Читать книгу Treatise on Poisons - Robert Sir Christison - Страница 5

By a variety of causes the action of poisons may be modified both in degree and in kind. The most important of them are—quantity; state of aggregation; state of chemical combination; mixture; difference in tissue; difference in organ; habit; idiosyncrasy; and lastly, certain states of disease.

1. Quantity affects their action materially. Not only do they produce their effects more rapidly in large doses; it is sometimes even quite altered in kind. A striking example has just been related in the case of oxalic acid; which, according to the dose, may corrode the stomach, or act on the heart, or on the spine, or on the brain. In like manner arsenic in a small dose may cause gastritis of several days’ duration; while a large dose may prove fatal in two or three hours by affecting the action of the heart. White hellebore in small doses excites inflammation in the stomach and bowels, in larger doses giddiness, convulsions, coma; and in either way it may prove fatal.

2. As to state of aggregation,—poisons act the more energetically the more minutely they are divided, and hence most energetically when in solution. Some which are very energetic in the fluid state, hardly act at all when undissolved. Morphia, the alkaloid of opium, may be given in powder to a dog without injury in a dose, which, if dissolved in oil or alcohol, would soon kill several. Previously dissolving poisons favours their action in two ways,—by diffusing them quickly over a large surface, and by fitting them for entering the bibulous vessels. Poisons, before being absorbed, must be dissolved; and hence, those which act though solid and insoluble in water, must, as a preliminary step, be dissolved by the animal fluids at the mouths of the vessels. In this way the poisonous effects of carbonate of baryta and arsenite of copper are explained; for though insoluble in water, they are soluble in the juices of the stomach.

Differences in aggregation, like differences in quantity, may affect the kind as well as the degree of action. Camphor in fragments commonly causes inflammation of the stomach; dissolved in spirit or olive oil, it causes delirium or tetanus and coma.

The reduction of certain poisons to the state of vapour serves the same end as dissolving them. When poisons are to be introduced by the skin, no previous operation is more effectual than that of converting them into vapour.

3. The next modifying cause is chemical combination. This is sometimes nothing more than a variety of the last. If a poison, in combining with another substance, acquire greater solubility, it also generally acquires greater activity, and vice versa: Morphia, itself almost inert, because insoluble, becomes active by uniting with acids, for they render it very soluble: Baryta as a very active poison, becomes quite inert by uniting with sulphuric acid, for the sulphate of baryta is altogether insoluble.

In regard to the influence of chemical combination two general laws may be laid down. One is, that poisons which only act locally, have their action much impaired or even neutralized, in their chemical combinations. Sulphuric acid and muriatic acid on the one hand, and the two fixed alkalis on the other, possess a violent local action; but if they are united so as to form sulphates or muriates, although still very soluble, they become merely gentle laxatives. But the case is altered if either of the combining poisons also act by entering the blood. For the second general law is, that the action of poisons which operate by entering the blood, although it may be somewhat lessened, cannot be destroyed or altered in their chemical combinations. Morphia acts like opium if dissolved in alcohol or fixed oil; if an acid be substituted as the solvent, a salt is formed which is endowed with the same properties: The sulphate, muriate, nitrate, acetate of morphia all act like opium. Strychnia, arsenic, hydrocyanic acid, oxalic acid, and many more come under the same denomination: Each produces its peculiar effects, with whatever substance it is combined, provided it do not become insoluble.

Mr. Blake has recently laid down what may be considered a branch or corollary of the second of these general propositions, and has confirmed it by many appropriate experimental facts,—namely, that the salts of the same base produce the same actions, independently of the acids with which they are combined.^[50] The law, however, is a more general one, as given above, and was stated in former editions of the present work. It applies not only to bases, but likewise to acids, such as the hydrocyanic, oxalic, arsenious, and arsenic acids, and also to neutral organic principles which act through the blood, such as picrotoxin, colocynthin, elaterin, and narcotin.

The same author considers it to be also a probable conclusion from a variety of experiments on the salts of various bases, that those salts which are isomorphous, or possess the same crystalline form, are closely allied in action.^[51]

4. The effect of mixture depends partly on the poisons being diluted. Dilution, by prolonging the time necessary for their being absorbed, commonly lessens their activity; yet not always; for if a poison which acts through the blood is also a powerful irritant, moderate dilution will enable it to enter the vessels more easily: a small dose of concentrated oxalic acid acts feebly as an irritant or corrosive; moderately diluted, it quickly enters the blood and causes speedy death.^[52] The effect of mixture may depend also in part on the mere mechanical impediment interposed between the poison and the animal membranes. This is particularly obvious when the mass containing the poison is solid or pulpy; for then the first portions of the poison that touch the membrane may cause an effort of the organ to discharge the rest beyond the sphere of action,—if, for example, it is the stomach,—by vomiting. The effect of mixture in interposing a mechanical impediment is also well illustrated where the substance mixed with the poison is a fine, insoluble powder, capable of enveloping its several particles. Thus it is that small, yet poisonous doses of arsenic may be swallowed and retained with impunity, if mixed with finely powdered charcoal, magnesia, and probably cinchona-bark, or the like. Besides diluting and mechanically obstructing their application, the admixture of other substances may alter the chemical nature of poisons, and so change their action.

