experiment makes it difficult to carry on the investigation with scientific accuracy.

2. Even when the amounts of gain and loss exactly correspond we cannot say that we have the best dietary; because some of the income may be quite useless, and pass through the economy without performing any function, and yet appear in the output so as to give an accurate balance.

3. We have just seen that the relative amounts of outgoings and of material laid by as store are altered and regulated by the quantity of income. And we find that the quality of the income, i. e., the relative proportions of the various food stuffs, has a material influence on the quantities of material laid by and eliminated respectively. We must, therefore, consider the efficacy of each of the groups of the food stuffs when employed alone and mixed in different proportions.

4. Different animals seem to have different powers of assimilation; and under various circumstances the requirements and assimilative power of the same animal may vary.

Nitrogenous Diet.-An animal fed upon a purely meat diet requires a great amount of it to sustain its body weight. It has been found that from to of the body weight in lean meat daily is necessary to keep an animal alive without either losing or gaining weight. If more than this amount be supplied the animal increases in weight, and as its weight increases a greater amount of meat is required to keep it up to the new standard. that, to produce a progressive increase of weight with a purely meat diet, it is necessary to keep on increasing the quantity of meat given. The reason of this is found in the fact that albuminous diet causes an increase in the changes occurring in the nitrogenous tissues.

If an animal which is in extremely poor condition be given an ad libitum supply of lean meat, only a limited portion of the albuminous substance is retained in the tissues. By far the larger proportion of the nitrogenous food is given off and is represented in the urine by urea, and a comparatively small proportion is stored up. If this large supply of meat diet be continued for some time, less and less of the albuminous material is stored,

more and more being eliminated as urea, until finally the urea excreted just corresponds to the albuminous materials in the ingesta. When only meat is given, it must be supplied in large quantities to maintain the balance of nitrogenous income and expenditure, which is spoken of as nitrogenous equilibrium. Upon the occurrence of a change in the amount of nitrogenous ingesta this nitrogenous equilibrium varies, and it takes some time to become reëstablished, because a decrease in the meat diet is accompanied by a decrease in the weight of the animal, and an increase causes it to put on flesh. For each new body weight there is a new nitrogenous equilibrium, which is only attained after the disturbed relation between the nitrogenous ingesta and excreta has been readjusted.

The increase of weight which follows a liberal meat diet depends in a great measure on fat being stored up in the body. Much more of this material is made than could come from the fat taken with the meat; hence, we must conclude that it is made from the albuminous parts of the meat.

Non-nitrogenous Diet.-The effect of a diet without any albuminous food is that the animal dies of starvation nearly as soon as if deprived of all forms of food, with the exception that the weight of the body is much less reduced at the time of death.

Mixed Diet.-The addition of fat or sugar to meat diet allows of a considerable reduction in the supply of meat, both the body weight and nitrogenous tissue change preserving their equilibrium on a smaller amount of food. It has been estimated that the nitrogenous tissue change is reduced seven per cent. by the addition of fat, and ten per cent. by the addition of carbohydrate food to the meat diet; therefore less meat is wanted to make up nitrogenous tissues. Further, fats and sugars, which obviously cannot of themselves form an adequate diet, since they contain no nitrogen, seem to have the power of accomplishing some end in the economy which, in their absence, requires a considerable expenditure of nitrogenous materials to bring about. Fats and sugars, then, supply to the body readily oxidizable materials, and thus shield the albuminous tissues from oxidation, as well as reduce absolutely the nitrogenous metabolism.

It would further appear from the experience gained from the stall feeding of animals that a good supply of carbohydrates, together with a limited quantity of nitrogenous food, is admirably adapted to produce fat. Since much more fat has been found to be produced in pigs than could be accounted for by the albuminous and fatty constituents of their diet, we must suppose that from their carbohydrate food fat can be manufactured in their body.

