292 



THE MECHANICS OF THE HEART 



an excised segment of aorta by quickly forcing a column of water through the 

 semilunar orifice toward the ventricular cavity. 



The third sound is diastolic in its nature and occurs 0.13 sec. after 

 the beginning of the second. It is soft and low in pitch. Two causes 

 have been assigned to it. As it appears to follow in the wake of the 

 second, Einthoven has suggested that it is dependent upon the after- 

 vibration of the closed semilunar. valves. It is also claimed that it 

 is due to the vibration of the auriculoventricular valves^ which are 

 opened at this moment of diastole, and to the friction-noises occasioned 

 by the blood as it rushes int6 the ventricles {d). 



D. THE CHANGES IN INTRACARDIAC PRESSURE 

 The Filling of the Heart 



Methods of Registration. — By the term intracardiac pressure is 

 meant the pressure to which the blood is subjected while traversing 



the different chambers of the heart. 

 To begin with, it is to be noted that 

 the general character of the pressure 

 variations in the auricles is quite 

 different from that of the variations 

 taking place in the ventricles, but that 

 the two ante-chambers as well as the 

 two main chambers show an almost 

 complete correspondence. In addition 

 it should be remembered that the 

 former develop equal degrees of pres- 

 sure, while the latter do not, because 

 the pressure encountered in the left 

 ventricle, is much higher than that pre- 

 vailing in the right. 



The methods employed to determine 

 the intracardiac pressures may be ar- 

 ranged in two groups, the first em- 

 bracing those procedures which are 

 practicable only when the heart is fully 

 exposed to the view, and the second, 

 those which are also practicable when 

 the chest is still closed. In the first 

 instance, the cardiac chamber is con- 

 nected directly with a manometer. En- 

 trance to the auricular cavity is effected 

 through its appendage into which a cannula may be inserted without 

 causing the slightest disturbance in the heart's action. The right 

 auricular cavity may also be reached by introducing a hollow probe 

 through the superior vena cava, and the left cavity by introducing 

 1 Thayer and Gibson, Boston Med. and Surg. Jour., 1908. 



Fig. 148. — Schema to Illus- 

 trate THE Method of Recording 

 THE Blood Pressure in the Right 

 Auricle and Ventricle. 



A probe (<S) filled with saline 

 solution, is inserted through the ext. 

 jug. vein. The tambour {T) regis- 

 ters the pressure upon a kymograph 

 {K). The connecting tubing is 

 equipped with a stop-cock or clip 

 (C). 



