MODES OF INFECTION. 273 



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 leucocytes, all of which 

 are seen in the meshes of the finer fibrous tissues of the 

 part. At the same time alterations are produced in the 

 walls of the vessels of the locality ; this tends to occlude 

 them, and thus the process 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 or 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 an- 

 terior 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 sub- 

 cutaneously the resulting process finds its most pro- 

 nounced 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 lymph- 

 atic glands. Here they may remain and give rise to 

 no alteration further than that seen in the glands them- 

 selves, or they may pass on to neighboring glands, and 

 eventually be disseminated throughout the whole lymph- 

 atic system, ultimately reaching the vascular system. 



