THE CIRCULATION OF THE BLOOD 



671 



delicate adjustment of flow can be effected with a high driving pressure than with 

 a low one. 



The fact that mammals possess a high arterial pressure was shown by Stephen 

 Hales (1733). The second volume of his book, " Hsemastatics," opens with the 

 description of his famous experiment, which I will give in his own words : " In 

 December I caused a mare to be tied down alive on her back . . . having laid open 

 the left crural artery about 3 in. from her belly, I inserted into it a brass pipe 

 whose bore was 1 of an inch in diameter ; and to that, by means of another brass 

 pipe which was fitly adapted to it, I fixed a glass tube, of nearly the same 

 diameter, which was 9 ft. in length : then untying the ligature on the artery, the 

 blood rose in the tube 8 ft. 3 in. perpendicular above the level of the left ventricle 



FIG. 224. PORTRAIT OF STEPHEN HALES. 



of the heart : but it did not attain to its full height at once ; it rushed up about 

 half way in an instant, and afterwards gradually at each pulse 12, 8, 6, 4, 2, and 

 sometimes 1 in. : when it was at its full height, it would rise at and after each 

 pulse 2, 3, or 4 in. ; and sometimes it would fall 12 or 14 in., and have there for a 

 time the same vibrations up and down, at and after each pulse, as it had, when it 

 was at its full height; to which it would rise again, after 40 or 50 pulses." 



A portrait of Stephen Hales is given in Fig. 224. 



The circulatory system, then, consists of a branching system of tubes, the 

 arteries, arising from a pump, the heart. After dividing up into a fine network 

 of capillaries in the various organs, the vessels reunite to veins, which enter 

 the heart again at the opposite end. 



In mammals there are what amounts to two separate circulations with two 

 pumps, although the two pumps are combined together side by side in the heart, 



