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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 proclaimed 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 an adjacent healthy 
region. 
Xext in popularity to the plant poison theory of the cause of the 
disease is the mineral poison theory. 
Seaton (1841) very vigorously maintained that arsenic was the 
cause of the disease. Lead and cobalt have also been accused. 
I have been unable to bring about any condition in guinea pigs 
that even remoteh T resembles trembles by feeding experiments with 
cobalt, lead, or arsenic. IVhen the guinea pigs finally succumbed to 
the poison the tissues, from these pigs were without any harmful 
effect on animals 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 disease 
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 conditions, 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 
possibility of the conveyance of the infection through an intermediary 
host, such as arthropods or biting insects. 
A favorite theory many years ago was that the disease is produced 
by a gas or miasm rising from the earth in the affected region. The 
g*as was supposed to be generated by earth or vegetation. At the 
present day, no discussion of this theory is deemed necessary. 
