THE LUNGS. 191 



are united with one another by a fibre-elastic sheath continuous with the 

 perichrondrium of the segments. This membrane likewise connects the 

 ends of the "rings," as the pieces are called, and thus completes the tube 

 behind, where otherwise the framework of the posterior wall of the trachea 

 and bronchi would be deficient. In order to maintain the proper tonicity of 

 the fibro-cartilaginous tube, especially of its membranous portion, bundles of 

 unstriped muscle, the trachealis muscle, lie between the ends of the cartilages. 

 In the main these bundles are disposed circularly, connecting the ends of the 

 cartilaginous rings. 



The mucous membrane, smooth and attached with considerable 

 firmness to the cartilages by the submucous tissue, but looser and thrown 

 into longitudinal folds over the posterior wall, is clothed with stratified ciliated 

 columnar epithelium. Many of the surface cells contain mucus and are of the 

 goblet variety. The'/fttt&g propria is rich in elastic fibres and contains 

 numerous lymphoid cells, which are so abundant in places, particularly 

 around the openings of the tracheal glands, as to suggest lymph-nodules. 

 The submucous layer, composed of fibro-elastic tissue, lodges, in addition 

 to the larger blood-vessels and lymphatics, the tracheo-bronchial glands. 

 These occur as considerable masses (Fig. 236) and belong to the mixed 

 mucous tubo-alveolar type. Their ducts pierce the tunica propria and open 

 on the free surface by minute funnel-like depressions in the epithelium. The 

 blood-vessels, lymphatics and nerves follow essentially the same plan of dis- 

 tribution as described in connection with the larynx. 



THE LUNGS. 



In their mode of development and architecture, the lungs resemble 

 compound alveolar or saccular glands, the repeatedly subdividing air-tubes 

 (the bronchioles) representing the duct-system of a gland and the ultimate 

 compartments of the respiratory tissue (the 

 alveoli) corresponding to the glandular 

 alveoli. Instead of being almost filled with 

 secreting cells, however, after birth the pul- 

 monary alveoli are distended with air and ^ 

 the cells reduced in thickness to endothelium- 

 like plates. 



The Lobule and Lung-Units. The 

 surface of the lung (Fig. 237) is marked 

 with small polygonal areas, 10-25 mm. in 

 diameter, which are defined by lines of con- 

 nective tissue, often darkened by pigment. 

 These areas are the bases of pyramidal 

 masses of pulmonary tissue, the lobules, 

 each of which is entered by and surrounds 



a minute air-tube, inlralobular bronchiole, FlG 237 ._ Externa , surface of , ung , 

 from . <% i mm. in diameter, accompanied by showing polygonal areas, corresponding 



11 r .1 i <Tt to the lobules, mapped out by pigment 



a branch of the pulmonary artery. 1 he W ithm the connective tissue, 

 bronchiole enters the lobule near, but not quite 



at, its apex and divides into two a little above the middle of the lobule, having 

 previously given off two or three collateral branches to its upper part. In 

 the third quarter of the lobule, the two branches subdivide in a plane at 

 right-angles to the preceding splitting. Such division is repeated in three 

 or four successive bifurcations, a varying number of collaterals being given 



