296 BACTERIOLOGY. 



proceeds as long as the bacilli live and continue to 

 produce their poisonous products — there is in addition 

 a reactive change in the surrounding tissues, which 

 consists in the formation of the granulation zone at the 

 outer margins of the dying and dead tissue. This zone 

 consists of small, round granulation cells and of leuco- 

 cytes, all of which are seen in the meshes of the finer 

 fibrous tissues of the part. At the same time altera- 

 tions are produced in the walls of the vessels of the 

 locality; these tend to occlude them, and thus the pro- 

 cess of tissue-death is favored by a diminution of the 

 amount of nutrition brought to them. These changes 

 may continue until eventually conglomerate tubercles, 

 widespread caseation, or cavity-formation results ; or 

 from one cause or another the life-processes of the 

 bacilli may be checked and recovery occur. 



Modes of Infection. — Experimentally, tuberculosis 

 may be produced in susceptible animals by subcutaneous 

 inoculation; by direct injection into the circulation; 

 by injection into the peritoneal cavity; by feeding of 

 tuberculous material; by the introduction of the bacilli 

 into the air-passages, and by inoculation into the ante- 

 rior chamber of the eye. 



In the human subject the most common portals of 

 infection are, doubtless, the air-passages, the alimentary 

 tract, and cutaneous wounds. When introduced subcu- 

 taneously the resulting process finds its most pronounced 

 expression in the lymphatic system. The growing 

 bacilli make their way into the lymphatic spaces of 

 the loose cellular tissue, are taken up in the lymph 

 stream and deposited in the neighboring lymphatic 

 glands. Here they may remain and give rise to no 

 alteration further than that seen in the glands them- 



