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] As to the nervous tissue, it is a fact worthy of mention, that the poisons which appear to act on the sentient extremities of the nerves, do not act at all on the cut surface of the brain and nerves, or upon any part of the course of the latter. This has been proved with respect to most active narcotics. The power of the cellular tissue as a medium of absorption, has not been, and cannot easily be, ascertained. On the one hand it is difficult to apply poisons to it, without also applying them to the mouths of divided vessels; and, on the other hand, it is difficult to make a set of experiments for comparison with others on the stomach, pleura, or peritonæum, as the cellular tissue does not form an expanded cavity, and consequently, the extent of surface to which a poison is applied cannot be made the same in each experiment of a series. It is a ready medium, however, for admitting poisons into the blood, especially if an artificial cavity be made where the tissue is loose, as, for example, by separating the skin from the muscles of the back with the finger introduced through a small incision in the integuments. The variations caused by difference of tissue in the activity of poisons have been viewed in the previous remarks as depending chiefly on the relative quickness with which absorption goes on. But in this way it is impossible to explain the whole amount of the differences sometimes observed. Some poisons cause death when applied to a wound in the minutest quantity, but are quite harmless when swallowed in large doses: Others are diminished a little in activity, but still remain powerful and fatal poisons. There is not much difference in the power of arsenic when it is applied to different textures, the skin excepted. But oxalic acid injected into the peritonæum will act eight or ten times more rapidly than when swallowed and the poison of the viper may prove fatal to a man through a wound in almost invisible doses, while the whole poison of six vipers may be swallowed by so small a creature as a blackbird, with complete impunity.[58] Differences in the absorbing power of the tissues cannot explain these facts.