



