TUBERCLE BACILLUS 441 



also the spleen, kidneys, liver, meninges both of the cord and brain, 

 the pleural and pericardial cavities, the genito-urinary apparatus, 

 and, less frequently, joints and bones. The muscles are only very 

 rarely invaded. Various clinical names have been applied to tuber- 

 culosis of different tissues: tuberculosis of the lungs is commonly 

 designated consumption; of the spine, Pott's Disease; of the cervical 

 lymph glands, scrofula; and of the skin, lupus. The characteristic 

 initial lesion is a small nodule or tubercle which may undergo secon- 

 dary changes, as caseation, calcification, ulceration, or various types 

 of sclerosis. In the lungs the first organisms that reach the alveoli 

 may leave no trace. They are dissolved there apparently, but may 

 produce no progressive lesion. A second invasion in the same area 

 frequently causes a local inflammation which usually results in infec- 

 tion, apparently because the body has been sensitized by the first 

 bacilli that entered, and in some way is rendered locally susceptible 

 to the organism. 



The irritation caused by the extracellular toxin excreted by the 

 tubercle bacillus brings about a response on the part of the tissues 

 which is protective, as is manifested by a walling off of the bacilli. 

 First there is a proliferation of the connective tissue which forms a 

 spherical mass of epithelioid cells around the focus of infection. Out- 

 side of the epithelioid cells there is usually an infiltration of lympho- 

 cytes. The tissue is avascular and the young tubercles contain little 

 or no fats. 1 The central part of the tubercle soon begins to undergo 

 coagulation necrosis, probably due to the action of the intracellular 

 toxin, and it is gradually converted into a homogeneous, cheesy mass. 

 In many tubercles giant cells are found, which are formed either by 

 the coalescence of several epithelioid cells, or by atypical cell divi- 

 sion, the nucleus dividing faster than the cytoplasm. The nuclei of 

 the giant cell are arranged peripherally as a rule, either completely 

 around the cell, or in the shape of a horseshoe. The centre of the 

 giant cell likewise may undergo caseous degeneration, and tubercle 

 bacilli are not infrequently found in the middle of these cells. 2 Accord- 

 ing to Zeit, giant cells are essentially blind blood capillaries which 

 have extended into the tuberculous area, but have not become true 

 vessels because the toxins of the organisms have prevented the 

 final development of functional blood channels. Besides these small 



1 Joest, Virchow's Arch., 1911, cciii, 451. 



2 See Evans, Bowman, and Winternitz, Jour. Exp. Med., 1914, xix, 283, for a critical 

 experimental study of the histogenesis of the miliary tubercle in vitally stained rabbits 

 for the finer details of the process. 



