BLOOD-PRESSURE. 321 



another brass-pipe which was fitly adapted to it, " was attached a 

 glass tube of nearly the same diameter, which was nine feet in 

 length"; the blood rose to a height of 2.44 meters, and at each in- 

 spiration of the animal the column rose and at each expiration it fell ; 

 besides this movement due to respiration, there was a smaller 

 rise at each systole and a corresponding fall at each diastole. 

 This pressure under which the blood is in the artery which causes 

 it to rise so high in the tube is arterial pressure, or the blood- 

 pressure in the arteries. Such an instrument for measuring 

 pressure is a manometer. It is manifest that a glass tube 2^ 

 meters in height is not a very convenient instrument to manipu- 

 late, and besides the blood soon clots. To obviate both of these 

 difficulties the mercurial manometer was devised, together with the 

 use of a solution of sodium carbonate ; the latter preventing the 

 coagulation of the blood. Later a drum or kymograph was added 

 to the apparatus, on which a record could be made for study and 

 preservation (Fig. 166). The legend beneath the illustration is 

 sufficiently descriptive of the complete apparatus. The curve 



P 



BL 



FIG. 167. The trace of arterial blood -pressure from a dog anesthetized with 

 morphia and ether. The cannula was in the proximal stump of the common carot- 

 id artery. The curve is to be read from left to right: P, the pressure-trace written 

 by the recording mercurial manometer ; B L, the base-line or abscissa, representing 

 the pressure of the atmosphere. The distance between the base-line and the press- 

 ure-curve varies, in the original trace, between 62 and 77 millimeters, therefore the 

 pressure varies between 124 and 154 millimeters of mercury, less a small correction 

 for the weight of the sodium-carbonate solution ; T, the time-trace, made up of 

 intervals of two seconds each, aud written by an electro-magnetic chronograph 

 (Curtis). 



made by the pen is shown in Fig. 167. The longer waves are 

 due to respiration, the smaller ones to the heart-beat. By the 

 term mean pressure is meant the average pressure throughout the 

 observation. The mean arterial pressure in man is approximately 

 150 mm., an increase of 5 mm. occurring at the time of systole. 



The figures here given for the mean aortic pressure may be too 

 high ; it has, of course, never been measured in man. The 

 following table (Starling) gives the approximate heights in differ- 

 ent portions of the vascular system, calculated largely from obser- 

 vations in lower animals : 

 21 



