348 Proceedings of the Royal Society of Edinburgh. [Sess. 
compression, as was shown by Masing (30),* is not the absolute maximum. 
The determination of the minimum or diastolic pressure is not such an 
easy problem. Marey (12) originally suggested that the point at which 
the largest swing of the instrument occurred was an index of the mean 
pressure, and this was adopted and amplified by Roy and Adami (31). It 
has been proved experimentally, however, by Howell and Brush (27) that 
this does not indicate the mean, but really records the diastolic, pressure. 
Masing (30), who obtained some tracings from the artery below the seat 
of compression, believes that the greatest movement of the sphygmographic 
tracing marks the diastolic pressure. This must, however, be an error, as 
the greatest amplitude of oscillation of the sphygmographic curve is very 
commonly found after the pressure in the armlet has been allowed to fall 
nearly, if not quite, to zero, and the results obtained by this method of 
estimation are unmistakably erroneous. My own method of obtaining the 
diastolic pressure is to ascertain where the greatest amplitude of oscillation 
occurs in the kymographic curve, and to take the middle point of this as 
the expression of the diastolic pressure. 
In fig. 3, for example, while the systolic pressure by every observer 
would be reckoned as 180 mm. Hg., the diastolic pressure by my estimate 
would be 110, and by the method of Masing 90 mm. Hg. In fig. 4 the 
systolic pressure is 170 mm. Hg., and the diastolic by my computation 106, 
while according to Masing it would be 90. In some other tracings this is 
even more striking, as the maximum excursions of the transmission 
sphygmograph occur after the pressure in the armlet has been almost 
entirely removed. Fig. 5 is a good example of such a tracing. 
At first the lowest point of the greatest swing seemed to me that which 
might be considered as the index. There is one objection to this, however, 
which was suggested long ago by Marey (12), and which has been urged 
upon me by Dr Jane way in private correspondence. He points out that if 
the pressure in the apparatus is allowed to fall continuously, the inertia of 
the mercury will be apt to carry the lowest point of oscillation below the 
true level. This has been carefully tested by arresting the escape for a few 
seconds, after each five or ten millimetres of descent. It has been observed 
that if the escape was recommenced at an instant when the curve was 
falling, the inertia sometimes caused the curve to fall as much as 10 mm. 
If, on the contrary, the pressure recommenced its escape during an ascent 
of the kymographic index, the result never exceeded 5 mm., and was 
usually 2 or 3 mm. There is, therefore, real weight in the criticism, and it 
* Masing allows that the difference between the lateral and terminal pressure in one of 
the larger arteries is inconsiderable. 
