1907-8.] 
The Arterial Pressure in Man. 
345 
vantage of the principle of circular compression in order to estimate the 
maximum or systolic arterial pressure, while it at the same time gives the 
minimum or diastolic pressure. It has a mercurial manometer, the lumen 
of which is exactly that of the ordinary physiological kymograph. The 
air contained in the armlet can be increased, and the pressure on the limb 
therefore elevated, by means of a large syringe, and the pressure may be 
raised quickly or slowly according to requirements. By means of a valve 
the pressure may also be lowered quickly or slowly. A float rests upon 
the mercury, surrounded, as is usual in the physiological laboratory, by 
alcohol, and an upright rod of aluminium leads to a horizontal arm which 
writes on the revolving cylinder. In order to have the absolute zero, a 
fixed arm traces the abscissa upon the cylinder, which is driven by a 
clockwork placed horizontally, as in the instrument of Erlanger. The 
pulsations of the artery below the point of compression are recorded by 
means of a transmission sphygmograph. This consists of a tambour 
brought into contact with the brachial or radial artery, as may be most 
convenient, by a pelotte resting upon the vessel. It is adjusted to the arm 
by means of a spring provided with a screw. This tambour is brought 
into communication by rubber tubing with another tambour, the move- 
ments of which are recorded on the cylinder simultaneously with the 
movements of the kymograph. The best tracing is obtained when the 
tambour in contact with the artery is larger than that connected with 
the recording lever, by means of which the movements are amplified. The 
whole apparatus is shown in fig. 1. 
In using the instrument the pressure within the cuff may be raised 
gradually or quickly, the latter being the more usual course. If it is 
slowly raised, the tracing of the kymograph shows at first a line of ascent 
with small oscillations, but as it rises the pulsations become more and 
more marked, and the excursion of the index more extensive, until a 
maximum point of amplitude is attained, when they begin to diminish 
and gradually disappear. Simultaneously the transmission sphygmograph 
records a gradual diminution in the amplitude of the pulsations, which 
finally cease. When all the movements of the kymograph, as well as of the 
sphygmograph, have come to an end, the pressure is allowed to fall by the 
escape of air from the valve, and the events which follow are the converse 
of those just described. Such a tracing is shown in fig. 2. In this tracing 
the systolic pressure on the ascending curve, estimated by the disappearance 
of the pulsations of the transmission sphygmograph, was 180, and on the 
descending curve, measured by the reappearance of the pulsation, it was 
exactly the same. The diastolic pressure, estimated by the middle point 
