_A medicine introduced into the system elsewhere acts in the same way as when introduced into the stomach._
This proves at least that contact with the mucous surface is not an essential requisite for the operation of a remedy. We may cause a medicine to be absorbed by the skin, or inject it directly into an opened vein. The result of this latter experiment proves not only that entry into a distant part of the system is sufficient for the action of a medicine, but that it will operate when introduced into the blood. Numberless proofs of this affirmation may be adduced. Tartar Emetic injected into the veins produces vomiting. Croton Oil rubbed on the surface of the abdomen causes purging. Mercurial ointment applied by friction to the skin will produce salivation. Extract of Belladonna applied to the temples causes dilatation of the pupil of the eye; and tincture of Opium dropped on the eyeball causes the pupil to contract. Ammonia inhaled as gas into the lungs will relieve fainting in the same way as when swallowed. The breathing of Prussic acid, causing its vapour to be applied to the pulmonary surface, is sufficient to kill. Prussic acid, dropped in a concentrated state into the eye of a dog, causes speedy death. Solution of Aconitina, applied to the skin, will produce numbness, and tingling of distant parts. Injection of Nux Vomica, or any powerful poison, into the veins, is rapidly followed by symptoms of poisoning, like those which would have followed its introduction into the stomach.[17] Thus contact with the stomach is not necessary, but introduction into the system any where is sufficient. But still, may not the poison in either case act by influencing the nerves? Even when it has entered the blood, it may not travel along in it, but act in a more direct way. So in the second part of the proof we must show that a remedy cannot act by an impression conducted from the surface of the stomach by means of the nerves.
_The continuity of nerve is not necessary for the propagation of such effects; but vascular connexion is necessary._
This alone, if established, would be sufficient to prove that a medicine must be introduced into the circulation, in order to act on distant parts. When confined to a surface, it can operate on the remote part only by its contact with the superficial extremities of the nerves. For vascular connexion to be established, it must first enter the vessels. Many experiments have been made which demonstrate that the vessels are the only channel by which medicinal effects can be propagated.