246 CLINICAL APPLIED ANATOMY. 



The triangular area of superficial cardiac dulness corresponds 

 to that part of the pericardium which is uncovered by lung. 

 Its limits are the fourth costal cartilage above ; the impulse of 

 the heart externally; and internally, for all practical purposes, 

 the left edge of the sternum. The extension of this dull area in 

 pericarditis is due to distension of the sac, and gradual retraction 

 outwards of the lung margins. It is stated that the costo- 

 mediastinal pleural reflections do not always move outwards with 

 retreating lung margins, in such cases the parietal pleura still 

 covers the pericardium in front. The normal upper limit of the 

 pericardial sac corresponds to the mid-point of the manubrium 

 sterni, this being the summit of a tubular prolongation of the 

 pericardium common to the ascending portion of the aorta and 

 the pulmonary artery up to its point of bifurcation. (Fig. 26, 

 p. 249.) In moderate pericardial effusions the area of percussion 

 dulness is pyriform in outline, the upper narrower extension 

 corresponds to this tubular prolongation, the lateral limits corre- 

 spond with, or may pass beyond, the lateral limits of the heart 

 as projected on the chest wall, and the lower limit is lower than 

 the inferior border of the heart, for just as occurs in the case of 

 the pleural sac, there is a reserve space in the pericardial sac 

 and this space lies in front and below, so that the line along 

 which the pericardium leaves the upper surface of the diaphragm 

 in front crosses a little below the base of the ensiform cartilage. 



The pericardial sac when distended, exercises pressure on the 

 heart, and also on the structures in relation with it externally. 

 The low pressure in the auricles and pulmonary veins and the 

 comparative thinness of their walls, renders them more suscep- 

 tible to pressure than are the other parts within the sac. Of the 

 parts outside the sac the lungs suffer most easily. Pressure on the 

 lower part of the left lung often gives rise to signs near the inferior 

 angle of the left scapula, simulating pneumonia. Occasionally 

 similar signs appear in the fourth and fifth interspaces of the 

 right side near the sternum, and correspond in situation with 

 the anterior part of the middle lobe of the right lung. Disten- 

 sion of the sinus obliquus which lies between the points of entry 



