18 



THE TUBERCLE BACILLUS. 



The germ of the disease, the tubercle bacillus, is a tiny, slender, rod- 

 shaped body. Several thousands of them placed end to end would be needed 

 to measure an inch, so that they are quite invisible to the naked eye. A 

 powerful microscope is needed to see them. 



Once the bacillus has gained lodgment inside the body of an animal, it 

 begins to grow and multiply. It gets longer, and when full grown, divides 

 crosswise, making two out of one. Each of these goes through the same pro- 

 cess, the two become four, the four eight, the eight sixteen, and so on indefi- 

 nitely. 



This multiplication takes place quite rapidly when conditions are favour- 

 able, a few hours only being required for the birth of each generation. Nature, 

 however, does not permit this process to continue long without offering some 

 resistance. The forces of the body are roused to action and a battle begins 

 between the tissues of the body and the army of the invaders. 



The first line of defence is composed of the white cells of the blood which 

 liurry to the scene of action and endeavour to destroy the invaders by eating 

 them up. Sometimes they are successful and the bacilli are destroyed, the 

 infection checked. Often they fail in their object and are themselves destroyed 

 and the multiplication of the germs continues. 



The second line of defence is found by the cells of the tissue invaded by 

 the germs. These cells arrange themselves in a circle around the germs and 

 try to form a living wall between them and the rest of the body. This barrier 

 gradually becomes thicker and thicker and forms a little hard lump or tubercle, 

 from which the disease gets its name. If this wall is complete and successfully 

 imprisons the bacilli, these gradually die and the disease in that particular 

 spot is arrested. 



Frequently, however, both these safeguards are overcome. The germ^^ 

 break through the barriers and are carried in the blood stream or lymph chan- 

 nels to other parts of the body. New points of attack are selected and the 

 process begins again, but with less chance on the side of the animal. As the 

 tubercles increase in number, the power of the body to grapple with them 

 becomes less and less, and gradually the animal falls a prey to the disease. 



The tubercle bacillus does not multiply outside the body of an animal. It 

 can live for a long time in favourable surroundings, such as dark and dirty 

 slables. Sunlight soon destroys it. Freezing does not hurt it, but it can only 

 stand a moderate amount of heat; exposure to 149 degrees Fahrenheit for 

 twenty minutes kills it. Protected by a layer of dried mucus, such as is 

 coughed up from the lungs, it withstands drying, light, and ordinary disin 

 fectants, but is readily killed by steam or boiling water. 



