2O4 



THE CIRCULATORY SYSTEM. 



[CH. XVIII. 



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. 



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

 divided by a longitudinal partition in such a manner as to form 

 two chief chambers or cavities right and left. Each of these 

 chambers is again subdivided transversely into an upper and a 



Diaphragm. 



Fig. 207. 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 removed. The lungs 

 are partly collapsed. 



lower portion, called respectively, aiiricle and ventricle, which 

 freely communicate one with the other ; the aperture of communi- 

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

 blood to pass freely from the auricle into the ventricle, but not in 

 the opposite direction. There are thus four cavities in the heart 

 the auricle and ventricle of one side being quite separate from 

 those of the other (figs. 208, 209). 



Right Auricle. The right auricle is situated at the right 

 part of the base of the heart in front. It is a thin walled 

 cavity of more or less quadrilateral shape, prolonged at one corner 

 into a tongue-shaped portion, the right auricular appendix, which 

 slightly overlaps the exit of the great artery, the aorta, from 

 the heart. 



