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dry. The last outbreak in Tennessee occurred in April, 1907, and the 
general impression prevails among physicians and laymen in that 
State that the disease occurs only in the spring and the autumn. 
So far as milk sickness in man is concerned about the only etiolog- 
ical fact of importance is that the disease occurs as a result of the use 
of milk, butter, cheese, or flesh from an annual suffering from trem- 
bles. Even this has been questioned. Yandell (1867) states “that 
the relation of the disease to animal products is not on an impregnable 
basis.” The great mass of evidence, however, leaves little doubt but 
that the disease is practically always derived from a case of trembles. 
The favorite theory among physicians and laymen is that trembles 
is caused by a poisonous plant eaten by the animals. It is supposed 
that the poison is eliminated in the milk, or if the animal is not in 
lactation is stored up in its tissues. In support of this theory it is 
urged that the disease occurs only in seasons when animals are allowed 
to graze in the open, and only when they graze in certain special 
places that soon become known as milk sick. A number of plants, 
notably poison ivy, white snakeroot, and certain mushrooms, have 
been claimed to be the essential cause of the disease. These plants 
are all common in many localities that have never had. milk sickness, 
and in no case does the claim that any one of them is the cause of the 
disease appear to be well founded. Indeed the flora of a milk-sick 
region may be identical with that of the adjacent healthy land. 
Next in popularity to the plant-poison theory of the cause of the 
disease is the mineral-poison theory. 
Seaton (1841) very vigorously maintained that milk sickness was 
a form of arsenic poisoning. Lead and cobalt have also been accused. 
I have been unable to bring about any condition in guinea pigs 
that even remotely resembles trembles by feeding experiments with 
cobalt, lead, or arsenic. When these animals finally succumbed to 
the poison their tissues were without any harmful effect on animals 
(guinea pigs) to which they were fed. 
Two facts, apparently well established, may be urged against either 
the plant or mineral poison theory. In the first place, the flesh of 
animals dead of either trembles or milk sickness will, when eaten by 
another animal, cause that animal to develop trembles and the dis- 
ease may again be reproduced by feeding the flesh of the second 
animal. It is said that this transference of infection may thus be 
carried through a long series of animals. In the second place, the 
observation has been made very frequently that, under natural con- 
ditions, it is only exposure at night or in the morning while dew is 
on the grass, that is capable of infecting an animal with trembles. 
The limitation of trembles to certain well-defined areas, and the 
fact that night exposure only appears to be dangerous, suggest the 
