carried off with the excrement, or milk may be
spilt, or there may be a discharge from the vagina when the genital organs
are tuberculous. There may also be ulcers of the intestines, from which
many bacilli escape with the feces. The bacilli from these sources may
become dried and pulverized and carried in the air of the stable and into
the lungs of still healthy cattle, where the disease then develops.

The disease of the stomach, intestines, and mesenteric glands is very
probably the result of feed infection. Tubercle bacilli may have been
scattered upon the feed by diseased animals, but the most common source of
such infection is the milk of tuberculous cows. Calves may become infected
in this way. The disease may remain latent until the animal becomes older.
The not-infrequent occurrence of tuberculosis of the uterus and ovaries
makes it probable that the disease may be transmitted by a diseased bull or
carried by a healthy one from a diseased cow to a number of healthy cows.

The source of infection is always some previous case of the disease, for
the disease can never rise spontaneously; hence, in those stables in which
there is frequent change of cattle the introduction of tuberculosis by
cattle coming from other infected stables is the most frequent source of
infection. Since the bacilli when dried can be carried 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, rodlike body (see Pl. 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 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