798 MAMMALIA. 



(2327.) The lymphatic system of Mammals, as far as it has been 

 studied, conforms in its arrangement to that of Man. 



(2328.) Neither will it be at all necessary to describe at any length 

 the construction of the respiratory and circulatory organs in the class 

 now under consideration, seeing that the structure of the lungs, the 

 mechanism of respiration, the arrangement of the pulmonary vessels, the 

 cavities of the heart, and the general disposition of the arteries and 

 veins of the systemic circulation differ in no material circumstance from 

 what is met with in the human subject. 



(2329.) The lungs, occupying the two sides of the chest, are each 

 contained 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 diaphragm, is re- 

 flected 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 more- 

 over passes deeply into those fissures that separate the lung into several 

 distinct lobes. 



(2330.) In the interspace between the two pleurae, called the me- 

 diastina, is lodged the heart, contained in a fibre-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. 



(2331.) 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. 



(2332.) The inspiration and expiration of air are effected by the alter- 

 nate movements of the diaphragm and of the walls of the thoracic cavity, 

 whereby the atmospheric fluid is drawn into and expelled from the 

 pulmonary cellules, and is thus constantly renewed as it becomes dete- 

 riorated by the abstraction of the oxygen consumed during the process 

 of converting the venous into arterial blood. 



(2333.) The purified blood, after passing through the pulmonary 

 capillaries, 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 (fig. 403, c) and of a very muscular ventricle (a), the auriculo- 

 ventricular opening being guarded by mitral valves and columnce carnece, 

 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. 



(2334.) When again collected from the periphery of the body, the 

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



