PATHOGENIC BACTERIA. 357 



cell. Except possibly in the very youngest tubercles, a small 

 area of necrotic tissue will usually be found at the center of the 

 tubercle. 



Around the giant-cells and the necrotic area are seen large 

 cells with distinct nuclei which resemble epithelial cells, and are 

 often called epithelioid cells; they are also often termed granu- 

 lation cells, and represent an attempt at the formation of 

 granulation tissue. But no new-formed blood-vessels, such as 

 are found in granulation tissue as a rule, occur in the tubercle. 

 Tubercle bacilli may also be found among the epithelioid cells. 

 Outside of these epithelioid cells is another layer of small cells 

 called lymphoid cells, which represent leukocytes that have 

 appeared in this situation as a part of the inflammatory reaction 

 excited by the presence of the tubercles. The zone of lymph- 

 oid cells may be very indistinct or wanting. Frequently it 

 may be very difficult to make out that the cells are arranged in 

 distinct zones at all, for instead of distinct tubercles, the tuber- 

 cle bacillus may produce a diffuse form of inflammatory tissue. 

 The cells are imbedded in a matrix consisting of the connective 

 tissue originally belonging to the part, to which some fibrin 

 may be added. In addition to the fact that no new blood- 

 vessels are formed to maintain the nutrition of these newly 

 formed cells, the small vessels included in the tubercle and 

 around it suffer from inflammatory changes. Owing to these 

 causes and to a toxic substance formed by or in the tubercle 

 bacilli, degenerative changes and necrosis take place at the 

 central part of the tubercle. As a result of these degenerative 

 changes the center of the tubercle becomes converted into a 

 dry, yellowish-white, friable mass, resembling dry cream- 

 cheese. Such material is said to be caseous, and the process 

 is called caseation. Prudden and Hodenpyl found that the 

 injection of dead tubercle bacilli into animals produced lesions 

 having the histological characters of tubercles, but caseation 

 did not take place. 



