Although five-sixths of the total mineral matter of the body is found in bone and in spite of the fact that bone has all the appearance of being firm and resistant the evidence available goes to show that the bony structures must be regarded as active store houses of mineral matter. When the need arises the body as a whole can draw upon the bones for constituents like lime and phosphates. Under certain conditions the bones indeed may give up so much of their mineral matter that they become soft and can no longer function as an effective frame work.
The other one-sixth of the mineral constituents found in the body are not distributed uniformly throughout the remainder of the tissues. As regards this varying distribution of salts in the tissues and the blood Macallum, in his study of palaeochemistry, produced some most interesting evidence in favour of his view that the present composition of the blood plasma, in so far as its inorganic constituents are concerned, is probably identical with that of the sea water just before the Cambrian period and that the salt concentration in protoplasm represents conceivably the salt concentration of the primaeval ocean in which life first appeared. At any rate the curious ratio of potassium and calcium to sodium, which is characteristic of protoplasm, is reflected in the salt relationship in water drawn from pre-Cambrian formations.
The mineral constituents play an important part without doubt in digestion in that acid must be secreted for gastric digestion and alkali for intestinal digestion.
So far as is known water undergoes no change in the body, but nevertheless all the water present in the organism does not come from some source external to the body but is in part formed in the tissues during the metabolism of the various f oodstuffs and of the tissues themselves. According to one calculation this internal source of supply accounts for about 16% of the water excreted. It has been also maintained that this intracellularly formed water is of greater value to the living structure than the imbibed water as it presumably does not bring about drastic os motic changes. It is impossible, however, with the information now available to decide whether the metabolic water in the cell differs in any way from the water received from external sources.
It is of interest to note that animals which live in arid regions, in contradistinction to those which live where water is abundant, excrete as their main nitrogenous waste product uric acid which is voided with the minimum waste of water instead of urea, which has to be excreted in solution.
If the extent and degree of metabolic activity of a subject have to be determined we may do so by direct calorimetry. The sub ject is enclosed in a special chamber or calorimeter so devised that all the heat given off by the subject can be directly collected and measured (fig. 1). These calorimeters, which we owe mainly to the ingenuity and skill of Atwater, Rosa and Benedict, are very delicate and costly pieces of apparatus. In order that there be no escape of the body heat to the environment, the double copper walls of the chamber are fitted up with elaborate electrical equipment which permits of so delicate a balance that heat can neither pass out nor into the chamber. In order to measure the heat given off by the subject, a current of cold water is circulated within the box in continuous piping. If the temperature of the incoming and outgoing water is carefully measured and if the quantity of water passing through be also known the heat lost by radiation and conduction by the subject in a given time can be determined. As part of the heat lost, amounting to about one fourth of the whole, is eliminated by the subject in the form of water vapour this must be and is also determined by absorbing the water lost in sulphuric acid and subsequent weighing of the acid container. This method of direct calorimetry is very accu rate and reliable, but the method is difficult and the apparatus is very liable to get out of order. In addition to this direct meas urement of the heat output, the metabolic activity of the subject can be calculated from a determination of the amount of carbon dioxide given off and the amount of oxygen utilized by the sub ject in a given period. This method of Indirect Calorimetry can be carried out simultaneously with the direct method and serve as a check upon it. The calorimeter chamber in which the sub ject is enclosed is gas-tight and the air is circulated through a gas tight absorbing system (fig. i ) by means of a rotary blower. The carbon dioxide given off is absorbed by means of soda lime, the amount absorbed being determined by weighing the soda lime container at the beginning and the end of the experiment. The carbon dioxide free air is returned to the chamber after the de ficiency in oxygen, which is approximately determined by reduc tion in volume, has been made good from a cylinder of pure oxygen. The amount of oxygen used during the experiment is determined either by measuring by a meter the amount of oxygen passed in or by weighing the cylinder before and after the experi ment. The heat lost by the subject can be determined from the amount of oxygen used. The caloric value of a litre of oxygen used in the tissue combustion has been determined. This value varies with the carbon dioxide—oxygen ratio, called also the respiratory quotient (R.Q.), from 4-795 calories with an R.Q. of .713 which is held to represent the combustion of fat alone to 5.058 calories with an R.Q. of i.00, which is accepted as repre senting the combustion of pure carbohydrate by the tissues. The two methods of direct and indirect calorimetry have been found to give almost identical results.