948 ANIMAL PHYSIOLOGY. 
able for experiment and in man is so slight that it becomes ne- 
cessary to exaggerate them in the tracing, hence long levers are 
used to accomplish this. 
The sphygmograph is the usual form of instrument em- 
ployed for the purpose. It consists, essentially, of a clock-work 
for moving a smoked surface (mica plate commonly) on which 
the movements of a lever-tip, 
answering to those of a button 
placed on the artery, are re- 
corded. 
Considering the nature of the 
pulse and the apparatus em- 
Fic. 218.—Diagrammatic schema showing 
the essential part of the instrament Ployed to wfite its characters, it 
when in working order. The knife-edge * : 
B’, of the short lever is in contact with Will be seen that the possible 
were esi Gis at 4 Sommanca, sources of error are numerous. 
1 ea aene ever em eae Different observers have, as 
a matter of fact, even with the 
same sort of instrument obtained tracings differing not a little 
in character. As the subjoined figures show, the pressure ex- 
erted upon a vessel may so alter the result that entire features 
of the tracing may actually disappear. The sphygmograph, 
even in the most skillful hands, has proved somewhat disap- 
pointing as a physiological and especially as a clinical instru- 
ment, though it is not without a certain value. 
We shall do well to inquire whether there are any features 
in common in tracings obtained in various ways, and which 
have therefore in all probability a real foundation in nature. 
An inspection of a 
large number of pulse- 
tracings, taken under 
diverse conditions, 
seems to show that in 
all of them there oc- 4 
curs, more or less 
marked, the follow- 
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ing: 1. An upward D® 
curve. 2. A downward eee ee ee TA aerate captain 
curve, rendered irreg- ony wae : Gi relbeane tenons wie ae 
ular by theoccurrence hove is that made borg tani ponte with ten double 
of peaks or crests and vibrations in a second. 
notches. The first of these are termed the predicrotic notch and 
crest, and the succeeding ones the dicrotic notch and crest. 
The latter seem to be the more constant. 
