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The same conclusion followed naturally from a consideration of the properties of irritants. Greatly as they differ in their nature, whether physical, as mechanical violence, heat, and the electric shock, or chemical of the most varied characters, one feature they have all in common; if pushed far enough they destroy the tissues on which they act. Extreme inflammatory congestion is the state which they produce when their action is just short of the lethal degree; and it could hardly he doubted that the state of the tissues just short of death must be one of impairment of vital power.
This view was beautifully confirmed by a series of observations to which I was led by a most unlooked for experience. Before I had adopted the method, which I have described, of obtaining a perfectly tranquil state of the frog s foot, I sought to study the local effect of an irritant by placing on the middle of the web a small piece of moistened mustard, which could not be shifted in position like a drop of liquid when the animal struggled. On removing the mustard after a while to observe its effects, I was astonished to see the part of the web on which it had lain, not only affected with inflammatory congestion, but totally different in colour from the rest of the web in consequence of a difference of arrangement of its pigmentary constituents. Where the mustard had rested, the pigment appeared as a delicate black network among the tissues, causing an extremely dark appearance; whereas in the rest of the web it was gathered into
