117 



level and sinks again. Get a tracing- of this blood pressure 

 curve upon a very slowly revolving drum. The actual pres- 

 sure, in millimeters of mercury, is obtained by multiplying 

 the mean height of the curve, above the atmospheric line, by 

 two. 



368. Stannius's Experiments on the Frog's Heart. 

 Some of the early and important experiments relating to the 

 beat of the frog's heart were performed by Stannius, and bear 

 his name. 



If the sinus venosus is separated from the rest of the heart 

 by a ligature of thread passed under the aorta and drawn 

 tightly around the sinus at its junction with the auricle, .the 

 sinus venosus continues to pulsate, but the auricles and ven- 

 tricle are quiescent. If the auricles are now separated from 

 the ventricle by a thread ligature tied around the auriculo- 

 ventricular groove, the auricles remain motionless, but the 

 ventricle begins to beat, so that the sinus venosus and ventricle 

 are pulsating, but with a different rhythm, while the auricles 

 are at rest. The rate of the ventricular beat is usually much 

 slower than that of the sinus. 



Fig. 29. Aur, auricle ; V, ventricle ; S V, Sinus Venosus. The figure to 

 the left shows the appHcation of the Hgature between the sinus and the auricle. 

 In the figure to the right there is shown the second ligature between the auricle 

 and ventricle. 



The quiescence of the auricles and ventricle, in the first 

 case, has been supposed to show that the motor centers for the 

 entire heart reside in the sinus, and that from them the motor 

 impulses originate which keep up the rhythmical pulsations 

 of the organ. But the fact that the ventricle begins to pul- 

 sate on its own account, as in the second case, when separated 



