A second factor which may also play its part in the loss of blood volume in the general circulation is the exudation of some of the plasma into the surrounding tissue spaces. As the stagnation increases, oxygenation decreases, and the walls and the capillaries become more permeable, so that some fluid is probably lost in this way. This permeability may also be accentuated by the increased hydrogen-ion concentration in the blood, which often accompanies shock, but it seems to be clear that this is a secondary phenomenon resulting from imperfect oxygenation in the tissues, and it will therefore not be regarded as one of the factors responsible for shock. Further fluid is lost by the copious perspiration commonly seen in shock. There seems, therefore, to be a conspiracy between a whole set of different factors all tending to deprive the patient of his circulating fluid. The net result is a condition so closely resembling hæmorrhage that it may be impossible to distinguish the two, this difficulty being increased by the fact that they so often occur together. In the foregoing account of the production of shock the fate of the lost blood has been discussed, but nothing has been said of the factors initiating the capillary stagnation. This is a subject which is of great interest and some obscurity, and is of evident importance in considering how shock may be avoided. The present treatise, however, is primarily concerned with the treatment of shock when already established, and it is therefore not proposed to follow out the other question in detail. An injury may be followed immediately by a condition of “primary wound shock,” in which the patient becomes suddenly pale and pulseless. This is a physiological reaction, which may be transient, and it is to be distinguished from the much more serious condition of “secondary wound shock” which appears some time later. It is this secondary shock alone which has been under consideration in the preceding pages. The chief importance of the primary shock lies in the fact that it may initiate the conditions which predispose to secondary shock, so that under certain circumstances the one may become merged in the other. These predisposing conditions are increased evaporation from the skin, a general fall in the temperature of the body, mental anxiety, and the continued stimulation of the higher centres by afferent impulses as is manifested by pain. The condition of secondary wound shock was shown in a striking degree, during the earlier years of the war, by the men suffering from fracture of the femur. In the later part of the war warmth was supplied more systematically than before to the seriously wounded, and all fractured femurs were treated at an early stage with Thomas’s splints. Two of the factors predisposing to shock, namely cold and pain, were in this way to some extent eliminated, and it was very striking how much better than before was the general condition of the patients on arrival at the hospitals.