CH. XVIII.] 



THE HEART 



207 



Tulmoiiary 

 Artery. 



Left Lung. 



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. 



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 ^s 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. 217, 218). 



The right auricle is situ- 

 ated at the right part of the 

 base of the heart in front. 

 It is a thin walled cavity 

 of quadrilateral shape, pro- 

 longed 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 cavse, or great veins, 

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

 The opening of the inferior 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 fossa ovalis, which corre- 

 sponds to an opening between the right and left auricles which 

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

 left 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 forms 

 the right margin of the heart. It takes no part in the formation of 

 the apex. On section its cavity, in consequence of the encroachment 



Larynx 



Trachea 



Aorta - 



Right 

 Lung 



Diaphragm. 



Fio. 216. View of heart and lungs in situ. The front 

 portion of the chest-wall and the outer or parietal 

 layers of the pleurae and pericardium have been re- 

 moved. The lungs are partly collapsed. 



