of lime
salts. When these tubercles continue to form in large numbers they run
together, forming 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[4] 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 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 generalized, 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 bronchitis with abundant catarrhal secretion; this plugs up the
smaller air tubes, and the lung tissue supplied with air by the tubes
collapses. Subsequently it becomes filled with yellowish, cheesy matter,
which greatly distends the small air tubes and air vesicles
(bronchopneumonia). The connective tissue between the lung lobules, around
the tubercles, and around the air tubes becomes thickened and indurated. In
the larynx and the bronchi tubercles may vegetate upon the mucous membrane,
and ulcers may result from their b