1 62 WOOLFOLK : 



To come back to our subject again. The source of infec- 

 tion may vary widely, but as stated in the beginning, always 

 comes from direct contact with the tubercle bacilli. The 

 bacilli are distributed about them from some previously 

 affected animal or man. They may have been expectorated 

 from the lungs or may pass out from the body in some form 

 of secretion or excretion, as through the milk, if the mam- 

 mary glands are affected ; the urine, if the kidneys are 

 affected ; or in the feces, if an}^ of the digestive organs are 

 affected; or in meat, if the animal from which the meat was 

 taken was affected with generalized tuberculosis. 



From these sources the bacillus may enter the body through 

 numerous channels and thus cause infection. With cattle it 

 is frequently drawn into the air tubes in form of dust floating 

 in the atmosphere of the stable, but in most cases it is ?i07v 

 believed to enter through the alimentary canal, with food that 

 has been soiled with saliva or other secretions of diseased 

 animals. It may also gain entrance through a milk duct or 

 through the vaginal opening or b}- means of a wound. 



The bacillus appears to be able to penetrate the mucous 

 membrane, at least in certain places, even where there is no 

 wound or abrasion, and it may pass through the membrane 

 without leaving an}' tubercular material or other sign to show 

 where it gained entrance. However, in its progress through 

 the tissue it is usually soon arrested by a lymphatic gland or 

 in some other manner, and then it multiplies and causes the 

 formation of a tubercle. 



To go into a minute detail as to the exact manner in 

 which this tubercle is now formed would be rather tedious, 

 but a little niaj'^ be said. When the tubercle bacilli have 

 lodged in or invaded an organ, their irritating effect upon the 

 tissue surrounding them sets up changes similar to those seen 

 in ordinary inflammation. The fixed connective cells and 

 cells of the endothelium of the capillaries begin to multiply 

 and produce a large number of new cells which form a hollow 

 sphere around the bacilli. After the tubercle has made some 



