more rarely appears in the throat
and at the base of the tongue. The tumor, at first small and painful,
spreads very rapidly both in depth and extent. When it is stroked or
handled a peculiar crackling sound is heard under the skin; this is due to
a collection of gas formed by the bacilli as they multiply. At this stage
the skin becomes dry, parchment-like, and cool to the touch in the center
of the tumor. If the swelling is cut into, a frothy, dark-red, rather
disagreeable-smelling fluid is discharged. The animal manifests little or
no pain during the operation.

As it is frequently desirable to know whether the disease is anthrax or
blackleg, a few of the most obvious post-mortem changes may here be cited.
The characteristic tumor with its crackling sound when stroked has already
been described. If after the death of the animal it is more thoroughly
examined, it will be noted that the tissues under the skin are infiltrated
with blood and yellowish, jellylike material and gas bubbles. The
muscular tissue beneath the swelling may be brownish or black, shading into
dark red. (Pl. XLII.) It is soft, easily torn and broken up. The muscle
tissue is distended with numerous smaller or larger gas-filled cavities,
often to such extent as to produce a resemblance to lung tissue. Upon
incision it does not collapse perceptibly, as the gas cavities are not
connected with one another.

In the abdomen and the thorax bloodstained fluid is not infrequently found,
together with bloodstaining of the lining membrane of these cavities. Blood
spots (or ecchymoses) are also found on the heart and lungs. The liver is
congested, but the spleen is always normal in appearance.

_Differential diagnosis._--Among the features of this disease which
distinguish it from anthrax may be mentioned the unchanged spleen and the
ready clotting of the blood. It will be remembered that in anthrax the
spleen (milt) is very much enlarged, the blood tarry, coagulating feebly.
The anthrax carbuncles and swellings differ from the blackleg swellings in
not containing gas, in being hard and solid, and in causing death less
rapidly.

It is difficult to distinguish between the swellings of blackleg and
malignant edema, as they resemble each other very closely and both are
distended with gas. Malignant edema, however, generally starts from a wound
of considerable size; it usually follows surgical operations, and seldom
results from the small abrasions and pricks to