THE CAEDIAC IMPULSE. 247 



of the pulmonary veins gives rise to some difficulty in swallowing, 

 for the oesophagus lies immediately behind this part of the sac. 

 The vagi are in close contact with the oesophagus and some 

 authors refer the vomiting which occasionally occurs in peri- 

 carditis to pressure on these nerves. The weight of fluid in the 

 sac tends to depress the diaphragm, but the heart does not 

 travel downwards since it is fixed to the upper and posterior part 

 of the sac by the veins and arteries. 



The tendency of pericardial friction to persist along the line of 

 the sternum is due to the fact that there is no spare room for 

 antero-posterior distension, the heart being interposed between 

 the projecting spinal column and the sternum. Distension tends 

 rather to occur laterally and downwards below the cardiac level. 

 In tapping the pericardium the pleura may be avoided by 

 making the puncture through the fifth or sixth intercostal space 

 of the left side close to the sternal edge, i.e. internal to the 

 internal mammary vessels ; but since it is often a matter of great 

 difficulty to distinguish a dilated heart from a distended pericar- 

 dium, it is safer to proceed surgically by resection of cartilage 

 and exposure of the sac before puncturing it, or the under aspect 

 of the sac may be reached through the abdominal wall in the 

 subcostal angle without opening the peritoneum. 



VALVULAR DISEASE OF THE HEART. 



An accurate conception of the effects of the various valvular 

 lesions on the heart can only be formed by a knowledge of its 

 surface anatomy. The normal position of the cardiac impulse, 

 or apex beat, the outlines of the cardiac chambers as projected 

 on the anterior chest wall, and the positions of the various valves, 

 must be thoroughly understood. (Fig. 26, p. 249.) 



The cardiac impulse varies in position with age and with the 

 posture of the body. In the infant the heart lies higher and is 

 more horizontal than in the adult, possibly on account of the 

 higher position of the diaphragm. The impulse is consequently 

 found in the fourth intercostal space, but descends into the fifth 



