242 TUBERCULOSIS 



when completely dried they can resist a temperature of 100 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 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, the 

 structure of which varies in different situations and according to 

 the intensity of the action of the bacilli. 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 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 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 epithelioid cells become swollen and somewhat hyaline, their 

 outlines become indistinct, whilst their nucleus stains faintly, 

 and ultimately loses the power of staining. The cells in the 

 centre, thus altered, gradually become fused into a homogeneous 

 substance and this afterwards becomes somewhat granular in 

 appearance. If the central necrosis does not take place quickly, 

 then giant-cell formation may occur in the centre of the follicle, 

 this constituting one of the characteristic features of the tuber- 

 cular lesion ; or after the occurrence of caseation giant-cells may 

 be formed in the cellular tissue around. The centre of a giant- 

 cell often shows signs of degeneration, such as hyaline change 

 and vacuolation, or it may be more granular than the rest of the 

 cell. 



Though there has been a considerable amount of discussion 

 as to the mode of origin of the giant-cells, we think there can 

 be little doubt that in most cases they result from enlargement 

 of single epithelioid cells, the nucleus of which undergoes pro- 

 liferation without the protoplasm dividing. These epithelioid 

 cells may sometimes be the lining cells of capillaries. Some con- 

 sider that the giant-cells result from a fusion of the epithelioid 



