94, 95 and 96). Lymphatic glands may become several times larger than
normal and the liver and lungs greatly enlarged. The pleura and peritoneum
may be thickened and covered with tubercles about the size of a millet
seed, or larger. Pleural and peritoneal adhesions to the organs within the
body cavities are common.

[Illustration: FIG. 91.--The carcass of a tubercular cow. Note the
condition of the carcass, and the tubercular nodules on the chest wall,
showing that the disease was well advanced.]

[Illustration: FIG. 92.--A section of the chest wall of a tubercular cow
showing a better view of the diseased tissue.]

The tubercle usually undergoes a cheesy degeneration. Old tubercles may
become hard and calcareous. Sometimes the capsule of the tubercle is filled
with pus. A yellowish, cheesy material within the capsule of the tubercular
nodule or mass is typical of the disease.

THE TUBERCULIN TEST.--The only certain method of recognizing tuberculosis
is by this test. There is no other method of recognizing this disease that
is more accurate than the above test.

The substance used in testing animals for tuberculosis is a laboratory
product. It is a germ-free fluid prepared by growing the tubercle bacillus
in culture medium (bouillon) until charged with the toxic products of their
growth. The culture medium is then heated to a boiling temperature in order
to destroy the germs. It is then passed through a porcelain filter that
removes the dead germs. The remaining fluid is tuberculin.

[Illustration: FIG. 93.--A very large tubercular gland that had broken down