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ON THE PHYSIOLOGICAL ACTION OF MEDICINES. 117 
researches have been conducted with the salts of mercury, but they are not 
yet completed. I trust on a future occasion to Jay the results before this 
Section of the Association. 
The experiments that have been performed on the action of substances 
when introduced directly into the blood, embrace salts of iron, nickel, man- 
ganese and cadmium, thus completing, with the experiments already pub- 
lished, the whole of the magnesian class*. I have also investigated the 
action of the acids of arsenic and phosphorus, between which well-marked 
isomorphous relations are known to exist. 
As the action of each substance in the same isomorphous group closely 
resembles that of the others, I shall, for the first class, state generally the 
effects they give rise to, referring to the experiments which have already been 
published for more minute details. The most marked symptoms that follow 
the introduction into the blood of any of the salts of the magnesian family, are 
evidently due to an action they exert on the nervous system, an action so well- 
marked and so peculiar, as readily to distinguish this class of salts from any 
other substance derived either from the vegetable or mineral kingdoms with 
which I have yet experimented. Whilst most poisons that act on the nervous 
system, seem to exert their influence more particularly on the motor or sensi- 
tive properties of the nerves, these, on the contrary, afford an example of a 
specific action on the voluntary functions of the brain, or on the power of 
volition. When these salts are injected into the veins in proper quantities, 
the first effect that follows is generally vomiting ; the animal then either falls 
or lies down, and will remain for many minutes in the same position, or in 
any position in which it may be placed, without once attempting to move, 
although it possesses the power of standing and walking about: the state in 
which it lies may be compared to that of catalepsy, were it not that, whilst 
in catalepsy the position of the limb remains unchanged from a want of power 
to move it, in the state induced by the injection of these salts, the limb is 
capable of being moved, but retains its position from the animal making no 
effort to change it. During the whole of the time the animal lies in this inert 
state, the sensibility remains perfect, and it appears quite free from pain; it 
turns its eyes in the direction of the person who may speak to it, and shows 
its sensibility when caressed, by slight movements of the tail, although lying in 
all other respects like an inert mass, with the exception of the respiratory 
movements, which continue with the greatest regularity. Sometimes the 
animal will remain in a most constrained position for many minutes, although 
so placed as to require a considerable degree of muscular exertion to retain 
it. I have, for instance, seen a dog remain for full five minutes with its fore 
legs bent under it, resting on the head and thorax and hind legs, although 
at the time it could walk about very well. Such are the symptoms that 
characterize the action of the whole of these substances on the nervous 
system: besides this, they exert a decided effect on the heart, destroying the 
irritability of that organ, when injected into the veins in larger doses. I have 
described fully in my former memoirs the action of this class of substances 
on the vascular system, and have only to add, that the salts of iron, nickel, 
manganese and cadmium, are perfectly analogous in this respect with those 
of magnesia, copper and zinc. The only difference between them is, as to the 
quantities required to produce the same train of symptoms; the quantity of 
sulphate of iron, dissolved in six drachms of water, which, when introduced into 
the jugular, will arrest the action of the heart, is from thirty to forty grains ; 
of sulphate of nickel from ten to twenty grains ; of sulphate of cadmium from 
five to eight grains; and of sulphate of manganese from ten to twelve grains. 
* See Edinb, Med. and Surg. Journal, No, 148. 
