TUBERCULOSIS 155 
young animals than in adults, and is more common among swine 
than in cattle. 
Morbid anatomy. The usual direct anatomical change following 
the invasion of tubercle bacteria is the formation of nodules or tuber- 
cles. A tubercle has been defined as ‘‘a small nonvascular nodule 
composed of cells varying in form and size with some basement sub- 
stance between them and with an inherent tendency to undergo 
central necrosis." In a large number of cases the individual tubercles 
are distinct and easily recognizable, while in others they are coalesced, 
forming a mass of necrotic tissue. The lesions vary, therefore, from 
Oe 990 %58 9g 
0) 
FP LOe LLY 0 © 19.0 29 a * KS 
© oes gee egy ee ao 8 @ 
/ es inom. 
RHI ne a eee 
Fig. 30. A DRAWING OF A SECTION OF VERY YOUNG TUBERCLES IN SPLEEN (Thoma). 
well isolated minute or larger nodules to masses or cavities containing 
a purulent, caseous, or calcified substance. 
The location of the primary lesion depends upon the channel of 
infection. If the specific organisms are lodged in the oral cavity or 
pharynx they may pass through the mucosa and be taken to some of 
the lymphatic glands about the head; if they are taken directly through 
the respiratory passages into the lungs they either develop nodules 
in the lung tissue proper, or they are carried through the lymphatic 
system to the lymph glands draining the lungs where the lesions first 
appear. If the specific bacteria are first lodged in the intestinal 
mucosa, primary tuberculous ulcers may develop or they may pass 
