the blood-vessels; although it is not so clear, that after it has entered them, it acts by being carried with the blood to the brain. The newest doctrine supposes that it enters the blood-vessels, and produces on their inner coat an impression which is conveyed along the nerves.
According to the experiments of Professor Orfila, it is more energetic when applied to the surface of a wound than when introduced into the stomach, and most energetic of all when injected into a vein.[1] The inference generally drawn from these and other analogous experiments[2] is, that the blood transmits the poison in substance to the brain. They certainly, however, do not prove more than that the poison must enter the blood before it acts.
The old doctrine, that the blood-vessels have no concern with its action, and that it acts only by conveyance along the nerves of a peculiar local torpor arising from its direct application to their sentient extremities, has been long abandoned by most physiologists as untenable. But some have adopted a late modification of this doctrine, by supposing that opium may act both by being carried with the blood to the brain, and by the transmission of local torpor along the nerves. They believe, in fact, that opium possesses a double mode of action,—through sympathy as well as through absorption. It would be fruitless to inquire into the grounds that exist for adopting or rejecting this doctrine, because sufficient facts are still wanting to decide the controversy. So far as they go, however, they appear adverse to the supposition of a conveyance of impressions along the nerves, without the previous entrance of the poison within the blood-*vessels. The difficulties, in the way of the theory of the sympathetic action of opium, would be removed by the doctrine of Messrs. Morgan and Addison. According to their views, the experiments, which appear at first sight to prove that this substance operates by being carried with the blood to the part on which it acts, are easily explained by considering that the opium makes a peculiar impression on the inside of the vessels, which impression subsequently passes along the nerves to the brain.[3] But, as stated in the introductory chapter on the physiology of poisoning, this theory requires support.
The effects of opium, through whatever channel it may produce them, are exerted chiefly on the brain and nervous system. This appears from the experiments of a crowd of physiologists, as well as from the symptoms observed a thousand times in man. In animals the symptoms are different from those remarked in man. Some experimentalists have indeed witnessed in the higher orders of animals, as in the human subject, pure lethargy and coma. But the latest researches, among the rest those of M. Orfila, show that much more generally it causes in animals hurried pulse, giddiness, palsy of the
