THE HEART BEAT. 549 



rises smoothly to a maximum, and then falls off in diastole so that 

 the apex of the curve is rounded and shows no plateau. The re- 

 sults that have accumulated from the use of the more modern in- 

 struments seem to show that under normal conditions the pressure 

 curve does give evidence of a flattened top, as shown in Fig. 

 230a, e to /, although the plateau, so-called, may have an upward 



Fig. 230o. Two curves of intraventricular pressure. Right ventricle, dog's heart. Record 

 B taken with a more sensitive manometer than A; ab, period of auricular systole; bd, period of 

 tension before opening of semilunar valves (isometric period); df, period of emptying (ejection 

 period); fg, closure of the semilunar valves. (Wiggers.) 



or a downward slope according to the conditions of pressure in the 

 aorta. 



The Volume Curve and the Ventricular Output. In man the 

 volume of the heart under different conditions may be studied by 

 means of the x-ray (p. 545). In the lower animals the thorax 

 may be opened with suitable precautions as regards anesthesia 

 and artificial respiration, and the heart may be placed within a 

 plethysmograph (see p. 604) to measure its changes in volume 

 during systole and diastole. If the whole heart is treated in this 

 way the curve of volume changes is complicated by the fact that 

 one chamber, the auricle, is filling, while the other, the ventricle, 

 is emptying. A more useful disposition of the apparatus is to 

 enclose only the ventricles. Several different forms of plethysmo- 

 graph have been devised for this purpose, and they are usually 

 spoken of as cardiometers. The form described by Henderson* 

 is simple and easily applied to the heart. Its structure and the con- 

 nections of the recording apparatus are indicated in the diagram 

 given in Fig. 231. The apparatus consists of a rubber ball or glass 

 chamber with a circular opening at one point. Over this opening is 

 placed a membrane of rubber dam with a central opening through 

 which the heart is introduced, as shown in the diagram. The rubber 



* Henderson, "American Journal of Physiology," 16, 325, 1906, and 23, 

 345, 1909, contain also the literature. 



