NUTRITION 679 whatever kind the blood entering the tissue always has a different composition from the blood which leaves it. The tissues, therefore, are laboratories in which materials abstracted from the blood are transformed. To these chemical operations of transformation which occur in living tissues the term " metabolic " has been applied, a term first used by Schwann, and happily reintroduced by Michael Foster. What now becomes of the products of the metabolic activity of the tissues 1 We have hitherto considered the tissues as taking matter from the blood, changing the form of it, and giving it back to the blood ; but this is far from being the true account of the process. It does indeed represent all that we know of the metabolism of many tissues. In muscle, for example, matters are drawn from the blood, converted to other shapes within the tissue, and sooner or later cast out into the blood-current again. The same may be said of nerve-tissue and possibly of some other tissues. In secreting glands the case is different. Some only of the products of tissue-metabolism are returned to the blood ; others are poured into the ducts of the glands as the glandular secretion, and so leave the body altogether. This happens, for example, in the digestive glands, the milk -glands, and the kidneys. In a third order of tissues the case is different again, for here some of the products of metabolism may be retained in the tissue for an indefinite time. This occurs in certain tissues which have been called storage tissues, and of which fat is a typical example. Lastly, the liver is a complex organ whose metabolic products are disposed of in all three ways, part being cast at once into the blood, part being accumulated in the tissue itself as glycogen and passing into the blood at intervals as the body needs it, and a third part being poured into the biliary ducts in order that it may escape into the intestine. It will be well, therefore, to recognize three methods of the disposal of metabolic products, and to classify the tissues accordingly. Metabolism of Muscular Tissue. To arrive at a know ledge of the chemico-vital changes occurring in any tissue we must compare the matters entering the tissue with the matters issuing from it. It is only from such a compari son that we can infer the changes which go on within the tissue. In the case of muscle there is yet another method by which we can obtain inferences as to the nature of the metabolic changes. We have reason to believe that the tissue-changes of active muscles are simply the exagger ated form of changes which constantly occur. If we may assume this, a comparison of the chemical composition of muscle before and after a period of activity will help us to a knowledge of the changes that occur in muscular action, and, by implication, a knowledge also of the com mon metabolic changes of the tissue. Now a comparison of muscle before and after action shows that during activity the quantity of CO 2 becomes very largely increased ; at the same time the muscle becomes of acid reaction from the development of lactic acid ; the amount of bodies soluble in water decreases, while the amount of bodies soluble in alcohol increases ; the amount of glycogen decreases, while that of sugar increases ; and, finally, bodies develop ^which have a strong affinity for oxygen. If we compare the matters entering and issuing from muscular tissue we shall find that muscle gives up much CO 2 during activity, that the issuing blood is rich in reducing substances, and that it acquires sarcolactic acid. The circumstance of most importance in this comparison is that carbon dioxide is liberated in large amount without the immediate inter action of oxygen ; the CO 2 is produced from some body in the muscle which already contains within itself the O necessary for its formation. The fact of next importance is the absence of all indications of a large expenditure of nitrogen. Relying on these discoveries, physiologists have supposed that the metabolic changes of muscle follow some such course as the following. Non- nitrogenous organic material, along with oxygen, is absorbed into the tissue from the blood. In the tissue-cells it is elaborated with nitrogenous matter already in the tissue into a complex body called inogene substance, containing a nitrogenous factor linked to carbon, hydrogen, and oxygen. During the life of the tissue, and to a much greater extent during muscular activity, this inogene substance is split up into a nitrogenous portion and a non-nitrogenous portion, of which the former is retained in the tissue to be again worked up into inogene substance, while the latter is resolved into more stable bodies which escape. The full number of the effete bodies is not at present known to us ; it doubtless includes more than carbon dioxide and sarcolactic acid, but these are the only bodies whose existence has been definitely determined. Metabolism of Glandular Organs. Of late years very much light has been shed on the processes of gland ular cells, but it has been chiefly on their anatomical aspect. We know very little, indeed, of the nature of the chemical changes going on within the glandular cells, because, although we can ascertain approximately the com position of those products of the changes which escape into the glandular duct, we as yet know so little of those products which escape into the blood. We know, how ever, that mucin is capable of being formed in the interior of epithelial cells out of some non- mucous antecedent. We also have some reason for supposing that the gastric glandular cells the border -cells select phosphate of sodium and sodium chloride from the blood, and these bodies have been shown to be capable of interacting in such a way as to yield free hydrochloric acid. In the secretion of milk it is tolerably certain that the fat and the casein are both formed out of proteicl matter within the glandular cells. Even the milk-sugar has been thought to be elaborated out of some non-saccharine substance, or even out of some substance that is not a carbohydrate. In the case of the pancreas we know nothing of the actual chemical decompositions that occur, but we do know that they occur in two well-marked stages. In the first a body is formed which is stored in the gland-cells, as the inogene substance is supposed to be stored in muscle; in the second this body is resolved into other but still complex bodies, of which the trypsin ferment of the secretion is one. Among the glandular organs we may mention the kidney, respecting whose secretive activity it is far from certain that it entirely consists of physical filtration or powers of mere selection. No doubt many of the con stituents of urine are simply filtered off from the blood, while others are undoubtedly abstracted owing to some peculiar attraction which the renal cells exert upon them. But there is some reason to think that others may be formed within the kidney-tissues as products of cell-meta bolism. As yet we have no definite knowledge concerning the nature of the metabolic changes. Metabolism in the Storage Tissues. Of these the first that we shall describe is adipose tissue or fat. Fat appears to be accumulated in ordinary connective-tissue corpuscles. What metabolic changes occur in connective -tissue cor puscles which are not accumulating fat we do not know ; but under certain conditions of overfeeding the metabolism is so varied that fat is deposited within the cell. At first it appears in the form of fine granules ; these soon coalesce to globules, which afterwards enlarge by the confluence of granules subsequently formed. The exact changes of which this fat is the outcome, or one of the products, are entirely obscure. It is enough if we can establish the probability that fat may be derived from the proteid proto-
