ORGANS OF RESPIRATION. 379 



AERTFEROUS APPARATUS. 



The aeriferous apparatus of the lungs consists of bronchial 

 tubes and air cells. 



Bronchial Tubes. The bronchial tubes are formed 

 of cartilaginous rings united by - elastic tissue : the rings, 

 however, are imperfect, and are only present in the larger 

 tubes, as those of the first, second, and third or fourth dia- 

 meters : in the smaller tubes they are absent, these being- 

 composed entirely of a nucleated form of fibro -elastic tissue : 

 they divide repeatedly in a dichotomous manner : it is not 

 easy, however, to determine the number of divisions which 

 each bronchial tube undergoes after its entrance into each 

 lobule : it would appear, however, that the intra-lobular 

 ramifications are less numerous than the interlobular branch- 

 ings, the bulk of the lobule consisting chiefly of air cells. 



The bronchial tubes are lined throughout by mucous mem- 

 brane, which is invested by a stratum of cylinder ciliated 

 epithelium. The smaller bronchial tubes are eminently elastic 

 and contractile by virtue principally of the elastic tissue of 

 which they are chiefly formed. This elasticity is still fur- 

 ther increased in the case of the larger tubes by the presence 

 of muscular fibrilke of the unstriped kind. 



Air cells. The air cells are minute cavities of variable 

 size, of an angular form, not dilated, which communicate 

 freely with each other and the parieties of Avhich are formed 

 of a diaphanous, tough, highly elastic and delicately fibrous 

 membrane, which contains and supports the capillary blood- 

 vessels, and the interior of which is lined by a modified 

 mucous membrane also invested with a layer of ciliated epi- 

 thelium. 



From this description, it becomes evident that the air cells 

 contain the same structural elements as the smaller bronchial 

 tubes- themselves, and that they therefore differ from these 

 solely in form and arrangement, possessing precisely the same 

 general physical properties, being in the same manner highly 

 elastic, a property mainly dependent upon the abundance 



H H 



