420 DISEASES OF CATTLE. 



until the animal becomes older. The not infrequent occurrence of 

 tuberculosis of the uterus and ovaries makes it probable that the 

 disease ma}^ be transmitted by a diseased bull or carried by a healthy 

 bull from a diseased cow to a number of healthy cows. 



The source of infection is always some previous case of the disease, 

 for the latter can never arise spontaneously. Hence, in those stables 

 in which there is frequent change of cattle the introduction of tuber- 

 culosis by cattle coming from other infected stables is the most fre- 

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

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

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

 the greatest number of cases occuy 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 discoven^d by Kobert Koch in 1882. 

 It (see PI. XXIX, fig. 6) is a slender, rod-like body from one-third to 

 two-thirds the diameter of a red-blood coi-puscle 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 composed of several 

 kinds of tissue cells. Soon a change takes place within the tubercle. 

 Disintegration begins, and a soft, cheesy substance is formed in the 

 center which may contain particles of lime salts. When these 

 tubercles continue to form in large numbers they run together, form- 

 ing 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 surrounded by or embedded in 

 layers 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 



1 These membranes comprise the smooth, very deUcate, glistenins? 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 of the cavity and covers the bowels, liver, spleen, etc. 



