34 TffE MECHANISM OF THE CIRCULATION. 



the standing position, the apex beat shifts downwards for about \ to 

 1 in., and by the same amount to the left. 



When a man rolls over on to his right side, the seat of the impulse 

 moves to the right, and may completely disappear beneath the sternum. 

 It has been observed to shift 3f in. from left to right. Similarly, when 

 a man rolls over on to his left side, the impulse is found considerably 

 to the left side of the nipple line (see Fig. 22). 



This shifting of the impulse is due to the effect of gravity on the 

 heart within the pericardium. On passing needles through a certain 

 given spot in the thoracic wall into the heart, with the body placed in 

 the three positions supine, on the right, and on the left side the needles 

 are found to pierce the heart at different places. In each position a 

 different part of the heart, owing to the weight of the organ, is brought 

 into close contact with the chest wall. The heart rotates in the peri- 

 cardium. This membrane is too strong to allow of much lateral dis- 

 placement of the whole heart. 



If the finger be pressed up against the apex, it is shot down by the 

 systole. If the whole heart be grasped by the hand, the fingers sink into 

 the flaccid walls during diastole, but are driven asunder with great force 

 by systole. Wherever the heart is touched it appears to drive the finger 

 outward, and yet the heart diminishes in two if not in all its diameters. 



The origin of the impulse has been attributed to many factors (1) 

 That the curvature of the aorta is straightened out by the blood shot 

 therein. Since the heart is attached to the aorta, the apex is forced to 

 describe an arc of a circle and impinge against the chest wall. (2) That 

 the diastolic filling of the left auricle, which lies between the spine and 

 the base of the heart, pushes the heart forward. 1 Haller experimentally 

 showed that injection of the left auricle caused the heart to " approach 

 the mamma with vivacity." (3) That the systolic out- thrust of the 

 auriculo-ventricular valves raises the pressure in the left auricle, and 

 thus drives the heart forward. 



A more reasonable suggestion, and one put forward among others 

 by Harvey, is (4) that the heart erects itself from the collapsed 

 position, which it assumes in diastole. A fulcrum for this movement 

 is afforded by the base resting on the pericardium (Ludwig). 2 In 

 consequence of this erection, the heart strikes the chest wall. This 

 explanation is sufficient for the conditions which exist when the body 

 is supine or vertical, but it is scarcely satisfactory when the body is 

 prone, for then the heart is in apposition with the thoracic wall, and the 

 impulse is most diffuse and intense. (5) The recoil of the heart, due to 

 the expulsion of the blood into the aorta, and the straightening out of 

 the spiral figure formed by the aorta and pulmonary artery, have 

 respectively served their turn as an explanation of the cardiac impulse. 3 

 These suggestions are at once negatived by the fact that the output 

 of blood from the ventricle occurs later than the commencement of 

 the impulse. (6) During the period of rising tension within the ven- 

 tricle, the blood presses upon every point of the internal surface with 

 a force equal to that by which it is itself compressed. The pressure 

 of the blood reacts thus on the wall, for the heart contracts against the 



1 Senac, "Traite de la structure du cceur," Paris, 1749, p. 356. 



2 Ludwig, "Lehrbuch d. Phys.," Leipzig, 1861, Bd. ii. S. 86. 



3 Skoda, "Abhandl. ueber Percussion und Auscultation," Wien, 1842, S. 147; Kornitzer, 

 Sitzungsb. d. k. Akad. d. IVisscnsch., math.-natunv. C7.,AVien, 1857, Bd. xxiv. S. 120. 



