THE CIRCULATION OF THE BLOOD. 



159 



their united base (the sectional area of the capillaries) being 400-800 

 times as great as that of the truncated apex representing the aorta. 

 Thus the velocity of blood in the capillaries is not more than j^of that 

 in the aorta. 



(a.) In the Arteries. The velocity of the stream of blood is greater 

 in the arteries than in any other part of the circulatory system, and in 

 them it is greatest in the neighborhood of the heart, d during the ven- 

 tricular systole. The rate of movement dimin- 

 ishes during the diastole of the ventricles, and in 

 the parts of the arterial system most distant from 

 the heart. Chauveau has estimated the rapidity of 

 the blood-stream in the carotid of the horse at over 

 20 inches per second during the heart's systole, and 

 nearly 6 inches during the diastole (520 150 

 mm.) 



FIG. 141. -Ludwig's 

 Stromuhr. 



Estimation of the Velocity. Various in- 

 struments have been devised for measuring the 

 velocity of the blood stream in the arteries. Lud- 

 wig's "Stromuhr" (Fig. 141) consists of a U- 

 shaped glass tube dilated at a and a 1 ', the ends of 

 which, h and i, are of known calibre. The bulbs 

 can be filled by a common opening at k. The 

 instrument is so contrived that at b and #', the 

 glass part is firmly fixed into metal cylinders, at- 

 tached to a circular horizontal table, c c', capable 

 of horizontal movement on a similar table d d' 

 about the vertical axis marked in figure by a dotted 

 line. The opening in c c', when the instrument 

 is in position, as in fig., corresponds exactly with those in dd'\ but \ic c'be 

 turned at right angles to its present position, there is no communication 

 between h and a, and i and a' ', but h communicates directly with i\ and 

 if turned through two right angles c' communicates with d, and c with 

 d', and there is no direct communication between h and i. The experi- 

 ment is performed in the folio wing way: The artery to be experimented 

 upon is divided and connected with two canulae and tubes which fit it 

 accurately with h and ih the central end, and i the peripheral; the 

 bulb a is filled with olive oil up to a point rather lower than k, and a' 

 and the remainder of a is filled with defibrinated blood; the tube on &is 

 then carefully clamped; the tubes d and d' are also filled with defibrin- 

 ated blood. 'When everything is ready, the blood is allowed to flow into 

 a through h, and it pushes before it the oil, and that the defibrinated 

 blood into the artery through i, and replaces it in a' ; when the blood 

 reaches the former level of the oil in a, the disc c c' is turned rapidly 

 through two right angles, and the blood flowing through d into a' again 

 displaces the oil which is driven into a. This is repeated several times, 

 and the duration of the experiment noted. The capacity of a and a' is 

 known; the diameter of the artery is also known by its corresponding 

 with the canulae of known diameter, and as the number of times a has 



