STRUCTURE OF THE LUNGS OF MAMMALS. 



57 



not exceed one millimeter, is in the structure of the outer fibrous 

 layer; for independently of the almost total absence of 

 cartilage and of mucous glands, this becomes so remarkably 

 attenuated in the smallest bronchia, that its thickness in Man 

 does not exceed from 0'4 to 0'02 of a millimeter, and near 

 their terminations almost entirely disappears. The external 

 fibrous layer is here composed of longitudinal fasciculi of con- 

 nective tissue, with a few elastic fibres interspersed amongst 

 them, and running in the same direction. The succeeding 

 muscular layer, with its circularly disposed smooth muscular 

 fibres, becoming gradually thinner, breaks up ultimately in the 

 finest bronchia into isolated circular bands, separated by 

 intervals of greater or less width, which often consist of only 

 a single layer of muscular-fibre cells interwoven with fine 

 elastic fibres, that pursue a similarly circular direction. 



The compact bundles of longitudinal elastic fibres, so charac- 



Fig. 129. 



Fig. 129. Part of a transverse section of a bronchial tube from the 

 Pig, having a diameter of 0'4 of a millimeter. Magnified 240 diame- 

 ters, a, External fibrous layer ; 6, muscular layer ; c, internal 

 fibrous layer ; d, epithelial layer ; /, one of the surrounding alveoli. 



teristic of the inner fibrous coat of the larger bronchi, are con- 

 tinued into the smaller branches, where they run in the form 

 of closely compressed fasciculi with a clear delicately fibrous 

 basement membrane on the inner side of the tunica muscularis, 



