THE CIRCULATION OF THE BLOOD. 307 



greatest oscillations occur in the mercury column of the manometer. 

 The pressure is applied to the arm by the rubber armlet H, which is 

 3^ inches wide. This is the widest armlet that can be adjusted to 

 the average-sized arm and presents distinct advantages over the 

 narrow armlet hitherto employed. This armlet is prevented from 

 expanding outward by a cuff, F, of double thick canvas with inserted 

 strips of tin, which is held in place by two straps which completely 

 encircle the cuff. On the rigidity of this depends to a large extent 

 the transmission of pulsation. The rubber armlet is connected by 

 glass with a stiff- walled rubber tube, G, which in turn connects with 

 the manometer. The manometer is perhaps the most important part 

 of the apparatus. It is constructed entirely of metal except for the 

 glass tube containing the mercury column. The chamber c com- 

 municates by means of a metal tube with the glass column D, which 

 is connected by a screw-thread at 3, the caliber of c being approx- 

 imately 100 times that of D. The cap of the chamber, which screws 

 on, is provided with a metal T which is connected at 2 with the 

 rubber armlet and at i with the bulb, used as an air-pump. At A 

 is a stop-cock shutting the rubber bulb completely from the rest 

 of the apparatus, while at B is a screw-valve which allows the air 

 to escape from the closed system. When desired, the manometer 

 can be made portable (without removing the mercury) by screwing 

 the caps i and 2 into either end of the T at i and 2. The manometer 

 is then tilted away from the glass column D until all the mercury 

 has run into the chamber, the glass is then unscrewed and cap 3 

 screwed in. Before removing cap 3 the manometer must always be 

 tilted, else the mercury will be lost. 



The rubber bulb is similar to those found on atomizers. 



In using this apparatus the pressure is raised by the air-bulb 

 forcing air into the closed system distending the rubber armlet 

 and with the same degree of force displacing the mercury in c, 

 driving it up the glass column D. When the pulse is no longer felt, 

 the bulb still being compressed, the arm of valve A is turned 

 until it is at right angles with the thumb and finger. The valve B 

 is now slowly unscrewed until the mercury column begins to fall. 

 With the eye on the scale the point at which the pulse reappears is 

 mentally noted as the systolic pressure. Often considerably before 

 the reappearance of the pulse to palpation, a pulsation is seen in 

 the mercury column. As the column slowly falls this increases up 

 to its greatest oscillation and then diminishes. The lowest point 

 of the greatest pulsation is noted as the diastolic pressure. 



From experimental data and from theoretic reasoning it is certain 

 that the pressure in the carotid and femoral arteries is less than in 

 the aorta and greater than in the small peripheral arteries. In other 

 words, there is a fall of pressure from the beginning of the aorta to 