It is important to keep in view, that the influence of mixture may be exerted in consequence of the cavity into which a poison is introduced being at the time filled with contents. Some of the most powerful and unerring poisons may in such circumstances altogether fail to produce their usual effect, if speedily vomited. Thus Wibmer notices the case of a man, who swallowed an ounce and a half of arsenic after a very hearty meal, had merely a severe attack of vomiting with subsequent colic, and got quite well in four days.^[53] And a still more pointed instance has been briefly mentioned by Dr. Booth of Birmingham, where an ounce of corrosive sublimate was swallowed after a full meal without any material ill consequence, vomiting having been speedily induced.^[54]

5. Difference of tissue is an interesting modifying power in a physiological point of view, but does not bear so directly on medico-legal practice as the rest, and may therefore be passed over cursorily.

On the corrosives and irritants a difference of tissue acts but indirectly: their effects vary not so much with the tissue as with the organ of which it forms part. But as to poisons which act through the blood, their energy must evidently depend on the activity of absorption in each texture.

The cutaneous absorption is slow, on account of the obstacle presented by the cuticle, and by the intricate capillaries of the true skin. Accordingly many active poisons are quite inert when applied to the unbroken skin, or even to the skin deprived of the cuticle. Hydrocyanic acid, perhaps the most subtle of all poisons, was found by Coullon to have no effect when dropped on the skin of a dog.^[55] Some authors have even gone so far as to deny that poisons can be absorbed at all through the skin, unless they are pressed by friction through the cuticle. But this is an error; most gaseous poisons, such as carbonic acid and sulphuretted hydrogen, and some solid poisons when volatilized, such as the vapours of cinnabar, will act though simply placed in contact with the skin; and there is distinct evidence that corrosive sublimate will bring on mercurial action in the form of a warm bath, or when used as a liniment.

On the mucous membrane of the stomach and intestines, poisons act much more energetically than on the skin; which clearly depends in a great measure on the superior rapidity of absorption there,—or, according to some, on the facility with which poisons come in contact with the sentient extremities of nerves.

The serous membranes possess an activity of absorption which hardly any other unbroken texture can equal. Accordingly many poisons act much more rapidly through the peritonæum than through the stomach. When oxalic acid is introduced under the same collateral circumstances into the stomach of one dog and the peritonæum of another, the dose may be so apportioned, that the same quantity, which does not prove fatal to the former, kills the latter in fourteen minutes.^[56]

While the preceding modes in which poisons enter the blood are indirect, they may be introduced directly by a wound in the vein. There is no way in which poisons, that act through the blood, prove more rapidly fatal. Some which act very slowly through the stomach cause instant death when injected into a vein. A peculiar variety of this mode of introducing poisons deserves to be distinguished, namely, the application of them to a wound. If the surface bleeds freely, they may not act at all, because they are washed away. But if they adhere, they soon enter the divided veins. Hence, if they act in small doses, this mode of applying them is hardly less direct than if they were at once injected into a vein.

So far the effect of difference in tissue has been determined. Poisons that act through the blood act least energetically on the skin, more actively on the alimentary mucous membrane, still more so on serous membranes, and most powerfully of all when introduced directly into a vessel. There are other textures, however, which merit notice, although their place in the scale of activity has not been exactly settled.

On the mucous membrane of the pulmonary air-cells and tubes, poisons act with a rapidity which is scarcely surpassed by their direct introduction into a vein. This is plainly owing to the exceeding delicacy and wide surface of the membrane. Hence three or four inspirations of carbonic oxide gas will cause instant coma. A single inspiration of the noxious gas of privies has caused instant extinction of sense and motion. Nay, liquid poisons have been known to act through the same channel with almost equal swiftness. For M. Ségalas found that a solution of extract of nux-vomica caused death in a few seconds when injected in sufficient quantity into the windpipe; and that half a grain will thus kill a dog in two minutes, while two grains will rarely prove fatal when injected into the stomach, peritonæum, or chest.^[57]