SIMPLE CARDIO-SPHYGMOGRAPH. —~ 29. 
The commencement of the two traces is indicated by the curved 
lines to the left of each trace as they are looked at without moving the 
page, and these curved lines are produced by letting the levers move 
without the watchwork, whilst the instrument is being fixed in posi- 
tion. Synchronous points in the two traces must evidently be at 
equal distances from the starting points in the traces, and therefore 
the one can be projected on the other by compasses or by super- 
position. 3 
In all cases it is necessary, both in the cardiograph and in the 
sphygmograph trace, to project all the main points, such as the origin 
of the main rise, and the deepest point in the seeondary fall, on to one 
line in the trace ; for, as the levers move in part of a circle, any point 
at the summit of the trace, if projected straight downwards, would 
not be correctly related to the lower parts of the trace. This correc- 
tion is best made by a most simple arrangement; a flat piece of board 
has a straight slip of wood fixed close to one edge; against this the 
tracing rests, being supported on the board. Two nails are fixed on 
the board, so that they bear the same relations to its supported trace 
as that did to the axes of the levers which marked on it in the cardio- 
sphygmograph. The marking apparatus consists of two pieces of 
string, each fixed at one end to the nails, and at the other carrying 
needles ; these pieces of string must be of the same length as the 
_ levers to which they correspond, the points of the needles must pierce 
them, and the other ends of the needles must be attached to the nails 
by a thread to prevent them from moving irregularly. 
The cardio-sphygmograph can be best applied when the person Page 268. 
using it is sitting, as it can then be made to rest on the arm of a chair, 
and in practice it is better not to have the main part of the instru- 
ment press against the chest-wall, as if it does the heart’s movement 
imparts itself to the whole apparatus, and so complicates the trace. 
In considering the results arrived at by the use of this instrument, 
it will be necessary to define a few of the terms that have to be 
employed in explaining them. 
(1.) The first cardiac interval is that which occurs between the 
commencing systolé and the closure of the aortic valve at the heart. 
(2.) The first arterial interval is that which occurs between the 
indications of the commencing systolé and the closure of the aortic 
valve in an artery. The radial artery at the wrist is the only one that 
is here considered. 
As the commencement of the arterial rise is somewhat later than 
the commencing systolé at the heart, and as the difference between 
the first cardiac interval and the first arterial interval is not great, 
these two events coincide in part of their duration, and give rise to 
minor divisions, which may be thus named and defined. 
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