334 TUBERCULOSIS 



of soft cheese. Caseation was looked upon by Weigert and others as 

 a particular form of coagulation necrosis. It begins, in fact, in the 

 tubercle with the appearance of the giant cells. Their protoplasm 

 early shows evidences of a coagulation necrosis, and not infrequently 

 in giant cells nuclear fragments are found in a necrotic coarsely 

 granular protoplasm. When the process of caseation progresses in 

 the giant and other cells of the tubercle its centre becomes completely 

 necrotic. The cells and their nuclei break up into a granular detritus 

 which take both the eosin stain and to some extent, but diffusely or 

 irregularly, the nuclear stain. During the formation of the caseous 

 material in the centre of the tubercle there is a certain amount of 

 transudation from the neighborhood of the tubercle and the necrotic 

 material becomes infiltrated with a coagulable substance which, while 

 not the true fibrin, is evidently much like it, and hence is called a 

 fibrinoid substance. The latter is to a large extent responsible for the 

 particular rather dry character and consistency of the caseous material. 

 Its characteristics are also partly due to the lack of vessels and 

 depending upon it the lack of fluids in the tubercle. The tubercle 

 which has undergone caseation in the centre now appears somewhat 

 yellowish; it has lost its transparency and has become opaque and 

 cloudy. At this stage of retrogressive changes, giant, epithelioid, and 

 lymphoid cells are seen peripherally to the caseous centre and between 

 them appear polynuclear leukocytes which even wander into the 

 caseous centre itself, where some of them may likewise become 

 necrotic or may remain unchanged and active to perform their 

 phagocytic function. Still later the caseous material becomes softened 

 and more creamy. This softening is probably brought about by 

 digestive proteolytic ferments, which are transported to the caseous 

 centre by the transudate and by the wandering leukocytes. If by 

 this time the activity of the bacilli has become lessened, the develop- 

 ment of fibrous connective tissue at the periphery becomes more 

 abundant and a fibrocaseous tubercle develops. If the caseous material 

 becomes completely liquefied and absorbed the inflammatory cells 

 disappear more and more and the originally cellular and partly 

 caseous tubercle becomes completely replaced by fibrous connective 

 tissue, and in place of the original cellular tubercle there is now a 

 fibrous nodule. As the tubercle disappears its elements may be 

 replaced by a deposit of lime salts, and this process is termed calcareous 

 degeneration of the tubercle. 



The inflammatory cells of the tubercle in the degenerative processes 

 generally become necrotic. According to Hektoen's observations, 

 sometimes in healing tuberculosis, young not yet necrotic giant cells 

 may break up into mononuclear cells, and these as fibroblasts form 

 connective-tissue fibers. 



The change from a tubercle into a fibrous nodule generally occurs 

 in healing tuberculosis. Even while this occurs new tubercles may 

 be formed in the neighborhood and the process spread. Even calcified 



