SECT. 177.] STRUCTURE OF THE BRONCHIAL TUBES. 385 



partly possessing cartilages, and a mucous coat, in which is a layer 

 of smooth muscular fibres. The outer coat, formed of connective 

 tissue and elastic fibrils, is as thick on the first bronchia within the 

 lung as in the two main bronchi from which they spring. It 

 becomes, however, gradually more and more attenuated, and is 

 scarcely demonstrable with the knife on bronchi below %"' in 

 diameter. Traced onwards, this coat is found to coalesce eventually 

 with the mucous membrane and the loose connective tissue, which 

 connects the bronchi with the pulmonary parenchyma. In it are 

 seated the cartilages of the bronchia, which, instead of semicircles, 

 are here irregular angular plates, distributed over all parts of the 

 circumference of the tube. These plates are at first large and 

 closely disposed, but become further apart from each other 

 where the bronchial tubes send off branches, becoming pro- 

 gressively smaller and smaller, till at length they disappear from 

 bronchia below %" in diameter. This is certainly the rule; but 

 Gerlach believes he has seen them upon bronchia I ^' // in dia- 

 meter. These cartilages are not unfrequently reddish, and their 

 structure, at first, exactly resembles that of the tracheal rings ; as 

 the cartilages become smaller, the differences between the super- 

 ficial and deeper cells disappear, and the tissue becomes alike 

 throughout, and resembles the interior portion of the larger car- 

 tilages. The muscular fibres are observed in bronchia of all sizes, 

 forming flat fasciculi, which completely encircle the tube, and form 

 a perfectly unbroken layer; in very old people, however, spaces 

 are found to exist between the fibres. These muscular fibres have 

 been observed upon branches ~^' ,r to ^" in diameter, and, there- 

 fore, probably occur even up to the pulmonary lobules. — Intimately 

 connected with the muscular coat is the mucous membrane, which, 

 at first, has the same thickness as in the trachea; but this, like 

 the other component coats, gradually becomes attenuated, so that 

 bronchia below half a line in diameter have altogether only a very 

 thin wall. At its attached surface, this membrane is composed 

 throughout of elastic longitudinal fibres, whose bundles give to 

 the inner surface of the bronchi the characteristic longitudinally 

 striped appearance, and also give rise to a longitudinal folding of 

 the mucous membrane more or less distinct. Secondly, there is a 

 homogeneous layer, o - oo2'" to 0-003" ^ n thickness, on which is 

 situated, thirdly, the ciliated epithelium. This epithelium in the 

 larger bronchia, as far as those of \" in diameter, is composed of 

 several layers, but gradually becomes reduced to a single layer of cili- 

 ated cells, o - oo6'" in length (fig. 8, p. 36) . — The bronchial tubes possess, 



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