VELOCITY OF THE CIRCULATION. 179 



quantity wliich can pass through the heart in each of its 

 actions. But the conclusions arrived at by this method 

 are less satisfactory. For the estimates both of the total 

 quantity of blood, and of the capacity of the cavities of 

 the heart, have as yet only approximated to the truth. 

 Still, the most careful of the estimates thus made accord 

 with those already mentioned ; for Valentin has, from 

 these data, calculated that the blood may all pass through 

 the heart in from 43 J to 62 seconds. 



The estimate for the speed at which the blood may be 

 seen moving in transparent parts, is not opposed to this. 

 For, as already stated (p. 162), though the movement 

 through the capillaries may be very slow, yet the length 

 of capillary vessel through which any portion of blood has 

 to pass is very small. Even if we estimate that length at 

 the tenth of an inch, and suppose the velocity of the blood 

 therein to be only one inch per minute, then each portion 

 of blood may traverse its own distance of the capillary 

 system in about six seconds. There would thus be plenty 

 of time left for the blood to travel through its circuit in 

 the larger vessels, in which the greatest length of tube 

 that it can have to traverse in the human subject does not 

 exceed ten feet. 



All the estimates here given are averages ; but of course 

 the time in which a given portion of blood passes from 

 one side of the heart to the other, varies much according 

 to the organ it has to traverse. The blood which circulates 

 from the left ventricle, through the coronary vessels, to the 

 right side of the heart, requires a far shorter time for the 

 completion of its course than the blood which flows from 

 the left side of the heart to the feet, and back again to the 

 right side of the heart ; for the circulation from the left to 

 the right cavities of the heart may be represented as form- 

 ing a number of arches, varying in size, and requiring 

 proportionately various times for the blood to traverse 

 them ; the smallest of these arches being formed by the 



N 2 



