ARTIFICIAL PRODUCTION OF TUBERCLE. 11 



ingly rare. Once or twice I have seen it in the rabbit ; never distinctly in 

 the guinea-pig. 



The microscopic examination of these growths in tho lungs presents 

 the following features. In the first place, there are three main points 

 in which they appear to originate. One of these is around the bronchi, 

 another is around the blood-vessels, and another is in the tissue of the lungs, 

 where no particular connexion can be seen with either bronchi or vessels. 

 Around the bronchi (Plate II. fig. 1, A, B), they seem to extend from little 

 masses of a lymphatic character, which normally exist in the bronchial sheath, 

 and which are abundant in the guinea-pig, and are also stated by Kolliker 

 to exist in the human lung. These granulations in the bronchi consist of 

 masses of cells growing in their sheaths, l-2500th to l-3000th of an inch in 

 diameter, mostly round, but sometimes, when densely packed, showing nothing 

 but nuclei. These cells extend into the surrounding pulmonary tissue, and 

 produce effects which I will dwell on presently. 



The other place in which they commonly originate is in the perivascular 

 sheaths of the pulmonary arteries (Plate II. figs. 2, 3). Here the growth 

 appears to be nothing more than an accumulation of the cells lining the 

 perivascular canal (fig. 2). The growth may extend for a considerable distance 

 in length along both the peribronchial and perivascular sheaths ; and from 

 both these sources of origin a rapid extension ensues into the surrounding 

 walls of the alveoli and smaller bronchi. A thickening of these is thus 

 produced, and apparently by a double mode of growth — by a rapid develop- 

 ment of fusiform cells at the margins, clusters of which are seen passing 

 among the capillaries, and by an increase of rounder cells, which are seen 

 nearer the centre of the new formation. Coincidently with this growth, a 

 change of great importance occurs in the capillaries of the lungs (Plate II. 

 figs. 5, 6, 8). Their nuclei enlarge, and the vessels, otherwise apparently 

 unchanged, contain no more blood; that is to say, no injection will pass into 

 them; and yet their outline is still marked, even in the neighbourhood of 

 these tubercular masses, by lines of nuclei traversing the base of the air- 

 vesicles. This obstruction of the capillaries takes place through very much 

 wider areas than tbe space apparently occupied by the tubercle. So also with 

 the thickening of the walls of the alveoli. Around the grey granulations, 

 and for a space of three or four times their area, there is a circumference of 

 thickening affecting both the walls of the alveoli and of the smaller bronchi. 

 This appearance is more distinct in the guinea-pig than in many specimens of 

 tubercle in the human lung ; but it can be distinctly seen sometimes in the 

 latter, and is a fact of great pathological importance, explaining the increased 



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