INFECTIOUS DISEASES OF CATTLE. 401 



on the open pasture are less susceptible than stabled animals. This 

 may. however, be due to concentration of virus in the stables. The dis- 

 ease is likewise far more common in cows than in oxen, owing to the 

 strain to which bringing forth young and milking subject the females. 

 Animals subjected to special feeding, such as cows in distilleries, brew- 

 eries, and other manufactories having waste available as food, are the 

 most susceptible to the disease. 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 intro- 

 duction of new animals into herds, but where the sanitary conditions 

 may be regarded as the poorest. 



Nature of the disease. The bacillus of tuberculosis was discovered by 

 Robert Koch in 1882. It (see Plate xxix Fig. 6) is a slender rod-like 

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

 in length. 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, whence 

 the general name of .the disease tuberculosis. 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 various size. The disintegration 

 whirh 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 imbedded 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 these determine 

 the course of the disease. 



In a large number of cases the changes are limited to the lungs ai:d 

 the serous membranes * of the thorax and abdomen. Pathologists hav^ 

 been in the habit of calling the lung disease tuberculosis and the <lis 

 ease of the serous membranes "pearly disease." Statistics have shown 

 that in about one-half the cases both lungs and serous membranes 

 ;m> ili-cased, in one-third only the lungs, and in oiie-fifth only the 

 serous membranes. At the same time the lymphatic glands near tin- 

 <li- MS( (1 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, affect- 

 ing all organs of the body. 



"These comprise the smooth, very delicate, glistening lining of the large body 

 cavities. In the thorax thepcrous membrane (plcnrn) covers the ribs and diaphragm 

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

 neum) lines tin- interior of the cavity and cover* the bowels, liver, spleen, etc. 

 24697 L'O 



