STRUCTURE AND DEVELOPMENT OF HUMAN LUNG. 



[Fig. 219. 



577 



A view of the Bronchise and Blood-Vessels of the Lungs as shown by dissection, as well as the rela- 

 tive position of the Lungs to the Heart ; 1, end of the left auricle of the heart ; 2. the right auricle ; 3, 

 the left ventricle with its vessels; 4, the right ventricle with its vessels; 5, the pulmonary artery ; 6, 

 arch of the aorta ; 7, superior vena cava ; S, arteria innominata ; 9, left primitive carotid artery ; 10, left 

 subclavian artery ; 11, the trachea ; 12, the larynx ; 13, upper lobe of the right lung ; 14, upper lobe of the 

 left lung ; 15, trunk of the right pulmonary artery ; 16, lower lobes of the lungs. The distribution of the 

 bronchia and of the arteries and veins, as well as some of the air-cells of the lungs, is also shown in 

 this dissection.] 



other Reptile ; the whole lung of the Mammal being made up of a multitude 

 of such lobules, which are almost exact repetitions of each other. There is, 

 however, this difference ; that the air-cells in the lung of the Reptile are mere 

 sacculated depressions in the walls of the cavity, opening very freely into it ; 

 whilst the air-cells of each lobule of the lung of the Mammal are arranged 

 around the central passage in such numbers, that the outer ones can only 

 communicate with this passage through the medium of those which are nearer 

 the middle of the cluster. Those cells which communicate directly with the 

 bronchial tubes and intercellular passages, open into them by large circular 

 apertures ; and they are themselves similarly opened into by other cells, which 

 again communicate with others beyond them ; so that each of the openings 

 in the air-passage leads to a series of air-cells, extending from it to the surface 

 of the lobule. These cells have also lateral communications with each other. 

 The walls of the air-cells are formed, of a very thin and transparent mem- 

 brane, which is folded sharply at the orifices of communication, so as to 

 form a very definite border to them; and the capillary plexus is so placed 

 between the two layers, which form the walls of two adjacent air-cells, as to 

 expose one of its surfaces each ; by which provision, the full influence of the 

 air upon it is secured. 

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