174 THE CIRCULATION. 



the arterial and capillary system tlie heart itself, indeed, 

 must overcome this resistance ; so that the excess of the 

 force of the blood's motion in the arteries is expended in 

 overcoming this resistance, and the rapidity of the circu- 

 lation in the arteries, even from the commencement 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 capil- 

 laries 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 systemic 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, probably 

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

 required for each quantity of blood to traverse its own 

 appointed portion of the general capillary system will 

 scarcely amount to a second : while in the pulmonic capil- 

 lary system the length of time required will be much less 

 even than this. 



The estimates given above are drawn from observations 



