to
destructive agents than the bacteria which produced them. The anthrax spore
may live several years in a dried state, but the anthrax bacillus perishes
in a few days under like conditions. This matter will be referred to again
when we come to discuss the subject of disinfection.

Of the protozoa which cause disease very little is at present known. One
which produces Texas fever is pictured on Plate XLV, in figs. 4 and 5.
These parasites have a more complex life history than bacteria; and as they
can not be grown in artificial media, their thorough investigation is at
present hampered with great difficulties.

The differences in the symptoms and lesions of the various infectious
diseases are due to differences in the respective organisms causing them.
Similarly the great differences observed in the sources from which animals
become infected and the manner in which infection takes place are due to
differences in the life history of these minute organisms. Much discussion
has taken place of late years concerning the precise meaning of the words
"infection" and "contagion."

* * * * *

VARIOUS BACTERIA WHICH PRODUCE DISEASE IN CATTLE.
DESCRIPTION OF PLATE XXVIII.

The bacteria on this plate are partly from tissues, partly from cultures,
and stained artificially with aniline colors (fuchsin or methylene blue).
Figs 6 and 7 are copied from Fraenkel and Pfeiffer's atlas. All but fig. 7
are magnified 1,000 times; fig. 7, 500 times.

Fig. 1. Bacteria from pneumonia in cattle. These are also the cause of
hemorrhagic septicemia and are closely related to swine-plague bacteria.
These bacteria were drawn from a piece of spleen pulp (rabbit).

Fig. 2. Micrococci (streptococcus) which produce inflammation of the lining
membranes of the abdomen, thorax, heart, brain, and joints. Frequently
associated with the preceding bacteria in abscesses.

Fig. 3. Micrococci (staphylococcus) which produce inflammation and
suppuration; also pyemia.

Fig. 4. Bacilli of blackleg. The pale oval bodies as well as the light
spots in one end of the bacilli represent spores.

Fig. 5. Bacilli which produce tetanus or lockjaw. The light spot in the
enlarged end of each rod represents a spore.

Fig. 6. Bacilli of tuberculosis. Microscopic sections of a pearly nodule
from the lining membrane of the chest cavity. The bacilli are stained red
and appear as small straight rods within the cells of the nodule or
tubercle.