130 THE miners' phthisis OF THE RAND. 



to a sort of ragged network of pigmentation ; in the advanced 

 stage the whole tissue is more or less uniformly pigmented. 



The onset of infection by the tubercle bacillus, which may 

 occur at any stage of silicosis, is usually indicated by the appear- 

 ance of small areas of light grey fibrous tissue of very charac- 

 teristic appearance and structure. As time advances these areas 

 of new, grey tissue are liable to undergo necrosis — that is, local 

 death and disintegration — thus giving rise to cavities or to 

 collections of fluid, which are dark grey or almost black in 

 colour owing to the pigmented particles which have now been 

 liberated from the dead tissue. 



Silica is a very hard and brittle substance, and when 

 crushed breaks up into a powder of minute angular fragments, 

 many of which are elongated. We are all familiar with the 

 conchoidal fracture of flint, and it is interesting to remark that 

 traces of this characteristic fracture are also to be found in the 

 microscopic fragments which are given off when siliceous rocks 

 are abraded by drilling or shattered by blasting. 



If we make an ordinary microscopic examination of a sec- 

 tion of silicotic lung, we shall find evidence of fibrosis — that is 

 to say, a great increase in the number of connective-tissue cells 

 normally present, and an encroachment by these cells upon the 

 cavities of the air vesicles. We shall also find that the collec- 

 tions of pigmented matter, which are so conspicuous to the un- 

 aided eye, have been first laid down around the smaller blood 

 vessels and air tubes. The pigmented matter itself we shall 

 find to consist of carbonaceous particles, many of which, when 

 viewed under higher powers, are fovmd to be contained within 

 the bodies of phagocytic cells. When we come to look for 

 fragments of silica, however, we shall fail to see them, unless 

 we are very experienced, for they are translucent, and do not 

 hold the stain with which the section is coloured. In order to 

 make the siliceous particles visible, we must first polarise the 

 light, and then view our section through an analyser ; with the 

 Nicol prisms crossed most of the particles of silex stand out 

 as bright specks and spicules. We shall now be enabled to 

 measure the particles with a micrometer, and to discover the 

 fact that they are of very small size. 



The peculiar areas of light grey tissue, which usually indi- 

 cate the onset of tuberculous infection, are found, by micro- 

 scopic examination, to enclose in their meshes collections of 

 pigment and mineral fragments. The silica among these frag- 

 ments will not become obvious, of course, until we polarise the 

 light and put on the analyser. 



It is probable that all the fragments of silex found in the 

 lung tissue have first been taken up within the bodies of living 

 leucocytes and other wandering cells, and then conveyed by these 

 sightless porters along the lymphatic channels of the organ. 

 These wandering cells are unable to transport particles, which 

 are much larger than themselves, and it is this fact which ex- 



