Memoranda on Poisons

Toxicology is that branch of medical science which treats of the nature, properties, and effects of poisons. It appears scarcely possible to give any definition of a poison which will bear a critical examination; insomuch that some have preferred to deal with the evil effects of any substance, that is poisoning

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Poisons may be introduced into the body in various ways and in various forms. Thus they may be administered by the mouth or by the rectum, and they may be given in the form of solids, liquids, or gases, uncombined, or mixed with various matters. Some agents are more readily absorbed than others; whilst some textures permit of absorption taking place more quickly through them than other tissues.

Thus, the most diffusible poisons prove most rapidly fatal, especially when introduced directly into the circulation by a wound in a vein, or when they are injected into the subcutaneous connective tissue. Their action is also speedy when applied either in a gaseous state to the pulmonary air-cells, or as a fluid to that of the stomach or intestines. The serous membranes, too, possess an activity of absorption almost superior to that of the mucous membranes; while absorption through the skin is slow, on account of the cuticle. Poisons taken into the stomach when that viscus is empty, necessarily act much more speedily than when it is full. It is remarkable that the agents which most affect the nervous system do not appear to act at all when applied directly to the brain or trunks of nerves. There are also some poisons, as that of the viper, which, although most deadly when introduced into the blood through a wound, are harmless when swallowed.

The effects of poisons may be considered as local and remote.

The local effects are mainly of three kinds, viz., corrosion, or chemical decomposition, as is seen in the effects of the strong mineral acids and alkalies; irritation or inflammation, varying from simple redness, in its mildest, to ulceration and gangrene, in its most severe degree, such as may result from the use of corrosive sublimate; and a local specific effect, produced on the sentient extremities of the nerves, as is felt on the local application of prussic acid, aconite, &c.

The remote effects are those influencing organs remote from the part to which the poison has been applied. These may be either common or specific; common, such as the constitutional indications of inflammatory fever, however produced; specific, like the constitutional effects of opium over and above its local influences in relieving pain, &c. Various narcotic poisons produce but little local change, though their remote effects are very remarkable. For example, belladonna, in whatever way it may be introduced into the system, paralyzes the ciliary nerves and so causes dilatation of the pupil. Many substances have both a local and remote action, as is well seen in the influence of cantharides upon the part to which they are applied, and their remote effects upon the urinary organs. These remote effects must be induced by one of two modes, or, as some contend, by both: by absorption, that is, by the passage of the poisonous particles into the blood; or by sympathy, that is, by an impression transmitted through the nerves.

In the present day every one allows that poisons may become absorbed, and that, provided they produce poisonous effects at all, they are absorbed, in whatever way they may have been applied to the body. But it is sometimes asked, Is this absorption necessary for their action? The following evidence may be briefly noticed as in some degree affording an affirmative answer to this question. Magendie divided all the parts of one of the posterior extremities of a dog, the artery and vein being reconnected by quills, so as to preclude the possibility of the effects being conveyed by the nervous filaments supplying the coats of the vessels; on applying a portion of upas tieuté to a wound in the foot, the symptoms of poisoning occurred, and death took place in ten minutes. If the veins leading from a poisoned part be tied, the arterial and nervous communication being complete, the symptoms of poisoning do not occur.

Mr. Blake introduced some prussic acid into the stomach of a dog, through an opening in its parietes, after he had ligatured the vessel entering the liver (the vena portæ, which, directly or indirectly, receives the gastric veins); no effect ensued until the removal of the ligature, within one minute of which proceeding the poison began to act. And lastly, not only has prussic acid been discovered in the blood of an animal which perished in thirty-five seconds, but in some experiments made by Mr. Erichsen, in a case of extroversion of the bladder, prussiate of potass was found in the urine within one minute of its being swallowed on an empty stomach.

The chief argument in favor of a sympathetic or direct nerve action, is the almost instantaneous manner in which some poisons act; fatal effects occurring, it is said, before sufficient time has elapsed to allow of absorption. It has, however, been proved that the round of the circulation may be accomplished much more speedily than has been imagined. Thus, the ferrocyanide of potassium injected into the jugular vein of a horse was discovered throughout the entire venous system in twenty-seconds; and Mr. Blake has inferred from his experiments that a poison may be diffused through the body in nine seconds. It may therefore be concluded that in most instances poisons act by being absorbed and conveyed with the blood to the different organs which they impair, or the nerve centres which rule the functions of these; some paralyzing the heart when they reach it, some affecting the brain or the spinal cord, some stopping the play of the lungs and others acting upon the different glands.

Nevertheless, in view of the extreme rapidity with which death is brought about in a few instances, the possibility of a direct shock to the nervous system causing death must not be overlooked.

The action of a poison may be variously modified, and the modifying circumstances must be carefully taken into consideration in the formation of a prognosis and in suggesting a line of treatment.