In discussing these questions, we shall need to refer to several of the current theories concerning proteid metabolism, notably, the theories of Voit, Pflüger, and Folin. In 1867 Carl Voit,[50] 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 absorption being 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, in 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 units. 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.” [50] See Voit: Hermann’s Handbuch der Physiologie, Band 6, p. 301.