We now pass on to consider the process of nutrition or metabolism, that is, the activity of the tissues, the development of force by protoplasm in the presence of blood. We shall find that this subject has an important bearing on the action and uses of many drugs and other therapeutic measures.

I. Physiological Relations

The best means of comprehending the obscure subject of metabolism is to take the instance of a muscle. A muscle has a definite structure; enjoys a free supply of blood; displays force during the period of its contraction, namely, mechanical energy, heat, and sound; and produces certain chemical sub-stances-carbonic acid, water, sarkolactic acid, kreatin, other allied nitrogenous bodies, and possibly urea. The Mood which passes through the muscle becomes venous, that is, loses oxygen and a small quantity of proteids, and takes up the waste products.

In doing this work, the muscle first incorporates the oxygen and other elements of the plasma with its own substance, however loose that combination may be. In this respect the molecules of the muscle are being constantly changed. It is a fact of the first importance to the pharmacologist, that when a muscle or any other living tissue incorporates nutrient materials, acts upon them, and forms force and other products from them, its own molecules are changed or altered. As the blood or plasma supplied varies, so will the materials vary that are incorporated, the amount and even the character of the force and the products, and the chemical-possibly even the anatomical-constitution of the active protoplasm. In one sentence, we may say that the muscle and the plasma act and re-act upon each other: that the protoplasm acts on or alters the lymph; the lymph acts on or alters the protoplasm.

This process of double decomposition appears to be going on in every organ and tissue of the body; though, naturally, the tissue being different in each case, so are the particular substances broken up by it, the products yielded by it, and the particular kind of force which it displays, for instance secretion, nervous energy, growth and development. The oxygen and the proteids are carried to the organs by the arterial blood; the heat is distributed and lost; the carbonic acid, water, and nitrogenous and other products are excreted by the lungs, skin, kidneys, and bowels; and the active organs are maintained in size and vigour amidst all the change.

There are various means of estimating the state of metabolism in the living body. "We may measure, first, the amount of force displayed--the muscular activity or tone, the rate of growth, the temperature, the mental capacity; or, secondly, the amount of material consumed - the food taken and the air inspired; or, thirdly, the products of metabolism, that is, the excretions. The first two means are by no means always available with accuracy. This is what makes the examination of the urine, the principal excretion, so important in the majority of clinical cases; for knowing the state of the urine, we can work backwards, as it were, and estimate the functional activity and even the anatomical state of the organs in which its constituents have been produced.

Unfortunately, metabolism is not the simple process which we have described, but in many respects still very obscure. Thus the proteids are not at once broken down into carbonic acid, water, and nitrogenous compounds, as represented above. In some of the tissues at least there are intermediate products, one of which is fat, which is in turn oxydised into carbonic acid and water. It is also probable that all metabolism is associated with ferments, if not actually due to their activity, like digestion and the coagulation of the blood. Lastly, the intimate protoplasmic changes which are the basis of vital force are controlled by the central nervous system, by trophic centres lying in the cord and cerebrum, with afferent and efferent trophic nerves.

II. Pharmacodynamics

This brings us to the second part of our inquiry-our power over metabolism in a healthy individual. This is greater than would at first appear.

1. Our influence on metabolism through the blood as a whole, has been fully discussed in the preceding chapter, and does not require to be more than mentioned here.

2. We can affect nutrition through the constituents of the blood which supply material to the particular organs. Experience taught us, long before science, how to feed a man in training for muscular exertion; which kinds of food are specially suited for the exercise of the brain, for the periods of growth and development, of pregnancy and lactation, of degeneration and decay. It is but expressing the same fact in other words to say that by supplying an excess of certain kinds of food, we can increase the activity of an organ, the cells of which appear to exercise themselves more vigorously when their natural source of energy and nutrition is freely supplied to them. Alcohol, Cod-liver Oil, Olive and Almond Oils are thus valuable foods, or nutritive tonics.

3. An increased supply of oxygen in the blood increases metabolism. The valuable influence of fresh air on active organs is familiar, and we have learned in this connection the use of Iron, which is thus a haematinic tonic.

4. An increased amount of work is an interesting means of increasing protoplasmic activity. By throwing more weight upon a muscle, up to a certain point, we can increase the force of its contraction. This is exercise; but it must be accompanied by a sufficient supply of plasma and oxygen. A man in training not only selects his food and air, but throws an increased amount of work on his muscles by exercising them regularly.

5. We can influence metabolism by means of the excretions, that is, by hastening the removal of its products through the lungs, kidneys, skin, and bowels, as we have already seen in the case of the stomach and liver. The same principle manifestly applies to all the tissues.