LUNGS. 



[ 478 ] 



LUNGS. 



not all possess the same minute structure. 

 The thyroid and cricoid cartilages consist of 

 hyaline cartilage, the basis being homoge- 

 neous, and containing disseminated cartilage- 

 corpuscles. The walls of the corpuscles are 

 usually thick. The basis often becomes 

 fibrous, and both corpuscles and basis in- 

 crusted with calcareous salts, or completely 

 ossified. Their perichondrium is firm, and 

 is composed of connective tissue, with fine 

 elastic fibres, vessels, and nerves. 



The epiglottis (PI. 49. fig. 40) and the 

 appendices of the arytenoid consist of fibro- 

 cartilage ; and the corpuscles are frequently 

 more or less filled up by secondary deposit. 



The mucous membrane, as also the sub- 

 mucous tissue of the larynx, consists of 

 connective tissue with networks of fine 

 elastic fibres; at the surface it becomes 

 more homogeneous, but does not form a 

 separable basement layer or membrane. It 

 contains a number of small racemose glands, 

 the vesicles of which are lined with pave- 

 ment, the ducts with cylindrical epithelium. 

 Its surface is covered with ciliated epi- 

 thelium, 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 



Fig. 417. 



Epithelial cells of the trachea in situ; human, a, 

 longitudinal elastic fibres ; 6, homogeneous outer (base- 

 ment) layer of the mucous membrane ; c, deep layers 

 of round cells ; d, intermediate layers : e, outer ciliated 

 cells. 



Magn. 350 diameters. 



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 longi- 



Fk. 418. 



tudinal 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. The deepest layers are 

 composed of roundish cells with distinct 

 nuclei, those succeeding being elongated, 

 while those next the surface are still longer, 

 greatly narrowed at the base, and with oval 

 nuclei. Those of the last row are covered 

 with vibratile cilia (fig. 418). 



The smaller bronchi differ 

 in structure somewhat from 

 the larger. Thus, the car- 

 tilage forms angular plates 

 distributed throughout 

 their circumference, while 

 the elastic and connective- 

 tissue coats become thin- 

 ner, and the transverse mus- 

 cular fibres smaller and less 

 closely placed. 



Air-cells. The terminal 

 bronchi open into a group f 

 of air-cells partly fused to- 

 gether, forming common 

 cavities the lobules, al- 

 veoli, or infundibula. These 

 are separated from each 

 other by connective tissue 

 mixed with muscular 

 fibres, containing, in adult Isolated {fchelial 



animals, black pigment m cells from the surface 



the form of granules, some- f the trachea ; hu- 

 times also crystals They ma ^ nifled 350 dia . 

 are best seen m the lungs meters, 



of young animals. 



The walls of the pulmonary air-cells 

 consist of a fibrous, and an epithelial layer. 

 The former is composed of a basis of homo- 

 geneous connective tissue, with numerous 

 elastic fibres, vessels and nerves. 



The elastic fibres surround the air-cells 

 in the form of elegant wavy bundles and 

 separate fibres, which anastomose and con- 

 stitute a dense network, most obvious at 

 those parts where several cells are in con- 

 tact with each other ; while in other parts 

 the areolar element supporting the nume- 

 rous capillaries predominates, and the elastic 

 elements are more sparing and slender. 

 The epithelium is of the pavement kind, 

 not ciliated, consisting of rounded or polygo- 

 nal nucleated cells, about ??rW in diameter. 



The smaller or primary lobules are ag- 

 gregated to form larger secondary lobules 

 the lobules of descriptive anatomists, and 



