64 ELEMENTARY PHYSIOLOGY 



of the mechanical conditions of the organs of the circula- 

 tion combined with the rhythmical activity of the heart. 

 This activity drives the fluid contained in these organs out 

 of the heart into the arteries, thence to the capillaries, 

 and from them through the veins hack to the heart. And 

 in the course of these operations it gives rise, incidentally, 

 to the cardiac impulse, the sounds of the heart, blood- 

 pressure, and the pulse. 



13. The Rate of Blood Flow. —It has been found, by 

 experiment, that in tlio horse it takes about half a minute 

 for any substance, as for instance a chemical body, whose 

 presence in the blood can easily be recognized, to com- 

 plete the circuit, ex. gr. to pass from the jugular vein 

 down through the right side of the heart, the lungs, the 

 left side of the heart, up through the arteries of the head 

 and neck, and so back to the jugular vein. 



By far the greater portion of this half minute is taken 

 up by the passage through the small vessels, where the 

 blood moves, it is estimated, at the rate only of about one 

 and a half inches in a minute, wlioreas through tlie camtid 

 artery of a dog it flies along at the rate of about ten inches 

 in a second. Of course to complete the circuit of the 

 circulation, a blood-corpuscle need not have to go through 

 so much as half of an inch of capillaries in either the lungs 

 or any of the tissues of the body. 



Inasmuch as the force which drives the blood on is 

 (putting the other comparatively slight helps on one 

 side) the beat of the heart and tliat alone, however much 

 it may be modified, as we have seen, in character, it is 

 obvious that the velocity with which the blood moves 

 must be greatest in the aorta and diminish towards the 

 capillaries. 



For with each l)ranching of the arteries the total area 

 of the arterial system is increased, the total width of the 

 capillary tubes if they were all put together side by side 

 being very much gi-eater than that of the aorta. Hence 

 the blood, or a corpuscle, for instance, of the blood Itoing 



