Moreover, it is now quite certain that of the two principles the intermediary body alone is a fixed, native element of the blood plasma, and the complement is subject to considerable fluctuations in quantity. The origin of the intermediary body has not been determined, while it is quite established that the complement is yielded by the white corpuscles, or leucocytes, of the blood. This matter of the origin of the complement is very important because the protective value of the blood fluid is determined by the quantity of complement available at any one time and not so much by the more constant intermediary body which is usually in excess of the complement. The complement would appear to arise from the leucocytes partly as a secretion; but the quantity derived in this way would not appear to be considerable. It also arises from leucocytes which are brought by any cause to degeneration and disintegration, and this would seem to be a richer source than the other. Leucocytes are constantly being worn out by physiological use and as constantly yielding up their complement to the blood as they go to pieces. It would appear, then, that the very essential complement which exists in the circulating blood and passes from the blood into the lymph and serous cavities, will be more or less determined in quantity by the number of blood leucocytes and the conditions to which they are exposed, and as they are brought to slower or faster degeneration; and it is extremely probable that the secretion of complement is influenced also by the nature of the stimuli to which even the living leucocytes are exposed. It has been shown beyond peradventure that the blood plasma contains less complement than blood serum, as would now be expected since the origin of complement from degenerating leucocytes has been abundantly shown, and because in the clotting of the blood the leucocytes are so greatly disintegrated. But I do not think that even the most ardent adversaries of the view that the fluids of the interior of the body do not exert direct bactericidal effects, have been able to show that the plasma contains no complement. The complement is such a labile body that doubtless it is constantly used up physiologically and must therefore as constantly be renewed, and it is highly probable that the balance between production and destruction may not always be maintained, whence a considerable fluctuation may occur even in health. Whether the fluctuations ever synchronize with intending infections in such a manner as to promote them is not really known, but is not impossible.
It is, however, patent that the naturally operative defensive mechanisms against bacterial invasion must contain other factors than these humoral ones. We are all now prepared to admit that in the phagocytes, or the devouring white corpuscles of the blood, the body possesses another defensive system of high efficiency. The motile nature of these cells and their presence in the circulating blood accord them a high degree of mobility, so that they can be quickly dispatched to any part of the body threatened by invaders, and are hardly behind the fluids of