CH. XIX.] 



THE HEART 



209 



The pericardium is a comparatively simple serous membrane, because the 

 organ it encloses is a single one of simple external form. All serous membranes 

 are of similar structure ; thus the pleura which encloses the lung, and the peritoneum 

 which encloses the abdominal viscera differ from it only in anatomical arrangement. 

 The great complexity of the peritoneum is due to its enclosing so many organs. 

 Every serous membrane consists of a visceral layer applied to the organ or organs 

 it encloses ; and a parietal layer continuous with this in contiguity with the parietes 

 or body-walls. 



Bight 



Lung 



Pulmonary 

 Artery. 



Left Lung. 





The Chambers of the Heart. The interior of the heart is 

 divided by a longitudinal partition into two chief cavities right and 

 left. Each of these chambers is again subdivided transversely into 

 an upper and a lower portion, called respectively, auricle and ventricle, 

 which freely communicate one with the other ; the aperture of com- 

 munication, however, is guarded by valves, so disposed as to allow 

 blood to pass freely from 

 the auricle into the ven- 

 tricle, but not in the oppo- 

 site direction. There are 

 thus four cavities in the 

 heart the auricle and ven- 

 tricle of one side being 

 quite separate from those 

 of the other (figs. 191, 192). 



The right auricle is a 

 thin -walled cavity of quad- 

 rilateral shape, prolonged 

 at one corner into a tongue- 

 shaped portion, the right 

 auricular appendix, which 

 slightly overlaps the exit of 

 the aorta, from the heart. 



The interior is smooth, 

 being lined with the general 

 lining of the heart, the 

 endocardium, and into it 

 open the superior and inferior venae cavae, or great veins, which 

 convey the blood from all parts of the body to the heart. The 

 opening of the inferior vena cava is protected and partly covered by 

 a membrane called the Eustachian valve. In the posterior wall of the 

 auricle is a slight depression called the/ossa ovalis, which corresponds 

 to an opening between the right and left auricles which exists in 

 foetal life. The coronary sinus, or the dilated portion of the coronary 

 vein, also opens into this chamber. 



The right ventricle occupies the chief part of the anterior surface 

 of the heart, as well as a small part of the posterior surface. It 

 takes no part in the formation of the apex. On section after death 



O 



Diaphragm. 



FIG. 190. View of heart and lungs in situ. The front 

 portion of the chest-wall and the outer or parietal 

 layers of the pleurne and pericardium have been re- 

 moved. The lungs are partly collapsed. 



