272 METHODS OF RECORDING PULSE. [BOOK i. 



surface and with the support of the radius below so that adequate 

 pressure can be brought to bear by the lever on the artery, is favour- 

 able for making observations. It can of course be applied to other 

 arteries. 



The membrane-manometer of Hiirthle may also be applied directly 

 to an unopened artery. The cannula is replaced by a small funnel, the 

 mouth of which is covered by membrane bearing at its centre a small 

 block of cork. If the cork be pressed lightly on an artery, the ex- 

 pansions of the artery move the membrane of the funnel, and the 

 movements of this are transmitted along the fluid of a rigid tube to the 

 recording tambour. 



A pulse tracing may also be indirectly obtained by the plethysmo- 

 graphic method. If the arm be introduced into a plethysmograph 

 ( 122) a tracing may be obtained of the rhythmic expansions of the 

 arm, that is of the rhythmic expansions of the arteries of the arm, due 

 to the heart beats. If the plethysmograph chamber be filled with air 

 instead of fluid, the changes of pressure in the chamber may be brought 

 to bear on a sensitive flame, the changes of which in turn may be 

 photographed. 



If the artery be laid bare other methods may be adopted. In some 

 cases, in that of the aorta for instance, it is sufficient to attach a light 

 hook into the outer coat of the artery, and to connect the hook by 

 means of a thread with a carefully balanced lever. The movements of 

 the coat of the artery are then recorded by the lever. 



The sphygmotonometer of Roy may also be used without opening 

 the artery. For this purpose a length of the artery is enclosed in a 

 tube with rigid walls, filled with fluid, which acts as a plethysmograph, 

 the movements of the fluid around the artery being recorded by means 

 of a piston working a lever. If the artery be ligatured and divided, 

 one end may be drawn into the tube for the distance required. The 

 tube may also be made of two halves, one of which is slipped under the 

 artery simply laid bare, the other placed above it, and the two halves 

 are brought together round the artery, the two ends of the tube being 

 closed with membrane. 



And still other methods may be employed. 



The several tracings obtained by these several methods differ 

 of course in minor features, but they agree in general features ; 

 and from a comparative study of the results obtained by different 

 methods we are able, in many cases at all events, to form 

 conclusions as to which of the minor features of a curve are due to 

 the instrument itself and which represent events actually taking 

 place in the artery. On the whole the curve obtained by directly 

 recording the pressure within the artery is concordant with that 

 obtained by recording the expansions of the artery; the curve 

 obtained by the manometer or by the sphygmoscope very closely 

 resembles that obtained by the sphygmograph, and the more com- 

 pletely the incidental errors of each instrument are avoided, the more 

 closely do the two curves agree. We may accordingly in treating of 

 the pulse confine ourselves largely to the results obtained by the 

 sphygmograph. Any of the various instruments applied to the radial 



