THE CIRCULATION OP THE BLOOD. 233 



II. 



Bight 

 auricle. 



Cardiac 

 impulse. 



Fig. SOO. — Simultaneous tracings from the interior of the right auricle, from the inte- 

 rior of the right ventricle, and of the cardiac impulse in the horse (after Chauveau 

 and Marey). Tracings to be read from left to right, and the references above are 

 in the order from top to bottom. A complete cardiac cycle is included between 

 the thick vertical lines I and II. The thin vertical lines indicate tenths of a sec- 

 ond. The gradual rise of pressure within the ventricle (middle tracing) during 

 diastole, the sudden rise with the systole, its maintenance with oscillations for an 

 appreciable time, its sudden fall, etc., are all well shown. There is disagreement 

 as to the exact meaning of the minor curves in the larger ones. 



also follow f rota the intercostal muscles being simply unsup- 

 ported when the heart recedes. 



INVESTIGATION OF THE HEART-BEAT FROM 

 , WITHIN. 



I By the use of apparatus, introduced within the heart of the 

 mammal and reporting those cKanges susceptible of graphic 

 record, certain tracings have been obtained about the details of 

 which there are uncertainty and disagreement, though they 

 seem to establish the nature of the main features of the cardiac 

 beat clearly enough. An interpretation of such tracings in the 

 light of our general and special knowledge warrants the follow- 

 ing statement. 



1. !3oth auricular and ventricular systole are sudden, but the 

 latter is of very much greater duration. 



2. While the chest wall feels the ventricular systole, the 

 auriculo-ventricular valves shield the auricle from its shock. 



3. During diastole in both chambers the pressure rises gradu- 