Much of the difficulty found in reconciling the opinions of different authors concerning the sources of fat in the body can be removed, and some knowledge of the manufacture of fats from the food stuffs can be gained by bearing in mind the properties of the protoplasm. There can be no doubt that protoplasm, if properly nourished, can manufacture fat. As examples, we may take the cells of the mammary gland and connective tissue. This fat production may be regarded as a secretion of fat, though only in one of the examples given does it appear externally as a definite secretion-milk. We cannot scrutinize the chemical methods by which this change is brought about in protoplasm, any more than those which give rise to the special constituents of other secretions. We know that protoplasm uses as pabulum, albumin, fat, and carbohydrate, and we have no reason to doubt that the proportion of these materials found to form the most nutritious diet for the body generally, is also the proportion in which protoplasm can best make use of them. Probably cells which secrete a material containing nitrogen, such as mucinyielding gland cells, require a greater proportion of albumin. Those cells which produce a large quantity of non-nitrogenous material may not require more nitrogen than is necessary for their perfect re-integration as nitrogenous bodies. In the manufacture of their secretion, they only require a pabulum which contains the same chemical elements as are to be found in the output. In the case of fat formation, a supply of fat or carbohydrate ought to suffice if accompanied by a small amount of albuminous substance. If these non-nitrogenous substances be withheld, the protoplasm could no doubt obtain the quantity of carbon, hydrogen, and oxygen requisite to manufacture fat from

albumin, but this would not be economical, for a large amount of nitrogen would be wasted.

Fat cannot be produced by the tissue cells without nitrogen in the diet, because the fat-manufacturing protoplasm cannot live without nitrogen, which is absolutely necessary for its own assimilative re-integration. A good supply of nitrogenous food aids in fattening, since it gives vigor to all the protoplasmic metabolism, and among them fat formation.

The albuminoid substance gelatine, which is an important item in the food we ordinarily make use of, is able to effect a saving in the albuminous food stuffs. Although it contains a sufficiently large proportion of nitrogen, it cannot satisfactorily replace albumin in the food. Indeed, in spite of the great similarity in its chemical composition to albuminous bodies, it can no better replace the proteids in a dietary than fat or carbohydrate; and, although an animal uses up less of its tissue nitrogen on a diet containing gelatine and fat than when it is fed on fat alone, it dies of starvation almost as soon as if its diet contained no nitrogenous substance.

EXCESSIVE CONSUMPTION.

The last case we have to consider is that in which the supply of food material is in excess of the requirements of the economy. This is certainly the commonest case in man.

Much of the surplus food never really enters the system, but is conveyed away with the fæces.

In speaking of pancreatic digestion, reference has been made to the possible destiny of excess of nitrogenous food. In the intestine, some of it is decomposed into leucin and tyrosin, which are absorbed into the intestinal blood vessels. In the body these substances undergo further changes, which probably take place in the liver. As a result of the absorption of leucin, a larger quantity of urea appears in the urine, and hence the leucin formed in the intestine by prolonged pancreatic digestion is an important source of urea. This view is supported by the almost immediate increase in the quantity of urea eliminated when albuminous food is taken in large quantity.

From the fact that a considerable amount of fat may be stored up by an animal supplied with a liberal diet of lean meat, we must conclude that part at least of the surplus albumin goes to form fat. It has been suggested that, after sufficient albumin has been absorbed for the nutritive requirements of the nitrogenous tissues, the rest is split up into two parts, one of which is immediately prepared for elimination as urea by the liver, and the other undergoes changes, probably in the same organ, which result in its being converted into fat.

It would further seem probable, from the manner in which the urea excretion changes during starvation, that, as before mentioned, the absorbed albumin exists in the economy in two forms: one in which it has been actually assimilated by the nitrogenous tissues and forms part of them, and hence is called organ albumin; the other, which is merely in solution in the fluids. of the body, being in stock, but not yet ultimately assimilated, and hence called circulating albumin. The latter passes away during the first few days of starvation, being probably broken up to form urea, and a material which serves the turn of nonnitrogenous food. The organ albumin appears to supply the urea after the circulating albumin has completely disappeared.

From the foregoing it will be gathered that we cannot say what are the exact destinies of the various food stuffs in the body. Proteids are not exclusively utilized in the re-integration of proteid tissues, as an excess gives rise to a deposit of fat. Carbohydrates are not employed simply to replace the carbohydrates constituting part of the tissues, but, as will be shown when speaking of muscle metabolism, they are intimately related to the chemical changes which take place during the activity of that tissue. If fats are chiefly devoted to the restitution of the fat of the body, they certainly are not the only kind of food from which fat can be made.

We may say, then, that all food stuffs are destined to feed the living protoplasm, whether it be in the form of gland cells, the cells of the connective tissues, or muscle plasma, so that all the food stuffs that are really assimilated contribute to the maintenance of protoplasm and subserve its various functions.