20 THE MECHANISM OF THE CIRCULATION. 



on the outside of the ventricle assume the shortest position which is 

 possible without change of the cavity, and therefore the external form 

 of the ventricle becomes more spherical in shape. As the tension rises 

 higher, the blood in the coronary vessels becomes more and more 

 expelled from the musculature of the heart. Chauveau and Marey took 

 synchronous curves of the intraventricular and aortic pressures. They 

 found the rise of aortic pressure occurred '1 sec. later than the beginning 

 of the systolic curve. 1 



This, therefore, was the period of rising tension in the horse. The 

 whole ventricular contraction, up to the point of steep descent at the 

 end of the plateau, covered a period of 4 sec. 



With improved apparatus Hiirthle found the time of rising tension 

 to be considerably shorter ; in the dog it was '02 to '04 sec. 2 



If the aortic pressure be very low this period becomes even 

 shorter. When the aortic pressure was high, Hiirthle found no constant 

 elation between the height of the pressure and the duration of the 



FIG. 14. A, aortic, and V, intraventricular pressure curves ; 0, abscissa; S, time trace = 

 . ; 0-5, corresponding ordinates. Hiirthle. 



period of rising tension. On taking measurements of the difference 

 between the diastolic pressure in the ventricle and the aortic pressure, 

 Hiirthle determined that with a pressure difference of 58 to 60 mm. 

 Hg, the period of rising tension was '017 sec., while it was '03 sec. 

 with a pressure difference of 111 mm. Hg. The variations of the 

 period of rising extension are therefore most insignificant, and 

 remain so even when the rhythm of the heart is altered by section of 

 the vagi or excitation of the accelerator nerves. This is equivalent to 

 saying that the heart can meet great demands on its powers, without 

 any appreciable loss of time. 



The period of systolic output. With what smoothness the cardiac 

 mechanism works is strikingly evidenced by the fact that neither the 

 opening nor closing of the valves produces any peak or point on the 

 curves of intracardiac pressure. Similarly, the ventricle fills from the 

 auricle so smoothly, that the auricular systole is scarcely marked on 

 the intraventricular pressure curve. 



The point on the intraventricular pressure curve at which the 

 semilunar valves shut, has been a subject of much discussion. The 

 moment when the second sound of the heart is heard has been 

 marked with a signal on tracings of such curves, and this, after 



1 Chauveau and Marey, Mem. Acad. de mtd., Paris, 1863, tome xxvi. p. 305. 



2 Arch. f. d. ges. PhysioL, Bonn, 1891, Bd. xlix. S. 59. 



