322 CIRCULATION OF BLOOD. LiECT. XVII. 



tion, how we may determine the velocity with which the 

 blood circulates in the aorta: I adopt the numbers given by 

 Hales. The quantity of blood expelled from the left ven- 

 tricle of the heart of a horse, is about 10 cubic inches; the 

 area of the aotfa is 1-036 square inches ; the fraction r 1 3 - 6 = 

 9'64 expresses the length of the cylinder of blood which is 

 formed in passing through the orifice of the aorta at each 

 systole of the ventricle ; and, as the heart of the horse 

 makes thirty-six pulsations per minute, that is, 2160 per 

 hour, it follows that a column of blood [2160 X 9-54 inches 

 or] 1735 feet long will be thrown into a vessel of the calibre 

 of the aorta every hour. Then, estimating the systole to 

 continue only one-third of the interval between the pulsa- 

 tions, the velocity will be found to be thrice as much, 

 namely, 86 feet per minute. 



Hales, who studied the phenomena of the circulation 

 with so much sagacity and skill, endeavoured to determine 

 experimentally the velocity of the blood in the capillary 

 vessels. For this purpose, he introduced warm water into 

 the descending aorta of a dog, by means of a tube thrust 

 into the artery. The pressure exercised by the column of 

 liquid, was nearly equal to that which the blood experiences 

 in this vessel [from the contraction of the heart.] The 

 intestines having been slit open from end to end, the water 

 was seen to ooze out, drop by drop, from the capillaries. 

 Hales varied the experiment, by successively dividing the 

 vessels nearer and nearer to the aorta; and, at the same 

 time, measured the water which passed off, in a given time, 

 by the various capillaries, whose diameter he knew. The 

 pressure of the column of liquid was kept constantly equal. 

 Here are some of the numbers obtained : 342 cubic inches 

 escaped from the capillary vessels in 400 seconds ; the same 

 quantity passed off by the mesenteric arteries 140 seconds; 

 and by the crural arteries in 20 seconds. 



