540 



BESPIRATOBY APPARATUS IN MAMMALIA. 



Form. — The bronchial tubes are not flattened like the trachea ; a transverse 

 section shows them to l^e regularly cylindrical. 



Volume. — The left bronchus is always smaller than the right, owing to the 

 left lung being the least : and both are much inferior in volume to the aggregate 

 of their respective branches. 



Relations. — Each bronchus enters the pulmonary lobe, or lung, along with 

 the blood-vessels, Avith which it forms what is called the root of the lung. The 

 divisions of this aborescent trunk are accompanied by the bronchial artery, vein, 

 and nerves, which ramify in the same manner. 



Fig. 320. 



A SMALL BRONCHIAL TUBE, WITH ITS BRONCHULES AND ULTIMATE RAMIFICATIONS. 



Near their origin, the bronchi are related to the bronchial glands, above 

 which, and to the left side, passes the oesophagus. 



Structure. — The structure of the bronchial tubes resembles that of the 

 trachea ; their walls being formed by cartilages, a muscular layer, mucous 

 memlrranp, and vessels and nerves. 



Cartilages of the hronchi. — These only exist in tubes of a certain calibre, the 

 minute tubes being deprived of them, and having only membranous walls. As 

 in the trachea, there is for each tube a series of transverse rings joined border 

 to border ; though these are no longer formed of a single ring, but each results 

 from the union of several lozenge-shaped pieces, the extremities of which over- 

 lap ; they are united to each other, like the segments of the neighbouring rings, 

 by means of connective tissue, and also by the membranes spread over their 

 internal surface. 



