THE CAPILLARIES. 137 



the arteries is expended in overcoming this resistance, and the 

 rapidity of the circulation in the arteries, even from the com- 

 mencement of the aorta, would be the same as in the veins 

 and capillaries, if the aggregate capacity of each of the three 

 systems of vessels were the same. 



But since the aggregate sectional area of the branches is 

 greater than that of the trunk from which they arise, the 

 rapidity of the blood's motion will necessarily be greater in 

 the trunk, and will diminish in proportion as the aggregate 

 capacity of the vessels increases during their ramification : in 

 the same manner as, other things being equal, the velocity of 

 a stream diminishes as it widens. 



The observations of Hales, E. H. Weber, and Valentin, 

 agree very closely as to the rate of the blood in the capillaries 

 of the frog ; and the mean of their estimates gives the velocity 

 of the systemic capillary circulation at about one inch per 

 minute. Through the pulmonic capillaries, the rate of motion, 

 according to Hales, is about five times that through the sys- 

 temic ones. The velocity in the capillaries of warm-blooded 

 animals is greater, but has not yet been accurately estimated. 

 If it be assumed to be three times as great as in the frog, still 

 the estimate may seem too low, and inconsistent with the facts, 

 which show that the whole circulation is accomplished in 

 about a minute. But the whole length of capillary vessels, 

 through which any given portion of blood has to pass, prob- 

 ably does not exceed -g^th of an inch ; and therefore the time 

 required for each quantity of blood to traverse its own ap- 

 pointed portion of the general capillary system will scarcely 

 amount to a second ; while in the pulmonic capillary system 

 the length of time required will be much less even than this. 



The estimates given above are drawn from observations of 

 the movements of the red blood-corpuscles, which move in the 

 centre of the stream. At the circumference of the stream, hi 

 contact with the walls of the vessel, and adhering to them, 

 there is a layer of liquor sanguinis which appears to be motion- 

 less. The existence of this still layer, as it js termed, is in- 

 ferred both from the general fact that such a one exists in 

 all fine tubes traversed by fluid, and from what can be seep in 

 watching the movements of the blood-corpuscles. The red 

 corpuscles occupy the middle of the stream and move with 

 comparative rapidity ; the colorless lymph-corpusc}es run much 

 more slowly by the walls of the vessel ; while next to the wall 

 there is often a transparent space in which the fluid appears 

 to be at rest ; for if any of the corpuscles happen to be forced 

 within it, they move more slowly than before, rolling lazily 

 along the side of the vessel, an4 often adhering to its wall. 



