A text-book of physiology, for medical students and physicians . the hear! of the horse [Tig. 217].) [twill be seen that the ure in the bearl rises suddenly with the beginning of the ven-tricular contraction and a certain time elapses before this pressure THE HEART BEAT. 527 is strong enough to open the semilunar valves. The moment thatthis occurs (1, on the ventricular curve in Fig. 222) is determinedby simultaneous measurement of the pressure in the aorta, itbeing evident that the pressure will begin to rise in this lattervessel the moment that the valves open. It is interesting to findthat
A text-book of physiology, for medical students and physicians . the hear! of the horse [Tig. 217].) [twill be seen that the ure in the bearl rises suddenly with the beginning of the ven-tricular contraction and a certain time elapses before this pressure THE HEART BEAT. 527 is strong enough to open the semilunar valves. The moment thatthis occurs (1, on the ventricular curve in Fig. 222) is determinedby simultaneous measurement of the pressure in the aorta, itbeing evident that the pressure will begin to rise in this lattervessel the moment that the valves open. It is interesting to findthat the yielding of the valves to the rising pressure in the ventricleis not indicated on the curve itself by any variation,—a fact whichindicates that the valves open smoothly, and are not thrown backwith a sudden shock. A very characteristic feature of the ventric-ular curve is its flat top, or plateau as it is called. In some casesthe plateau slopes more or less upward, in other cases downward,depending, doubtless, on the respective values of the force of the. Fig. 222.—Synchronous record of the intraventricular pressure (V), and the aorticpressure (A): S, The time record,—each vibration = ifc sec.; 0-5, corresponding ordi-nates in the two curves; 1 marks the opening of the semilunar valves; 3 (or shortly after)marks the closure of these valves and the beginning of diastole.—{Hiirthle.) heart contraction and the aortic tension, for during the wholetime of the plateau the semilunar valves are open and the ven-tricle is discharging a column of blood into the aorta. Thedifferent features of the ventricular systole as gathered from thesepressure curves are expressed by Hiirthle * as follows: I. Systole, phase of contraction of the muscle fibers (0 to 3 in Fig. 222, V). (a) Period of tension (0 to 1), during which the auriculo-ventricular andsemilunar valves are both closed and the heart muscle is squeezingupon the contained blood. This period ends at the opening of thesemilu
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