VELOCITY AND PRESSURE OF BLOOD-FLOW. 



443 



appropriate tubing filled with liquid and when the clamp was 

 removed from the vessel its pressure displaced the mercury in the 

 limbs by a certain amount. The difference in height between the 

 levels of the mercury in the two limbs in each experiment gives the 

 blood pressure, which is therefore usually expressed as being equal 

 to so many millimeters of mercury. By this expression it is meant 

 that the pressure within the artery is able to support a column 



Fig. 178. A, Schema to show the recording mercury manometer and its connection 

 with the artery: M, The manometer with the position of the mercury represented in black 

 (the pressure is given by the distance in millimeters between the levels 1 and 2; one-half of 

 this distance is recorded on the kymographion by the pen, P) ; F, the float resting upon the 

 surface of the mercury ; G, the cap through which the stem carrying the pen moves ; E, offset 

 for driving air out of the manometer and for filling or washing out the tube to the artery ; 

 R, the receptacle containing the solution of sodium carbonate ; c. the cannula for insertion 

 into the artery; w, the washout arrangement shown in detail in B. 



B, The washout cannula: c, the glass cannula inserted into the artery; r, the stem 

 connected with the reservoir of carbonate solution ; o, the stem connected with the manom- 

 eter. The arrows show the current of carbonate solution during the process of washing 

 out, the artery at that time being closed by a clamp. 



of mercury that many millimeters in height, and by multiplying 

 this value by 13.5 the pressure can be obtained, when desirable, 

 in terms of a column of blood or water. For continuous obser- 

 vations and permanent records the height of the column of mercury 

 and its variations during an experiment are recorded by the device 

 represented in Fig. 178. 



The distal limb of the U tube in which the mercury rises carries afloat 

 of hard rubber, aluminum, or some other substance lighter than the mercury. 



