404 DISEASES OF CATTLE. 



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

 two-tliirds the diameter of a red-blood corpuscle in length. As already- 

 explained, when the bacillus has become lodged in any organ or tis- 

 sue 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, forming masses of ravi- 

 ous sizes. The disintegration which attacks them leads to the forma- 

 tion of large cheesy masses of a yellowish color, containing more or 

 less of lime salts in the form of gritty particles. These large, tuber- 

 culous 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 mem- 

 branes 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 general- 

 ized, affecting all organs of the body. 



In the lungs (PL XXXIV) the changes observed vary according to 

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

 the appearance of very minute tubercles. These may appear in large 

 numbers on the surface of the lungs or within the lung tissue. Later 

 the contents become cheesy and partly calcified. When these tubercles 

 are sufficiently numerous to become confluent, large masses may be 

 formed, which undergo the same retrogressive changes of caseation 

 and calcification. In addition to the formation of tubercles in the 

 lung tissue, certain other changes take place. There is usually pres- 

 ent bronchitis with abundant catarrhal secretion. This plugs up the 

 smaller air tubes, and the lung tissue supplied with air by these tubes 



« These membranes comprise the smooth, very delicate, glistening lining of the 

 large tody cavities. In the thorax the serous membrane (pleura) covers the ribs 

 and diaphragm as well as the whole lung surface. In the abdomen a similar 

 membrane (peritoneum) lines the interior of the cavity and covers the bowels, 

 liver, spleen, etc. 



