244 TUBERCULOSIS. 



contain living bacilli after one hour. It may be stated that 

 raising the temperature to 100 C. kills the bacilli in fluids and 

 in tissues, but in the case of large masses of tissue care must be 

 taken that this temperature is reached throughout. They are 

 killed in less than a minute by exposure to 5 per cent carbolic 

 acid, and both Koch and Straus found that they are rapidly 

 killed by being exposed to the action of direct sunlight. 



Action on the Tissues. The local lesion produced by the 

 tubercle bacillus is the well-known tubercle nodule, but though 

 the typical structure is often described as consisting of a central 

 giant-cell surrounded by a zone of comparatively large and 

 somewhat spindle-shaped cells (epithelioid cells), and again by 

 an outer zone of lymphocytes or small uninucleated leucocytes, 

 the structure varies in different situations and according to the 

 intensity of the action of the bacilli. 



A considerable discussion has taken place as to the exact 

 origin of the elements composing the tubercle follicle. In the 

 case of the iris its formation was fully studied by Baumgarten, 

 and his views we consider to be correct regarding the ordinary 

 mode of formation. Before describing the exact changes which 

 occur in the tissues, it may be stated that the action of the 

 bacillus is twofold. On the one hand it induces tissue reaction 

 in the form of leucocytic infiltration and proliferative changes, 

 and on the other hand, it causes degenerative changes in the 

 cells around, which afterwards result in their death. 



After the bacilli gain entrance to a connective tissue such as 

 that of the iris, their first action appears to be on the connective- 

 tissue cells, which become somewhat swollen and undergo mitotic 

 division, the resulting cells being distinguishable by their large 

 size and pale nuclei. These constitute the so-called epithelioid 

 cells. These proliferative changes may be well seen on the fifth 

 day after inoculation or even earlier. A small focus of prolifer- 

 ated cells is thus formed in the neighbourhood of the bacilli 

 and about the same time numbers of leucocytes chiefly lym- 

 phocytes begin to appear at the periphery and gradually 

 become more numerous. 



Soon, however, the action of the bacilli as cell-poisons comes 

 into prominence, the changes first occurring in the centre of the 

 focus. The epithelioid cells become swollen and somewhat 

 hyaline, their outlines become indistinct, whilst their nucleus 



