In discussing these questions, we shall need to refer to several of the current theories concerning proteid metabolism, notably, the theories of Voit, Pfltiger, and Folin. In 1867 Carl Voit,1 of Munich, advanced the view that the proteid material of the body exists in two distinct forms, viz., as "morphotic" or "organized" proteid, representing proteid which has actually become a part of the living units of the body, i. e., an integral part of the living tissues; and "circulating " proteid, or that which exists in the internal meshes of the tissue, or in the surrounding lymph and circulating blood. The real point of distinction here is that while one portion of the body proteid is raised to the higher plane of living matter, i. e., becomes a component part of the living protoplasm, another and perhaps larger portion is outside of the morphological framework of the tissue, constituting a sort of internal medium which bathes the living cells, and acts as middleman between the blood and lymph on the one side and the living cells on the other. According to Voit's view, it is this circulating proteid that undergoes metabolism; the proteid of the food after digestion and absorptionbeing carried to the different tissues and organs, and then, without becoming an integral part of the living protoplasm of the cells, it is broken down under the influence of the latter. Obviously, small numbers of tissue cells are constantly dying, their proteid matter passing into solution, where it likewise undergoes metabolism. In other words, according to Voit, the great bulk of the proteid undergoing katabolism is the circulating proteid, derived more or less directly from the food, and which at no time has been a part of the tissue framework; while a smaller, but more constant amount, represents the breaking down of tissue cells. This conception of proteid metabolism is akin to our conception of morphological and physiological destruction. In the words of Foster: "We know that an epithelial cell, as notably in the case of the skin, may be bodily cast off and its place filled by a new cell; and probably a similar disappearance of and renewal of histological units takes place in all the tissues of the body to a variable extent. But in the adult body these histological transformations are, m the cases of most of the tissues, slow and infrequent, A muscle, for instance, may suffer very considerable wasting and recover from that wasting without any loss or renewal of its elementary fibres. And it is obvious that the metabolism of which we are now speaking does not involve any such shifting of histological unite. On the other hand, we find these histological units, the muscle fibre or the gland cell, for instance, living on their internal medium, the blood, or rather on the lymph, which is the middleman between themselves and the actual blood flowing in the vascular channels."

1 See Voit: Hermann's Handbuch der Physiologic, Band 6, p. 301.

Voit claims that the proteid dissolved in the Quids of the body is more easily decomposable than that which exists combined in organized form, or as more or less insoluble tissue proteid; and it is this soluble and circulating form which, under the influence of the living cells, undergoes destruction or metallism. We know, as has been previously stated, that oxidation does not take place to any extent in the circulating blood, and similarly there is every reason for believing that proteid metabolism does no occur in this menstrum Metabolism is limited mainly to the active tissues of the body, but according to the present conception of the matter it does not occur at the expense of the proteid of the living cells, but involves material contained in the fluids bathing the cells; i. e., it is not the organized proteid that undergoes metabolism, but the proteid circulating in and about the internal meshes of the cells and tissues, the living cell being the active agent in controlbng the process. Further, this view lessens the difficulty of understanding the elimination of nitrogen after a meal rich in proteid. If it was necessary to assume that ail the proteid of our daily food is built up into living protoplasm before kataboliam occurs, it would be exceedingly difficult to explain the sudden and rapid elimination of nitrogen which follows the ingestion of proteid. For example, we can hardly imagine that merely eating an excess of proteid food will lead to an actual breaking down of the living framework of the tissues, equivalent to the amount of nitrogen which the body at once eliminates. Voit's theory, on the other hand, supposes a twofold origin of the nitrogen excreted; one part, the larger and variable portion comes from the direct metabo-lism of the circulating proteid, being the immediate result of the ingested food and varying in amount with the quantity of proteid food consumed; the other, smaller and leas variable in amount, has its origin in the metabolism of the true tissue proteid, or the actual living framework of the body.

In a fasting animal, the tissues and organs of the body still contain a large proportion of proteid matter, yet only a small fraction of this proteid is eliminated each day, hardly 1 per cent. If, however, proteid is absorbed from the intestine, proteid metabolism is at once increased, and the excretion of nitrogen may be fifteen times greater than during hunger. In other words, the extent of proteid metabolism is not at all proportional to the total amount of proteid contained in the body as a whole, but runs parallel in a general way with the quantity of proteid absorbed from the intestine. Obviously, the newly absorbed proteid is quite different in nature from the proteid which in much larger amounts is deposited throughout the body, since it is not organized and is so much more easily decomposable (Voit). This is the circulating proteid of the body; it exists in solution, and it is a significant fact that, according to Voit, the chemical transformations that characterize proteid katabolism occur only in solution. The organized proteid, on the other hand, is in a state of suspension, and its katabolisiu, which is relatively very small, is preceded by solution of the proteid in the fluids of the tissue, after which its further breaking down is assumed to be the same as that of the circulating proteid. This latter view is a fundamental part of the Voit theory; in long-continued fasting, for example, the living protoplasm of the various tissues and organs is of necessity drawn upon for the nourishment of the more vital parts of the body, such as the brain, spinal cord, etc., consequently the organized proteid is gradually dissolved and then decomposed, after it has become liquefied and has thus lost its organized structure.