172 SPECIAL HISTOLOGY. 



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 mem- 

 brane are lodged the cartilages of the bronchia, which, in- 

 stead of being half-rings, are irregular angular plates, distributed 

 around the entire circumference of the tubes. These car- 

 tilaginous 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 l"' in diameter they usually cease to exist (Gerlach 

 would appear to have noticed some even in tubes of £'" in dia- 

 meter). The structure of these cartilages, which are not un- 

 frequently of a reddish hue, is at first exactly like that of the 

 tracheal rings, but in the smaller and smallest of them the dif- 

 ferences between the superficial and deeper cells disappear and 

 the tissue becomes homogeneous throughout, more like the in- 

 terior 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 completely continuous layer, 

 and as they are still seen in twigs of— — ^/" in diameter, pro- 

 bably exist even in the pulmonary lobules. The mucous mem- 

 brane 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'" have only an extremely 

 thin wall. This everywhere consists, externally, of elastic, 

 longitudinal fibres, the bundles formed by which give the cha- 

 racteristic 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 m thick; and thirdly and lastly, of a ciliated 

 epithelium, which, in the larger bronchial tubes and down to 

 those of 1"' in diameter, is distinctly composed of several 

 laminae, but is gradually reduced to a single layer of ciliated 

 cells 0*006'" in length. The bronchiae are at first also fur- 

 nished with racemose glands, even in considerable number, 

 which, however, are wanting in tubes of 1 — 1}. 



In the air-cells, I cannot admit the existence of more than 

 two layers — a fibrous membrane and an epithelium. The for- 

 mer is manifestly the much attenuated mucous membrane and 



