MEASUREMENT OF RATE OF FLOW. [BOOK i. 



and the oil once more driven from A to B. And this is repeated several 

 times, indeed generally until the clotting of the blood or the admixture 

 of the oil with the blood puts an end to the experiment. Thus the flow 

 of blood is used to till alternately with blood or oil the space of the bulb A, 

 whose cavity as far as the mark x has been exactly measured ; hence if 

 the number of times in any given time the disc D has to be turned round 

 be known, the number of times A has been filled is also known, and 

 thus the quantity of blood which has passed in that time through the 

 cannula connected with the tube G is directly measured. For instance, 

 supposing that the quantity held by the bulb A when filled up to the 

 mark x is 5 c.c., and supposing that from the moment of allowing the 

 first 5 c.c. of blood to begin to enter the tube to the moment when the 

 escape of the last 5 c.c. from the artery into the tube was complete, 100 

 seconds had elapsed, during which time 5 c.c. had been received 10 

 times into the tube from the artery (all but the last 5 c.c. being returned 

 into the distal portion of the artery), obviously -5 c.c. of blood had 

 flowed from the proximal section of the artery in one second. Hence 

 supposing that the diameter of the cannula (and of the artery, they 

 being the same) were 2 mm., with a sectional area therefore of 3-14 

 square mm., an outflow through the section of '5 c.c. or 500 c.mm. in a 

 second would give (|-JT), a velocity of about 159 mm. in a second. 



The Hfematachometer 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. A square or rectangular chamber, one side of which is of 

 glass and marked with a graduated scale in the form of an arc of a circle, 

 is connected by means of two short tubes with the two cut ends of an 

 artery ; the blood consequently flows from the proximal (central) 

 portion of the artery through the chamber into the distal portion of the 

 artery. Within the chamber and suspended from its roof is a short 

 pendulum, which when the blood-stream is cut off from the chamber 

 hangs motionless in a vertical position, but when the blood is allowed to 

 flow through the chamber, is driven by the force of the current out of 

 its position of rest. The pendulum is so placed that a marker attached 

 to its free end travels close to the inner surface of the glass side along 

 the arc of the graduated side. Hence the amount of deviation from a 

 vertical position may easily be read off on the scale from the outside. 

 The graduation of the scale having been carried out by experimenting 

 with streams of known velocity, the velocity can at once be calculated 

 from the amount of deviation. 



An instrument based on the same principle has been invented by 

 Chauveau and improved by Lortet, Fig. 33. In this the part which cor- 

 responds to the pendulum in Vierordt's instrument is prolonged outside 

 the chamber, and thus the portion within the chamber is made to form 

 the short arm of a lever, the fulcrum of which is at the point where the 

 wall of the chamber is traversed and the long arm of which projects out- 

 side. 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 this lever projecting within the tube is moved on 

 its fulcrum in the indiarubber ring by the current of blood passing 

 through the tube, the greater the velocity of the current, the larger being 



