The influence of poisons on the brain seems to be sometimes induced, not immediately, but indirectly through the intervention of a more direct influence on the pulmonary circulation. Thus Mr. Blake appears to have succeeded in proving that the insensibility and tetanic convulsions which immediately precede death, when certain substances, such as the salts of soda, are injected into the veins, depend simply on the obstruction directly produced in the pulmonary circulation causing increased pressure in the systemic veins, and consequently upon the brain and nervous centre generally. For when the jugular vein was opened after the development of tetanic convulsions, and blood was allowed to flow out, the nervous symptoms ceased, and the animal continued for two hours sensible and without any return of convulsions, dying eventually of hemorrhage.[[45]] But more generally the effect produced on the brain is direct and specific. Thus opium and its active principle morphia suspend the functions of external relation, which are peculiarly dependent on the brain; while for a long time the respiration and circulation are little affected. Even when the poison is admitted directly into the veins, the pulmonary capillaries are not obstructed, and the heart is only somewhat enfeebled in its contractions;[[46]] and in ordinary cases of poisoning with these substances the heart continues to pulsate, and the lungs also discharge their office, long after sensibility is extinguished and voluntary motion arrested,—until at length the circulation and respiration become affected consecutively by the depressed state of the nervous system.
Some poisons act specifically on the spinal cord. Those which are best known to possess such an action are nux-vomica, the other species of plants which, like it, contain strychnia, and also conia and the wourali poison. The tribe of poisons of which nux-vomica may be taken as the type excite violent fits of tetanus, during the intervals of which the mind and external senses are quite entire; and death takes place during a paroxysm, apparently from suffocation caused by spasmodic fixing of the chest. Their action on the spine is quite independent of any action on the brain; if indeed such action exist at all. For when the spinal cord is separated from the brain by dividing the medulla oblongata, the effects on the muscles supplied by the spinal cord are produced as usual.[[47]] Conia, the active principle of hemlock, according to my own researches, produces in the lower animals, howsoever introduced, gradually increasing paralysis, without insensibility or delirium, and without the circulation or respiration being for some time affected, till at length death takes place from stoppage of the breathing by palsy of the respiratory muscles; and after death the heart continues beating vigorously, the muscles contract when irritated, and arterialization of the blood in the lungs may be kept up long by maintaining artificial respiration. In this instance it would appear, that the first effect is arrestment of the functions of the spinal cord; that the paralysis does not depend upon a direct action on the muscles; and that neither the brain, heart, nor lungs can be influenced, except secondarily through the consequences of general muscular paralysis.[[48]] Many poisons which act on the brain also act on the spinal cord.
Other poisons apparently possess the singular property of impeding or arresting the general capillary circulation, and produce their tangible effects more or less through the medium of this operation. Such at least are the inferences which seem to flow from the researches of Mr. Blake; who found that many substances, soon after they are injected backwards by the axillary artery into the aorta, produce increased pressure in the arterial system indicated by the hæmadynamometer during life, and frequently congestion of the membranous textures as observed after death. Some substances have no effect of this kind. Others act on the general capillaries in concurrence with a similar action on the capillaries of the pulmonary circulation, such as the salts of strontia, baryta, lead, silver, and soda, euphorbium, tobacco and digitalis. But a few, such as potash and ammonia, with their salts, seem to influence the capillaries of the general circulation only.[[49]] These are important conclusions, if legitimate; but it cannot be denied, that the facts on which they are based must be very difficult to isolate and observe with accuracy and without bias.
The organs not immediately necessary to life may be likewise all acted on by poisons indirectly. On this subject details are not called for at present. It may be sufficient to remark that there is hardly a considerable organ in the body, except perhaps the spleen and pancreas, which is not acted on by one poison or another. Arsenic inflames the alimentary mucous membrane, mercury the salivary organs and mouth, cantharides the urinary organs, chromate of potass the conjunctiva of the eyes, manganese the liver; iodine acts on the lymphatic glands; lead on the muscles; and spurred rye causes gangrene of the limbs.
Some poisons, as was already mentioned, may act on one important organ only, every other being left undisturbed: thus nux-vomica in general acts only on the spine. But much more commonly they act on several organs at once; and the action of some of them is complicated in an extreme degree. I may instance oxalic acid and arsenic. Oxalic acid when swallowed irritates and inflames the stomach directly, and acts indirectly on the brain, the spine, and the heart. A large dose causes sudden death by paralyzing the heart; if the dose is somewhat less, the leading symptom is violent tetanic spasm, indicating an action on the spine, and death takes place during a paroxysm, the heart continuing to contract for some time after; if the dose is still less, the spasms, at first distinct, become by degrees fainter and fainter, while the sensibility in the intervals, at first unimpaired, becomes gradually clouded, till at length pure coma is formed without convulsions,—thus indicating an action on the brain. As for arsenic, coupling together the symptoms during life and the appearances in the dead body, it will be seen afterwards to have the power of acting on the brain, heart, and lungs,—the throat, gullet, stomach, and intestines,—the lining membrane of the nostrils and eyelids,—the kidneys, bladder, and vagina; and, what is remarkable, proofs of an action on all these parts may be witnessed in the course of a single case. The effects of mercury are hardly less multifarious.
Section II.—On the Causes which modify the Actions of Poisons.
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.