ARTIFICIAL PRODUCTION OF TUBERCLE. 19 



tubercle which, is more tenable than another, it is that of Virchow, who 

 has pointed out the analogy between tubercle and the lymphatic structures ; 

 viz., that they both consist of round cells, of many of which the nuclei alone 

 may be visible, imbedded in a network either of fibres or of a more trans- 

 parent but semi-cartilaginous-looking homogeneous tissue ; in which, however, 

 distinct fibrillation may be absent. 



Another point is, that these growths are agglomerated into little masses, 

 like the follicles of lymphatic glands, or the Malpighian bodies of the spleen, 

 or the solitary glands of the intestines ; that, wherever they occur, they tend 

 to form distinct granules, but that at their margins they blend insensibly with 

 the surrounding tissue ; that they extend in connective tissue by a change in 

 the mode of growth of this tissue, through which cells are produced resembling 

 rather those of a lymphatic gland than of the connective tissue itself; that 

 these cells have no specific character, in this resembling those of tubercle ; that 

 they pass into an early decay, which is not sloughing, but molecular death ; 

 and that this degenerative change appears to be caused partly by their mutual 

 pressure and by the density of the growth, and partly by the occlusion of 

 vessels. I think that we know of no other growth except tubercle of which 

 all these attributes can be predicated, and certainly none other which occludes 

 the vessels of the part to the extent which is effected by these formations in 

 the guinea-pig. 



The identity of the morphological structures of these growths in the lung 

 with those of human tubercle may, I think, be affirmed with some distinct- 

 ness. One fact may also, I think, be very distinctly stated — that we are not 

 dealing with any " lobular pneumonias," or with any " catarrhal pneumonia," 

 or with any infiltration of the interior of the alveoli (which are the very last 

 parts that are occluded). The growth proceeds by a progressive thickening of 

 the walls of the alveoli, which finally closes the air-vesicles. Even in the 

 densest masses, when examined with a binocular, a certain transparency can 

 be distinctly seen in some places, showing that the central portions of the 

 alveoli are still hollow. There is very little participation of the epithelium in 

 the growth. The epithelium seems, if I may say so, to disappear in the pro- 

 gress of tubercle, and to give place to cells of another and different character. 

 I think (if I may digress for a moment) that we have been dealing with the 

 question of the epithelium of the lung in too specific a sense ; and that in the 

 alveoli, it is only a derivative of the connective tissue, like the cells which I 

 have described in the normal omentum, and having but little resemblance to 

 the appearance of an epithelium in a mucous membrane, which rests on a 

 nucleated subjacent tissue. In the alveoli of the lung, no subjacent nucleated 



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