546 CIRCULATION OF BLOOD AND LYMPH. 



taneous measurement of the pressure in the aorta, it being evident 

 that the pressure will begin to rise in this latter vessel the moment 

 that the valves open. The different features of the ventricular 

 systole as gathered from these pressure curves are as follows: 

 The systole of the ventricle exhibits three phases or periods: 1. The 

 period of tension or the isometric period, lasting from the begin- 

 ning of the contraction to the time of opening of the semilunar 

 valves (2 to 3 in Fig. 232). During this period the blood is held 

 in the ventricle, since both the auriculo-ventricular and the semi- 

 lunar valves are closed, but it is squeezed upon by the contracting 

 muscle and the pressure rises very rapidly. When it exceeds the 

 pressure in the aorta the semilunar valves are thrown open, blood 

 begins to flow from the ventricle, and the cavity of the ventricle 

 diminishes in size. The opening of the semilunar valves occurs 5.t 

 (3), and at this point begins the second period, the period of empty- 

 ing. After the semilunar valves are open pressure in the ventricle 

 continues to rise (3 to 4) owing to the fact that the ventricle is dis- 

 charging blood into the aorta more rapidly than it can escape from 

 the aorta into the peripheral vessels. When this relation is reversed 

 the pressiire 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. 232 (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 2;-ray (p. 542). 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. 595) 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 apphed to the heart. Its structure and the con- 

 nections of the recording apparatus are indicated in the diagram 

 given in Fig. 233. The apparatus consists of a rubber ball or glass 



* 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," lOl.'). 



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

 345, 1909, contain also the literature. 



