THE CIRCULATION OF THE BLOOD 277 



auricle undergoes variations both in the way of increase and decrease as 

 shown by small undulations on the curve. 



With the close of the auricular systole, the ventricular systole occurs 

 quickly and energetically and endures for some time, after which the ven- 

 tricular walls quickly relax and remain at rest until the close of the next 

 auricular contraction. The summit of the ventricular tracing generally 

 spoken of as the plateau presents a series of elevations and depressions as 

 stated in a foregoing paragraph. 



A comparison of the two traces shows that between the close of the 

 auricular and the beginning of the ventricular systole there is a slight pause 

 known as the inter systolic pause (Chauveau). The tracings also show 

 that between the close of the ventricular contraction and the beginning of 

 the succeeding auricular contraction there is a period during which the 

 whole heart is at rest and during which the cavities are filling with blood. 



FIG. 118. A GRAPHIC RECORD OF (i) THE INTRA-AURICULAR PRESSURE; AND (2) THE INTRA- 

 VENTRICULAR PRESSURE OF THE HORSE. (Chauveau and Marey.) 



For the purpose of obtaining the time of all these events, the recording 

 surface was divided into equal spaces by vertically drawn lines. The rate 

 of movement of the surface was such that each division corresponded to 

 one-tenth of a second. The record thus indicates that the auricular con- 

 traction lasted approximately 0.2 second, the ventricular contraction 0.4 

 second, and the pause 0.4 to 0.6 second. The duration of the cycle was 

 therefore approximately one second. 



From similar experiments made on other animals, e.g., the dog, similar 

 results have been obtained ; but by reason of the employment of more 

 sensitive and more quickly responsive tambours, the curve of the auricular 

 contraction exhibits variations not recorded by the forms of tambour used 

 in earlier experiments. Reference to these variations will be alluded to in 

 subsequent paragraphs. The results obtained by recent observers now 

 generally accepted are in accord with the results obtained by Chauveau and 

 Marey by means of their cardiac tambours as shown in Fig. 118. 



From the similarity in the anatomy of human heart to the anatomy 

 of the horse's heart the inference may be drawn that if the same experiment 

 could be performed on a human being a similar record would be obtained, 

 but owing to the difference in the duration of the cycle in man, viz., 0.80 

 sec. approximately, the time relations of the separate acts would be shorter. 

 As determined by the rule of proportion the auricular systole would 



