ACTION ON THE TISSUES. 215 



ture of 1 00 C. for an hour, but, on the other hand, exposure 

 in the moist condition to 70 C. for the same time is usually 

 fatal. It may be stated that raising the temperature to 1 00 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 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 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 distinguish- 

 able 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 proliferated cells is thus formed 

 in the neighbourhood of the bacilli and about the same 

 time numbers of leucocytes of the small uninucleated 



