82 PULSE RECORDS IN THEIR RELATION TO 



tracings). As the cardiographic curve in its relationship to the 

 second sound in man shows no constancy, and as it shows a 

 similar divergence when compared at this phase to the carotid 

 curve, 1 it cannot be regarded as a safe criterion in fixing the S.C. 

 point. The constancy of the relationship of the S.C. point to the 

 aortic curve in animals shows the necessity of taking the carotid 

 curve as the standard in man. The nearest approach to accuracy 

 is obtainable by allowing for the transmission time from aorta to 

 carotid. The S.C. point in man is thus fixed at a point "03 sec. 

 in advance (to the left in Fig. 1 of this article) of the bottom of the 

 dicrotic depression. 



The actual determination of the A.O. point in man is im- 

 possible. In animals most writers are agreed in placing it at the 

 bottom of the intr a -ventricular downstroke. 2 On the aortic curve 

 it is marked at the summit of the dicrotic (Porter, Hiirthle, Bayliss 

 and Starling). Cardiographic curves show no constant relation- 

 ships at this phase. In man the A.O. point is best fixed on the 

 carotid tracing, '03 sec. in advance of the summit of the dicrotic 

 wave. It is to be noted, however, that this introduces a possible 

 source of fallacy. Galabin 3 states ( 15 ) that the dicrotic summit 

 recedes from the apex of the primary wave as the pulse travels 

 along the vessels. The error from this alteration is probably so 

 small as to be quite negligible in the carotid tracing, but it con- 

 stitutes an important element in radial tracings, which should 

 therefore never be employed in estimating this point, in exact 

 work. It must not be forgotten that the A.O. point in man is the 

 most difficult of all the standard movements to ascertain ; conclusions 

 drawn from it must always be received with due caution. 



II. THE RELATIONSHIP OF THE AURICULAR AND 

 VENTRICULAR CYCLES 



(a) The Evidence obtained from Intra-auricular Curves 

 in Animals. The events of the auricular cycle have received 



1 Thus in Keyt's tracings the bottom of the cardiographic downstroke corre- 

 sponds on the carotid to the bottom of the dicrotic depression, or to a point on the 

 upstroke of the dicrotic itself. In Edgren's tracings, as in Hiirthle's. it is placed 

 as a rule at the bottom of the depression ; but there is very considerable divergence 

 in the individual curves of these authors. 



2 Estimated by differential tracings. 



3 The statement of Galabin is supported by many of the tracings which 

 Keyt gives. The question is discussed by Frey (Untcrauch. cL Pvhes, 1892). 



