Physiology

The most striking instance of the mobilization of units is that of the capillaries as observed by Krogh. In a muscle for instance the capillaries run parallel to the muscle fibres. When the muscle is at rest the majority of these capillaries are entirely closed, no blood whatever running along them. As the muscle becomes more and more active, a correspondingly greater number of capillaries open till the maximum may be many times the number at rest.

Principle of Substantial Reserves.

The principal materials (gross) which the body requires are carbohydrate, fat, protein, water and oxygen. In the case of each the body carries some sort of store beyond that placed at the immediate seat of metabolism.

Carbohydrate.—The blood contains about 7 grams of sugar; that is a little more than half an hour's supply for the whole body. The remaining carbohydrate is stored as glycogen. Of this about half is distributed throughout the tissues generally and may therefore be regarded more or less as being at the seat of its metabolism. The remaining half, amounting to 75 grams or half a day's supply, is to be found in the liver which acts as a store for the body as a whole. Thus the body has a store of about one half day's supply of carbohydrate,-250-30o grams being the figure put down in the standard dietaries for daily consumption. That is not to say that after 12 hours' abstinence there is no carbohydrate left in the body, but it means that the organism is gradually falling back on something else.

Fat is stored on a much greater scale than carbohydrate. The amount of fat stored in the body naturally differs in different individuals within very wide limits, but it has been established as the result of numerous experiments that a portion of the fat laid down in the body is directly derived from what is eaten. Thus if some oil of low melting point be taken in the food it may at once be recognised as entering into the store of fat reserved in the body.

Whilst it is quite clear that carbohydrate eaten may be stored as carbohydrate and used as such, and while it is equally clear that fat eaten may be stored as fat, the more arresting question arises—Is not much of the fat of the body essentially a store of carbohydrate? In order to establish this thesis it would be nec essary to prove (I) that carbohydrate taken may be laid on as fat; and (2) that when carbohydrate is needed fat may be re converted into carbohydrate or at all events used as carbohydrate is used. That carbohydrate taken in the food may be laid on as fat was proved by the classical experiment of Lawes and Gilbert. In this two young pigs were chosen from the same litter and while one was killed and analysed, the other was fed on "grains," a diet which is practically free from fat, consisting of protein and carbohydrate. The amount of fat put on by the

growing pig was greater than could be accounted for on the assumption that it was formed from protein. Indeed since the days of Lawes and Gilbert it has become increasingly doubtful whether fat can be formed from protein on any considerable scale in the mammal.

To pass to the possible reconversion of fat into carbohydrate,— that is at the moment a subject on which the highest authorities are in controversy. It may be that when muscle is in want of carbohydrate and cannot get any from the usual sources fat is reconverted, but it may not be so. If it is not, however, there appears to be but one possibility, namely, that the muscle can derive energy from fat without its passing through the carbo hydrate stage. In either case the muscle would be availing itself of fat entering the body as carbohydrate. Therefore the fat would be a potential, if not an actual reserve of carbohydrate.

Oxygen.

Oxygen is stored to some extent in the body. At one time the conception was held that oxygen was stored actually in the tissues as so-called "intra-molecular" oxygen. That con ception was founded on two others—(i) that a frog's muscle could contract in an atmosphere free from oxygen; and (2) that the contraction was essentially an oxidation. Now the idea of "intra-molecular oxygen" has been given up and we must look for no further store of oxygen than is to be found in the haemo globin of the body. What is to be found there and in the lungs suffices to preserve consciousness for a few, perhaps two or three, minutes, and life for a somewhat longer time. The blood on the average is circulated round the body (at rest) rather more than once per minute and not so much as twice; on that circuit about one third of its oxygen is taken out. If therefore one assumes that all the blood goes round the body in 45 seconds and that there is enough oxygen in the blood for three circuits that would give oxygen enough for 135 seconds. In addition in such a case as that of drowning there is reserved oxygen in the lungs to the extent of what would supply the body for about two to three minutes. But if the body has no great store of oxygen on which to fall back, it has a considerable store of the material which transports oxygen and that is probably much more important. So far as is known about 8o per cent of the haemoglobin of the body is in circulation ; the rest is stored, some in the spleen, some probably in the red marrow and the rest in situations as yet not fully explored.