554 ON THE EXTERNAL CAUSES OF DISEASE. 
By several experiments with the muriate of barytes and emetic 
tartar, Mr. Brodie came to the same conclusions^. From these 
examples, then, it is probable, that these, as well as many other 
agents, are inert until they first mix with and give new proper¬ 
ties to the blood. They are then thrown out of the circulation 
into the various cavities,—the emetics into the stomach, the pur¬ 
gatives into the intestines, the diuretics by the kidneys ; and 
the increased action and inflammation which some of them pro¬ 
duce in the stomach and bowels, is merely the effect of the dis¬ 
eased secretion, produced by the deranged state of the circulat¬ 
ing current. 
A convincing proof that these agents enter almost imme¬ 
diately into the blood is seen by the rapid manner with which 
some of them pass through the circulation into the different 
secretions. 
Stehberger found that madder, indigo, and rhubarb could be 
detected in the secretions of the kidney in fifteen minutes from 
the time that they were swallowed. Turpentine, when rubbed 
into the skin, appeared in the urine in twenty-five minutes; but 
when inspired at the same time, it was found in the secretion of 
the kidneys in fifteen. It is likewise a fact, that prussiate of 
potash enters the circulation in the most rapid manner, and in a 
very short period gives a blue colour to the valves of the heart. 
There is something very mysterious in the modus operandi of 
those poisons that are generally believed to cause death by a 
direct impression on the brain, made through the medium of the 
nervous system. The prussic acid, for example, is supposed to 
produce its deleterious effects entirely in this manner. Dr. Ste¬ 
vens is of opinion, that these effects are produced through the 
medium of the blood, since it is as suddenly fatal when mixed 
with the blood in a vein as when it is injected into the carotid 
artery. 
It is destructive, likewise, to animals that have neither brains 
or a nervous system. It is very evident,” says the Doctor, 
that all those agents that enter the circulation must pro¬ 
duce their effects on the solids chiefly through the medium of 
* Pbilosophical Traiisactious, 1812. 
