TUBERCLE BACILLUS 479 
First there is a proliferation of the connective tissue which forms a 
spherical mass of e])ithelioi(i 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.^ 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 center of the 
giant cell likewise may undergo caseous degeneration, and tubercle 
bacilli are not infrequently found in the middle of these cells.- 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 
miliary tubercles, larger areas of caseation may develop; epithelioid 
cells, lymphocytes, and giant cells are usually found closely packed 
around these areas. 
The destruction of the capillaries and the resulting avascular tissue 
helps in the necrosis of the tubercle by cutting ofi' the blood supply. 
What is generally known as consumption or destruction of the lung 
tissue is probably not due to the action of the tubercle bacillus alone, 
but to secondary infection and liquefaction of tissue by other organ- 
isms, as the streptococcus, staphylococcus, pneumococcus, or IMicro- 
coccus tetragenus. If a caseous necrotic tubercle located near a 
bronchus ruptures into this bronchus, a large amount of tuberculous 
material is suddenly swept into the regional areas of the lung, over- 
whelming it and setting up a rapidly fatal infection which is known 
as galloping consumption or phthisis florida. If a caseous tubercle 
ruptures into a lymph or bloodvessel, the material may be carried 
very M'idely through the body, causing generalized miliary tuber- 
culosis, which resembles typhoid fever clinically. Hemorrhage not 
infrequently takes place from the lung, due to the erosion and subse- 
quent bursting of a bloodvessel which may have been included in the 
caseous area. In the human lung it is practically always possible to 
find old lesions at the apices when the infection is due to the human 
type of the tubercle bacillus. Uncommonly no old healed tubercles 
can be found, and the lungs are filled with miliary tubercles, in which 
case the infection is usually caused by the bovine type of the tubercle 
bacillus. Tubercle bacilli ingested with milk or other foods may cause 
1 Joest: Virchow's Arch., 1911, 203, 451. 
2 See Evans, Bowman and Winternitz (Jour. Exp. Med., 1914, 19, 2S3). A critical 
experimental study of the histogenesis of the miliary tubercle in vitally stained rabbits 
for the finer details of the process. 
