CHAP, iv.] THE VASCULAR MECHANISM. 



223 



repeatedly filled and refilled in succession. From the length of time 

 it takes to fill the bulb a certain number of times the flow through the 

 artery is calculated. 



FIG. 34. LUDWIG'S STKOMUHE AND A DIAGRAMMATIC REPRESENTATION or THE SAME. 



G and H fit into the cannulaB placed respectively into the proximal and distal 

 cut ends of the artery under examination. D is a metal disc revolving on a lower 

 similar disc E. A and B are glass bulbs (which can be filled through C) fixed upon 

 D ; the capacity of A up to the mark x is known. Holes are bored through D and 

 E in such a way that in the position shewn in the figure fluid passes from G 

 through a' and a into A and so by B, b and &' to H. If the disc D be turned 

 through two right angles, fluid passes from a' to b and so by , A, and a to 6'. If 

 it be turned through one right angle only the fluid passes directly from G to H 

 without entering the bulbs at all. A is filled with pure oil up to the mark x, B 

 with defibrinated blood. The blood is allowed to flow from G into A until the 

 whole of the oil is driven into B, the defibrinated blood occupying which is driven 

 into H. Then by a rapid turn the position of A and B is reversed, and the oil 

 driven back into A ; then again by another turn back from A into B, and so on 

 until clotting stops the observation. The time which it takes the flow through G 

 to fill A (up to the mark a;) alternately with blood and oil, being thus determined, 

 the sectional area of G and the capacity of A being known, the velocity of the flow 

 through G may be calculated. 



The Hsernatachometer of Vierordt is constructed on the principle of 

 measuring the velocity of the current by observing the amount of devia- 

 tion undergone by a pendulum, the free end of which hangs loosely in 

 the stream. 



An instrument based on the same principle has been invented by 

 Chauveau and improved by'Lortet, Fig. 35. A somewhat wide tube, 

 the wall of which is at one point composed of an indiarubber membrane, 

 is introduced between the two cut ends of an artery. A long light 

 lever pierces the indiarubber membrane. The short expanded arm of 



