170 SPECIAL HISTOLOGY. 



observes. The smallest air-vessels, 0*1 — 0-16'" in diameter, 

 arising by simple narrowing from the most minute lobules, are 

 at first still beset with simple air-cells, which may be termed 

 parietal, and at first also, have sinuous walls, a character, how- 

 ever, which is soon lost, to be replaced by the usual smooth 

 appearance which is afterwards retained. The size of the air- 

 cells varies very Considerably even in a healthy lung, amount- 

 ing after death, and when they are wholly undistended with air, 

 t° b — is — iV '• But owing to its elasticity, every air-cell may be 

 dilated to twice or three times its natural size without rupture, 

 and is capable afterwards of returning to its pristine condition. 

 It will not be wrong to assume, that, in life, when the lungs 

 are filled with the average quantity of air, the air-cells are at least 

 one third larger than we find them after death; and that, on the 

 deepest possible inspiration, the expansion reaches, perhaps, to 

 twice that dimension. In emphysema, dilatations to this and 

 even to a much more considerable extent, are permanent, and 

 ultimately lead to the rupture of the walls of the alveoli 

 belonging to a lobule, or even to the confluence of the lobules 

 themselves. The form of the alveoli , in a recent collapsed 

 lung, is most usually rounded or oval and, in one that has been 

 inflated or injected, in consequence of their mutual pressure, 

 rounded-angular; the air-cells of the surface of the lung are 

 invariably polygonal and their external sides are almost always 

 nearly plane. 



The lobulated structure of the lung is not nearly so distinct 

 in the human adult as in younger individuals and in animals. 

 It is therefore advisable, in the first instance, to seek for these 

 conditions in the lungs of a child. In this case the separate 

 lobules are still all distinctly parted from each other by con- 

 nective tissue and admit of being isolated, so that the tolerably 

 regular pyramidal form of the superficial, and the more irregular 

 one of the interior lobules can be satisfactorily perceived. 

 In the adult, also, these smallest lobules, in size \ — \ — 1'", still 

 exist, but are so intimately united, that, even on the surface of 

 the lung, their outlines are only perceived with difficulty and 

 imperfectly ; and, in the interior of the organ, a more homo- 

 geneous structure, something like that of the liver, is appa- 

 rently presented. Secondary lobules, on the other hand, of 

 j — * — 1" in s i Z e (lobules of authors) are, even in the adult, 



