576 



OF RESPIRATION. 



apparatus is restricted to the thorax: but the minute subdivision of their cavity, 

 and the mechanism by which a continual interchange of air is provided for, 

 render them very efficient for their designed purpose. The following, accord- 

 ing to the latest researches, especially those of Mr. Rainey,* appears to be 

 the nature of the ultimate structure of the lungs in Man and the Mammalia 

 in general. The bronchial tubes divide and subdivide, like the branches of a 

 tree, still retaining their ordinary characters, until they are no more than from 

 l-50th to l-30th of an inch in diameter ; and in these the longitudinal and 

 . annular fibres, together with the ciliated epithelium, come to an abrupt ter- 

 mination. Beyond this boundary, the tubular form of the air-passages con- 

 tinued from the bronchi is retained for some distance ; but it is gradually 

 changed by the irregular branching of the passages, and by the increase of 

 the number of apertures in their walls, which lead to the air-cells. Thus, at 

 last, each minute division of the air-passages becomes quite' irregular in form ; 

 air-cells opening into every part of it, and almost constituting its walls ; until 

 it terminates, almost without dilatation, in an air-cell. This terminal portion 

 of the air-passage, with its surrounding cluster of air-cells, may be regarded 

 as forming a sort of lobule, and as representing the entire lung of a Frog or 



[Fig. 218. 



The Larynx, Trachea and Bronchine, deprived of their fibrous covering, and with the outline of the 

 Lungs ; 1, 1, outline of the upper lobes of the lungs ; 2, outline of the middle lobe of the right lung ; 3, 3, 

 outline of the inferior lobes of both lungs ; 4, outline of the ninth dorsal vertebra, showing its relation to 

 the lungs and the vertebral column ; 5, thyroid cartilage ; 6, cricoid cartilage ; 7, trachea; S, right bron- 

 chus ; 9, left bronchus ; 10, crico-thyroid ligament; 11, 12, rings of the trachea; 13, first ring of the tra- 

 chea; 14, last ring of the trachea, which is corset-shaped; 15, 16, a complete bronchial cartilaginous 

 ring; 17, one which is bifurcated ; 19, double bifurcated bronchial rings; 19. 19, smaller bronchial rings; 

 20, depressions for the course of the large blood-vessels.] 



* Medico-Chirurgical Transactions, vol. xxviii. 



