THE ACID FAST GROUP 375 



temperature range is relatively narrow. It is difficult to 

 get growth below 30 c. or above 40 c. The bacteria are 

 readily killed by heat, the thermal point being given as 

 60 c. for 20 minutes. Desiccation destroys the organisms 

 slowly, although they have been known to remain alive in 

 dried sputum for several months. Certain physiological 

 characters have been noted as of assistance in differentiating 

 the human and bovine tubercle bacillus. It was noted by 

 Theobald Smith that when the human bacillus is grown in 

 glycerin broth somewhat acid to phenolphthalein, it brings 

 about a permanent acid reaction; while with the bacillus 

 of bovine tuberculosis the acidity finally diminishes and the 

 reaction eventually becomes alkaline. 



Pathogenesis. Tuberculosis is typically a chronic dis- 

 ease. It is characterized, no matter what part of the body 

 is affected, by the development of nodules having eventually 

 the same structure. When the tubercle bacillus lodges in a 

 tissue it causes a multiplication of the connective tissue 

 cells. These become surrounded by a more or less delicate 

 layer of epitheloid cells while at the center there are so- 

 called giant cells containing numerous nuclei. The whole 

 mass is termed a tubercle or a miliary tubercle. Eventually 

 autolysis of some of the cells in the interior of the nodule 

 takes place and it undergoes what is termed caseation, that 

 is, the interior becomes cheeselike in consistency. This is 

 particularly apt to happen when nodules are found in 

 large masses. In many cases the tissues surrounding the 

 nodules develop a firm fibrous capsule, and this may even- 

 tually become infiltrated with lime, that is, calcified. Such 

 granules will persist indefinitely in tissues which have been 

 tubercular even after the disease has been entirely cured. 

 The organisms are particularly apt to invade lymph nodes 

 and glands through the lymph channels. 



The different varieties of the tubercle bacillus show 

 marked variations in their pathogenesis for animals. In 



