_The specific cause_ of actinomycosis is commonly known as the ray fungus
(Fig. 118). This fungus grows on certain plants, and the animal usually
contracts the disease by eating plants or roughage that have the fungus on
them. Grasses having awns that are capable of wounding the mucous membrane
of the mouth and penetrating the gums are most apt to produce the disease.
Young cattle that are replacing and erupting their teeth are most prone to
"lumpy jaw." Conditions that favor bruising of the jaw and external wounds
favor the development of actinomycosis.

The fungus grows in the tissues, causing an inflammatory reaction and
destruction of the tissue. The ray fungus can be seen in the diseased
tissue or the pus as yellowish, spherical bodies about the size of a grain
of sand. Each of these bodies is formed by a large number of club-like
bodies arranged about a central mass of filaments.

[Illustration: FIG. 118.--The ray fungus.]

_The local symptoms_ are characteristic (Fig. 117). The tumor may involve
the soft tissues of the head. If the jawbone is affected the tumor feels
hard and cannot be moved about. Sometimes it is soft and filled with pus.
Tumors of long standing may possess uneven, nodular surfaces and fistulous
openings. When the tongue is affected, it is swollen and painful, and
prehension and mastication of the food may be impossible. When the pharynx
is the seat of disease, breathing and swallowing are difficult and painful.
Actinomycosis of the lungs may present the appearance of a chronic
pulmonary affection. If the disease involves the head and lungs, the animal
may become unthrifty and emaciated. In doubtful cases a microscopic
examination of a piece of the tumor, or some of the pus, may be necessary.