THE MAIN FACTS OF THE CIRCULATION. 



151 



in a large artery of a large animal, such as the carotid of a dog or horse, and 

 probably in the carotid of a man, the blood flows at the rate of 300 or 500 

 mm. a second. In the very small arteries the rate is probably only a few 

 mm. a second. 



Methods. The haemadromometer of Volkmann. [Fig. 45.] An artery e.g., a 

 carotid is clamped in two places, and divided between the clamps. Two canulse, 



[FiG. 45. 





Volkmann's Hsemadromometer: The conical portions of the instrument are inserted in the 

 cut ends of a vein or artery. By a simple arrangement of a double stopcock the blood-current 

 can be made to pass immediately through the transverse arm, as in A, or to pass through the 

 graduated U-shaped tube, as in B.] 



of a bore as nearly equal as possible to that of the artery, or of a known bore, are 

 inserted in the two ends. The two canulse are connected by means of two stop- 

 cocks, which work together, with the two ends of a long glass tube, bent in the 

 shape of a U, and filled with normal saline solution, or with a colored innocuous 

 fluid. The clamps on the artery being released, a turn of the stopcocks permits 

 the blood to enter the proximal end of the long U-tube, along which it courses, 

 driving the fluid out into the artery through the distal end. Attached to the tube 

 is a graduated scale, by means of which the velocity with which the blood flows 

 along the tube may be read off. Even supposing the canulse to be of the same 

 bore as the artery, it is evident that the conditions of the flow through the tube are 

 such as will only admit of the resu]t thus gained being considered as an approxi- 

 mative estimation of the real velocity in the artery itself. 



The rheometer (StronnihrT of Ludwig. This consists of two glass bulbs, A and 

 B, Fig. 46, communicating above with each other and with the common tube C, 

 by which they can be filled. Their lower ends are fixed in the metal disc />, 

 which can be made to rotate, through two right angles, round the lower disc E. 

 In the upper disc are two holes, a and b, continuous with A and B respectively, 

 and in the lower disc are two similar holes a' and &', similarly continuous with 

 the tubes G and H. Hence, in the position of the discs shown in the figure, the 

 tube G is continuous through the two discs with the bulb A, and the tube H 

 with the bulb B. On turning the disc D through two right angles, the tube G 



