BOVINE TUBERCULOSIS 165 



walls and be thrown oft', carrying with it the contained bacilli, 

 which then become a dangerous source of infection. 



INFECTION 



Tubercle bacilli gain entrance to the animal body, for the 

 most part, in two ways : either through the lungs in the air 

 breathed, or through the digestive organs in the food eaten. 

 The evidence aftbrded by the location of the tubercles points 

 to the lungs as the more frequent point of entrance. The 

 entrance into the system of a few bacilli does not necessarily 

 imply that tuberculosis will follow. It has l)een found that 

 the healthy tissues ofter considerable resistance to the invasion 

 of disease germs, that they even have the power of destroying 

 such germs when they are not present in too large numbeis. 

 This resisting power may be overcome b}' numbers, or may 

 be diminished by various conditions usually regarded as inju- 

 rious to health in general, so that the disease germs readily gain 

 a foothold. A moment's thought will show the importance 

 and bearing of these facts. 



Wherever may be the original site of the tubercles, they very 

 frequently, if not usually, spread to other organs, the bacilli 

 being carried by the lymph or the blood ; the fact that the 

 lymphatic glands are so frequently diseased points especially 

 to the former. In the beginnings of the disease, its traces may 

 be so slight that they readily escape detection wlien the animal 

 is slaughtered ; but in advanced cases, the tubercles mav be 

 found in nearly all of the internal organs; most frequently in 

 the lungs, intestines, liver, lymphatic glands, and the lining 

 membrane of the chest (pleura), and abdomen (peritoneum). 

 The surface of these organs is often thickly studded with tuber- 

 cular nodules which show a wide variation in size, as indicated 

 above; sometimes their appearance is sucli as to lead to the 

 use of such descriptive terms as " pearls " or '' grapes." (See 

 Fig I.) When these nodules are cut into they show tlie ciiar- 

 acleristic cheesy or gritty contents. The tubercles are often 

 found in the udder; they are seldom found in the muscles or 

 flesh ; important facts, as will be shown later. 



Tuberculosis is of much slower development than the other 

 germ diseases, and for a long time after infection the animal 



