590 INFECTION AND IMMUNITY. 



cells which have fused to form a multimiclear 

 mass. 



Still more remote from the center of the tuber- 

 cle, that is, surrounding the epithelioid cells, wan- 

 dering lymphoid and plasmal cells accumulate. 

 Certain retrogressive changes, especially necrosis 

 and caseation, characterize the further history of 

 the tubercle, although these changes do not occur 

 equally early nor with equal intensity in all cases. 

 Necrosis begins in the center of the lesion, and the 

 view is often expressed that the formation of the 

 giant cell is the first indication of the retrogressive 

 change. Cell degenerations, however, with karyor- 

 rhexis may occur before giant cells have formed. 

 With the death of the central tissue there occurs 

 sooner or later the death of many of the bacilli in 

 this portion of the tubercle. The progressive for- 

 mation of new tissue continues in the periphery as 

 the degenerative changes take place toward the 

 center; the tubercle enlarges, both epithelioid and 

 the surrounding lymphoid cells increase corre- 

 Formation spondingly, and new giant cells form at the periph- 



of Fibrous j! 4.1 ,. & u l JJ J 



Tissue, ery of the necrotic center, only to be included in 

 the degenerated area as the latter extends. In 

 favorable cases, in which the virulence of the or- 

 ganism is low or the resistance of the individual 

 strong, the tuberculous area is eventually sur- 

 rounded by adult fibrous tissue which in a sense 

 accomplishes the isolation of the infected area. 

 Without question such a capsule of scar tissue is an 

 obstacle to the extension of the tuberculous proc- 

 ess, whether it surrounds a nodule in a lymph 

 gland, a cold abscess or a tuberculous sinus. 

 Further steps in the healing consist of caseation of 

 the entire area, its partial or complete substitution 



