MAMMALIA. 683 



position of the arteries and veins of the systemic circulation differ 

 in no material circumstance from what is met with in our own per- 

 sons. 



The lungs, occupying the two sides of the chest, are each con- 

 tained in a distinct chamber, formed by the ribs and diaphragm, 

 without in any part adhering to its walls. Each lung is enclosed 

 in a serous cavity formed by the pleura, which, after lining the 

 ribs, the intercostal muscles, and the thoracic surface of the dia- 

 phragm, is reflected on to the lung itself at the point occupied by 

 the roots of the pulmonic vessels, and invests the entire surface 

 of the viscus ; it moreover passes deeply into those fissures that 

 separate the lung into several distinct lobes. 



In the interspace between the two pleurae, called the medias- 

 tina, is lodged the heart, contained in a fibro-serous envelope (the 

 pericardium) ; and behind this the oesophagus, accompanied by the 

 principal trunks of the vascular system, passes through the thorax 

 into the abdomen. 



(779.) Each lung is a closed bag, composed of innumerable 

 cells that communicate with the terminations of the bronchial 

 tubes, and collectively present an immense surface, over which the 

 blood contained in the capillaries of the pulmonary vessels is made 

 to circulate. 



The inspiration and expiration of air are effected by the alter- 

 nate movements of the diaphragm and of the walls of the thora- 

 cic cavity, whereby the atmospheric fluid is drawn into and ex- 

 pelled from the pulmonary cellules, and is thus constantly renewed 

 as it becomes deteriorated by the abstraction of the oxygen con- 

 sumed during the process of converting the venous into arterial 

 blood. 



The purified blood, after passing through the pulmonary capil- 

 laries, is collected in an arterialized condition by the pulmonary 

 veins, and conveyed to the systemic side of the heart, which offers 

 the same arrangement throughout the entire class, consisting of an 

 auricular chamber (Jig. 317, c), and of a very muscular ventricle, 

 a, the auricula-ventricular opening being guarded by mitral valves 

 and columns earner similar to those found in the human heart. 

 From the left ventricle the blood is driven into the aorta, e, the 

 commencement of which is guarded by three semilunar valves, and 

 thus it passes through the entire system. 



When again collected from the periphery of the body, the now 

 vitiated fluid is returned to the heart by the venous system, and 



