TUBERCULOSIS. 411 



quent source of infection. Since the bacilli when dried can be car- 

 ried by the air, it is not necessary that healthy animals come in 

 direct contact with cases of disease to become infected. In general 

 the greatest number of cases occur in the immediate environment of 

 cities, where there are not only abundant opportunities for infection, 

 owing to the frequent introduction of new animals into herds, but 

 where the sanitary conditions may be regarded as the poorest. 



The bacillus of tuberculosis was discovered by Robert Koch in 

 1882. It is a slender, rod-like body (see PI. XXVIII, fig. 6) from 

 one-third to two-thirds the diameter of a red blood corpuscle in 

 length. As already explained, when the bacillus has become lodged 

 in any organ or tissue it begins to multiply, and thereby causes an 

 irritation in the tissue around it, which leads to the formation of 

 the so-called tubercle. The tubercle, when it has reached its full 

 growth, is a little nodule about the size of a millet seed. It is com- 

 posed of several kinds of tissue cells. Soon a change takes place 

 within the tubercle. Disintegration begins, and a soft, cheesy sub- 

 stance is formed in the center, which may contain particles of lime 

 salts. When these tubercles continue to form in large numbers they 

 run together, forming masses of various sizes. The disintegration 

 which attacks them leads to the formation of large cheesy masses 

 of a yellowish color, containing more or less of lime salts in the 

 form of gritty particles. These large tuberculous masses are sur- 

 rounded by or embedded in laj^ers of fibrous tissue, which in some 

 cases becomes very dense and thick. 



The disease is thus a development of these tubercles in one or 

 more organs of the body. The distribution and number of the 

 tubercles determine the course of the disease. 



In a large number of cases the changes are limited to the lungs and 

 the serous membranes ^ of the thorax and abdomen. Pathologists 

 have been in the habit of calling the lung disease tuberculosis and 

 the disease of the serous membranes " pearly disease." Statistics 

 have shown that in about one-half of the cases both lungs and serous 

 membranes are diseased, in one-third only the lungs, and in one-fifth 

 only the serous membranes. At the same time the lymphatic glands 

 near the diseased organs are usually involved. Other organs, such as 

 the liver, not infrequently contain tubercles. Though the disease 

 may remain restricted to a single organ, it now and then is found 

 generalized, affecting all organs of the body. 



In the lungs (PI. XXXIV) the changes observed vary according 

 to the age and intensity of the disease process. They usually begin 



1 These membranes comprise the smooth, very deUcate, gUstening lining of the large 

 body cavities. In the thorax the serous membrane (pleura) covers the ribs and dia- 

 phragm as well as the whole lung surface. In the abdomen a similar membrane (perito- 

 neum) lines the interior o£ the cavity and covers the bowels, liver, spleen, etc. 



