578 SPECIAL HISTOLOGY. 



177. The intimate structure of the bronchia? and air-cells is as fol- 

 lows. The bronchia; are in general constituted in the same way as the 

 air-tubes and their branches, although from the very commencement 

 some differences are presented, which become greater and greater in 

 their further course. It is most proper to distinguish in them two 

 membranes, a fibrous, still in part containing cartilages, and a mucous, 

 with a smooth muscular layer. The former, constituted of connective 

 tissue and elastic fibrils, is at first thick, as in the bronchi, but gradually 

 becomes thinner and thinner ; in bronchia? less than J- a line in dia- 

 meter, it is scarcely demonstrable with the scalpel ; and ultimately, in 

 their terminations, it coalesces with the mucous membrane, and the 

 more lax connective tissue uniting the bronchia? with the parenchyma 

 of the lung. In this membrane are lodged the cartilages of the bron- 

 chice, which, instead of being half-rings, are irregular angular plates, 

 distributed around the entire circumference of the tubes. These carti- 

 laginous plates, at first large and closely approximated, are soon more 

 widely separated at the points of origin of the branches, and become 

 smaller and smaller, until finally in tubes under J a line in diameter 

 they usually cease to exist (Gerlach would appear to have noticed some 

 even in tubes of t^-of a line in diameter). The structure of these carti- 

 lages, which are not unfrequently of a reddish hue, is at first exactly like 

 that of the trachea! rings, but in the smaller and smallest of them the 

 differences between the superficial and deeper cells disappear and the 

 tissue becomes homogeneous throughout, more like the interior of the 

 larger cartilages. In the largest bronchia? the muscles present the form 

 of circular flattened fasciculi, which, except in old people, in whom 

 larger and smaller interstices occur between them, constitute a com- 

 pletely continuous layer, and as they are still seen in twigs of 1-10-1-12 

 of a line in diameter, probably exist even in the pulmonary lobules. The 

 mucous membrane is intimately united with the muscles, and at first is 

 of the same thickness as in the trachea, but this is gradually reduced, so 

 that tubes of less than J a line have only an extremely thin wall. This 

 everywhere consists, externally, of elastic, longitudinal fibres, the 

 bundles formed by which give the characteristic longitudinally striped 

 aspect to the inner surface of the bronchia?, and also produce a less 

 distinct longitudinal plication of the mucous membrane ; secondly, of a 

 homogeneous layer 0*002-0'003 of a line thick ; and thirdly and lastly, 

 of a ciliated epithelium, which, in the larger bronchial tubes and down 

 to those of 1 line in diameter, is distinctly composed of several laminse, 

 but is gradually reduced to a single layer of ciliated cells 0-006 of a 

 line in length. The bronchise are at first also furnished with racemose 

 glands, even in considerable number, which, however, are wanting in 

 tubes of 1-1 J lines. 



