276 



THE LUNGS. 



Fis. 1S4. 



but it varies from f^oth. to ^^jth of an inch ; they are larger on the 

 surface than in the interior, and largest towards the thin edges of the 

 organ : they are also said to be very large at the apex of the lung. 

 Their dimensions go on increasing from birth to old age, and they are 

 larger in men than in women. In the infant the diameter is usually 

 under ^-^oth of au inch. 



The small bronchial tube, as already stated, entering a lobule divides 

 and subdivides a variable number of times, according to the size of the 

 lobule ; its divisions, losing their cylindrical form, and being converted 

 into irregular lobular passages, are beset, at first sparingly, but after- 

 w\ards closely and on all sides"-with numerous little recesses or dilata- 

 tions, and ultimately terminate near the surface of the lobule in a group 

 of similar recesses. These small recesses, whether seated along the 



course or at the extremity of an air pas- 

 sage, are the air-cells, or alveoli; and 

 each group of alveoli, with the compara- 

 tively large passage between them, con- 

 stitutes an ullimaie lobule, or infundi- 

 huli/m, so called from the manner in 

 which it dilates towards its extremity. 

 The arrangement of these finest air- 

 passages and air-cells closely resem- 

 bles, though on a smaller scale, the 

 reticulated structure of the tortoise's 

 lung, in which large open passages lead 

 in all directions to clusters of wide 

 alveoli, separated from each other by 

 intervening septa of various depths. 



At the point where the small bron- 

 chial tubes lose their cylindrical cha- 

 racter, and become covered on all sides 

 with the cells, tlieir structural elements 

 also undergo a change. The muscular 

 layer disappears or almost so, the longi- 

 tudinal elastic bundles are broken up 

 into an interlacement of areolar and 

 elastic tissue, which surrounds the com- 

 mencements of the infundibula, and the 

 columnar ciliated epithelium gives 

 place to a stratum of cubical non- 

 ciliated cells. The walls of the alveoli, which mainly consist of an 

 indistinctly fibrillated connective tissue with corpuscles scattered here 

 and there, are supported and strengthened by scattered and coiled 

 elastic fibres, especially numerous near their orifices, in addition to 

 which, according to Moleschott and others, there is likewise an inter- 

 mixture of muscular fibre-cells. The air-cells are lined by a delicate 

 layer of tesselated epithelium, which is most easily demonstrated in the 

 young subject (fig. 185) but is present also in the adult : here, however, 

 the cells are less regular both in size and shape. Their outlines may 

 best be brought into view by treating the tissue with nitrate of silver : 

 their nuclei are for the most part round, and are thus distinguishable 

 from the more oval nuclei of the connective tissue and walls of the 

 blood-vessels. A number of granular rounded amoeboid cells are usually 



Fig. 184. — Semidiagrammatic re- 

 presentation OF Two Infundibu- 

 la, FROM NEAR THE SURFACE OF 

 THE LUNQ OP A NeW-BoRN ChILD 



(from Kolliker). 25 Diameters. 



a, exterior of tlie two lobuli or 

 infundibula ; b, pulmonary vesicles 

 or alveoli on these and on c, the 

 smallest bronchial ramifications. 



