TUBEMCULAB INFECTION. 255 



down of old caseous lymphatic glands is a common 

 example of this condition. 



Primary Infection. — The primary infection occurs 

 .through either the vascular or lymphatic circulation. 

 Through these channels the bacilli gain access to the 

 tissues and become lodged in the finer capillary ramifi- 

 cations or in the more minute lymph-spaces. Here 

 they find conditions favorable to their development, and 

 in the course of this process produce substances of a 

 chemical nature which act directly in bringing about the 

 death of the tissues in their immediate neighborhood. 

 This tissue-death is probably the very first effect of the 

 bacilli in the body, and represents the necrotic centre, 

 which can always be seen in even the most minute 

 tubercles. With the production of this progressive 

 necrosis — for progressive it is, as it continues as long 

 as the bacilli live and continue to produce their poison- 

 ous 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 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 oc- 

 clude them, and thus the process of tissue-death is 

 favored by a diminution of the amount of nutrition 

 brought to them. These changes continue until eventu- 

 ally the life processes of the bacilli are checked, or 

 conglomerate tubercles, widespread caseation, or cavity- 

 formation results. 



Modes of Infection. — Experimentally, tuberculosis 

 may be produced in susceptible animals by subcutaneous 



