Nevertheless, the elimination of these factors, which is a simpler matter in civil life than it was under conditions of war, will not avert all shock in a large proportion of cases. It is necessary, therefore, to find some additional factor which will initiate shock in addition to the predisposing causes. It is thought that this may have been identified in a substance of obscure nature which is derived from the damaged tissues themselves, and which, circulating in the blood, is able directly to affect the capillary system. Just as the shock following severe burns is believed to be due to the circulation of a toxic substance formed by the burning of the skin and other tissues, so the shock following severe trauma is believed to be of toxic origin, the toxin being derived from damaged tissues, muscle being particularly active in this respect. The condition may, therefore, be one of “traumatic toxæmia,” in which there is a general loss of capillary tone throughout the body, so that “the blood percolates into the network of channels as into a sponge.” The circulating blood is thus rapidly depleted, and the symptoms of shock become established. The investigation of this source of shock was carried out chiefly by Dale, Bayliss and Cannon (65), who were able to reproduce the condition of shock in animals by the injection into their circulation of a substance obtained from damaged muscles. To this substance the name histamine was given. It would be a mistake, however, to suppose that because a substance producing shock experimentally has been obtained from muscles, that therefore this is the identical substance which is responsible for every case of traumatic toxæmia. Extreme shock may be produced when but little damage has been done to muscles. Probably damage to any tissue of the body if extensive enough will produce a substance or substances which will give rise to the symptoms, and it may be a long time before these are isolated and identified. That the last word on the production of shock is still far from being uttered is shown by the fact that profound shock may be induced without doing any appreciable damage to tissue, namely, by handling and exposing the abdominal viscera. It may be this traumatic toxæmia which will account for many cases of post-operative shock, but it has been shown that some anæsthetics, such as chloroform or ether, will of themselves greatly accentuate shock initiated by other causes. It has already been mentioned that the increased hydrogen-ion concentration in the blood, which results from imperfect oxygenation in the tissues, is not itself a cause of shock, but it will aggravate shock due to other factors. A discussion of this will be found in the paper by W. B. Cannon already referred to.