LUNGS. 



[ 402 ] 



mogeneous, but does not form a separable 

 basement layer or membrane. It contains 

 a number of small racemose glands, the ve- 

 sicles of which are lined with pavement-, 

 the ducts with cylindrical epithelium. Its 

 surface is covered with ciliated epithelium, 

 agreeing in structure with that of the trachea. 



Trachea and larger bronchi. The incom- 

 plete cartilaginous rings of these tubes are 

 surrounded and connected together by a 

 firm, elastic, fibrous membrane, forming 

 their perichondrium, which also covers the 

 posterior part of the tubes as a somewhat 

 thinner layer. The cartilage is of the true 

 kind. At the posterior part of the tubes is 

 a layer of unstriated muscular fibres, most 

 of which form transverse, but a few longitu- 

 dinal bundles. The elastic tissue of the 

 mucous membrane is greatly developed, 

 forming a distinct internal layer of princi- 

 pally longitudinal anastomosing fibres. 



The epithelium is ciliated, and consists of 

 several layers. 



Fig. 421. 



Epithelial cells of the trachea in situ ; human. , lon- 

 gitudinal elastic fibres ; b, homogeneous outer (basement) 

 layer of the mucous membrane ; c, deep layers of round 

 cells ; d, intermediate layers ; e, outer ciliated cells. 



Magnified 350 diameters. 



The deepest layers consist of roundish 

 cells with distinct rounded nuclei, those 

 succeeding being elongated, whilst those 

 next the surface are still longer, greatly 

 narrowed at the base, and with oval nuclei ; 

 these forms are most distinct in the detached 

 cells. 



Those of the last row are covered with 

 vibratile cilia. 



The smaller bronchi differ somewhat in 

 structure from the larger. Thus the carti- 

 lage forms angularplates distributed through- 

 out their circumference, while the elastic 

 and areolar coats become thinner, and the 

 transverse muscular fibres smaller and less 

 closely placed ; the latter probably extend as 



LUNGS. 

 Fig. 422. 



Isolated epithelial cells from the surface of the trachea; 

 human. 



Magnified 350 diameters. 



far as the air-cells. The ciliated epithelium 

 extends to the termination of the bronchi, 

 forming, however, a single layer only of cells 

 in the smaller ones. 



The walls of the pulmonary air-cells con- 

 sist of two layers, a fibrous and an epithelial 



Fig. 423. 



Air-cells of a human lung, a, epithelium ; b, fibrous 

 portion, where the walls of several air-cells are confluent ; 

 c, thinner walls of air-cells. 



Magnified 350 diams. 



