234 TUBERCULOSIS. 



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 compara- 

 tively 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 Baum- 

 garten, 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, by its irritation it induces tissue reaction in the form 

 of proliferative changes and leucocytic infiltration, 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 prolifera- 

 tive changes may be well seen on the fifth day after inocula- 

 tion or even earlier. A small focus of proliferated cells is 

 thus formed in the neighbourhood of the bacilli and about 

 the same time numbers of leucocytes chiefly lymphocytes 

 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 



