THE HEART BEAT. 



553 



the pressure in the ventricle falls. The period of emptying closes 

 when the ventricles cease contracting and relaxation begins. The 

 third period, the period of relaxation or the period of diastole, be- 

 gins with the closure of the semilunar valves (5). From this 

 point the pressure within the ventricle falls very rapidly. The 

 time relations of the pressure changes in the left ventricle and the 

 aorta are indicated in Fig. 230a (Wiggers).* 



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. 548). 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. 608) 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 



;*+ ^-a 



Fig. 231. Diagram to show the arrangement of the Henderson cardiometer. The 

 recording tambour is inverted, so that the systole will give an up-stroke on the curve. 

 (After Hirschf elder.) 



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 cardiometer 's. The form described by Henderson f 

 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 



* For discussion, see Tigerstedt, "Skandinavisches Archiv f. Physiol.," 

 28, 37, 1912; also Wiggers, "American Journal of Physiology," 33, 382, 1914, 

 and "Circulation in Health and Disease," 1915. 



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

 345, 1909. contain also the literature. 



