Voit claims that the proteid dissolved in the fluids 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 metabolism. 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 not 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 controlling 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 all the proteid of our daily food is built up into living protoplasm before katabolism 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 metabolism 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 less variable in amount, has its origin in the metabolism of the true tissue proteid, or the actual living framework of the body.