CHAP, iv.] THE VASCULAR MECHANISM. 247 



2. The escape of .the contents of the ventricle into the aorta, 

 the contractions of the ventricular walls still continuing. 



3. Further maintenance of the contraction for some little 

 time after the main body, at all events, of the contents have 

 passed the aortic valves ; by this the complete emptying of the 

 ventricle seems assured. 



4. Sudden and rapid relaxation of the ventricular walls. 

 These four events together make up a large portion, and in a 



quickly beating heart the greater portion, of the whole cardiac 

 cycle. 



Meanwhile, that is during the time from b' to a, blood has 

 been flowing from the great veins into the auricle; during the 

 interval from b' to d none of this can pass into the ventricle since 

 this is still contracted, but with the commencement of relaxation 

 from d onwards there is no longer any obstacle, on the contrary, 

 as we shall see an inducement for the blood to pass from the 

 auricle into the ventricle. 



For a brief time, as we have seen, there is probably an unbroken 

 flow from the great veins (pulmonary or venae cavse) through the 

 auricle into the ventricle, leading to a steady but slight increase 

 of the front-to-back diameter, to a slight pressure of the apex on 

 the chest-wall, and to a slight increase of intraventricular pressure, 

 especially shewn in the curve of the slowly beating heart of the 

 horse (Fig. 38). In Fig. 40, the sudden rise due to the ventricular 

 systole is preceded by a rise b followed by a fall, forming thus, as 

 it were, a shoulder on the curve. This has been interpreted as 

 indicating the sharp transient auricular systole ; the sudden in- 

 jection of the auricular contents into the ventricle increases the 

 front-to-back diameter of the ventricle, and the momentum of 

 the rapid stroke being considerable, the lever is in each case 

 carried too far forward, so that the rise is followed by a fall, 

 producing a notch. A similar though somewhat different shoulder 

 is also seen in the cardiogram Fig. 41. In the curve of ventri- 

 cular pressure taken by means of the cardiac sound (Fig. 38) there 

 is a similar temporary increase b' in the ventricular pressure coin- 

 cident with the auricular stroke 6, and in the " piston " pressure 

 curve of the rapidly beating heart (Fig. 39 A) there is a similar 

 shoulder b just preceding the rise of the ventricular systole. The 

 meaning of the last curve is however doubtful, for in the similar 

 curve of the more slowly beating heart (Fig. 39 B) it occurs 

 immediately after the relaxation of the ventricle, some time 

 before the occurrence of the auricular systole, and in many curves 

 taken by the same method is absent altogether. The exact mean- 

 ing therefore of the shoulder b in the other curves must be left at 

 present undecided. 



135. We have still to consider the negative pressure shewn 

 by the minimum manometer. This instrument, as we have said, 



