i350 



HUMAN ANATOMY, 



Diagram showing relations of terminal sub- 

 divisions of air-tubes. B, bronchiole ending in 

 terminal bronchi ( TB) ; latter divide into atria 

 {A), each of which communicates with several 

 air-sacs {s) into which open the alveoli (a) ; 

 PA, branch of pulmonary artery follows bron- 

 chiole ; PV, pulmonary vein at periphery of 

 lung-unit. {After Miller.) 



vessels. Each lobule is entered by an intralobular bronchus (.5-1 mm. in diam- 

 eter), accompanied by its artery, — not quite at the apex of the pyramid, but slightly 

 to one side of it. The bronchus divides into two, at an angle of from 90°-ioo°, a 

 little above the middle of the lobule, having previously given off two or three col- 

 lateral branches to its upper part. In the third 

 quarter of the lobule the two subdivisions ( 2-3 

 mm. in length) again split, with about the same 

 degree of divergence as the parent stems, but in 

 a plane at right angles to that of the previous 

 splitting. This is repeated in three or four suc- 

 cessive bifurcations, a varying number of col- 

 lateral branches being given ofi. Thus the num- 

 ber of branches in the third quarter is much in- 

 creased ; but it is in the last quarter and towards 

 the periphery of the lobule throughout that the 

 tubes break up into the great number of truly 

 ultimate bronchi. The various collaterals, spread- 

 ing and even reascending, undergo subdivision 

 also. Laguesse and d' Hardiviller * estimate the 

 number of terminal bronchi (ductuli alveolares) 

 within a single lobule at from fifty to one hun- 

 dred or even more. The slightly dilated distal ex- 

 tremity of the terminal bronchus communicates 

 with from three to si.x spherical cavities, the atria 

 of Miller'^ (so named by him from the resemblance 

 to the arrangement of an ancient Roman house). 

 The atria, in turn, communicate with a group of 

 larger and irregular cavities or air-sacs (sacculi 



alveolares), into which directly open the ultimate air-spaces, the alveoli or air-cells 



(alveoli pulmonis). The latter open not only into the air-sacs, but also into the atria, the 



dilated distal part of the terminal bronchus being likewise beset with scattered alveoli. 



Miller holds that the terminal bronchus, the air-chambers connected with it, 



together with the vessels and 

 Fig. 1572. . nerves, is the true lung-unit, 



and calls it the lobule. We 

 cordially agree that this is 

 the true hing-iinit, and pro- 

 pose that name for it, retain- 

 ing the term ' ' lobule' ' for 

 the above-described more or 

 less isolated portion of the 

 lung which is surrounded by 

 connective tissue and vessels 

 and receives a single intra- 

 lobular bronchus and artery. 

 In some animals the lobules 

 are perfectly distinct ; they 

 niav be isolated in the infant, 

 and can be in the main easily 

 made out in the adult. The 

 lung-unit, on the other hand, 

 is not surrounded by areolar 

 tissue, and its limits can be 

 determined only by recon- 

 struction from microscopical sections ; hence, apart from its minuteness, it is practi- 

 cally too much of an abstraction to deserve the name almost universally applied to 

 something tangible. 



' Bibliographie Anatomique, 1898. 



"Journal of Morphology, 1893. Archiv f. Anat. u. Phys., Anat. Abth., 1900. 



Corrosion-preparation of lung, showing lung-units, a. minute bronchus 

 ending in terminal bronchi (A, b) ; c, atria; </, air-sac ; e, alveoli. X 8. 



