576 SPECIAL HISTOLOGY. 



the true condition of these parts is most readily shown when sections, 

 in various directions, of an inflated and dried lung are prepared, or a 

 preparation injected with a colored resinous material is corroded hy 

 hydrochloric acid. In preparations of this kind, vesicles either termi- 

 nal or otherwise pedunculated, or opening independently, are never met 

 with ; on the contrary, they always open in such a way, one into the 

 other, and coalesce to such an extent, as, in the aggregate, to form, 

 most usually, a pyriform sacculus, with sinuous walls. These sacculi, 

 which are also identical with the finest lobules, or the infundibula of 

 Rossignol, must not, however, be regarded as sacs, furnished on their 

 walls with closely placed simple cells or alveoli, the latter, on the con- 

 trary, being always grouped in such a way that many of them do not 

 open directly in the larger space, but first into other alveoli, and through 

 them into the common cavity. An idea of the whole relations of these 

 parts may be best arrived at if each pulmonary lobule be viewed as an 

 amphibian lung in miniature, or if it be conceived that the outer surface 

 of the dilated extremities of the bronchial tubes is thickly beset with 

 numerous racemose groups of vesicles, the constituents of which all 

 open into one another and into the common cavity. Understood in 

 this way the structure of the lungs, then, does not differ in the least, 

 in any important respect, from that of the other racemose glands, 

 except that in the former, at all events in the adult, a partial confluence 

 of the gland-vesicles or air-cells of a lobule, appears to have taken 

 place, the dissepiments between them being here and there broken 

 through and reduced to isolated traJbeculce, as Adriani correctly observes. 

 The smallest air-vessels, 0-1-0-16 of a line 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, however, which is soon lost to be re- 

 placed by the usual smooth appearance which is afterwards retained. 



The size of the air-cells varies very considerably even in a healthy 

 lung, amounting after death, and when they are wholly undistended 

 with air, to 1-6-1-10-1-18 of a line. 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. Thefonn of the alveoli, in a recent collapsed lung, 

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



