Having now roughly sketched out the scheme upon which such a body works, we can go on to a more detailed examination of the division of the labour, and the way in which each department supplies, and is dependent upon, the others. If we were to do this thoroughly, it would take a great deal of time and space, for the physiology of a potato plant, though essentially the same, presents many differences from that of a horse; but the physiology of the great human interest is also that of the most complicated animal, namely, man, so it is on him that we shall focus our attention.
Protoplasm is more easily studied the more specialized is the animal it composes. When all the events of life are taking place in a speck of matter, invisible without a microscope, it is impossible to analyze the changes which it is working in its surroundings, or to infer those which are going on in itself. But when large numbers of cells are examined collectively, we can deal with what they take in and what they give out in sufficient bulk to arrive at a fairly accurate determination. The study is rendered still easier in an animal with extremely specialized organs, like man, in which food is nearly all taken in by the mouth, and thus kept quite distinct from what is eliminated; the latter, again, being mostly given off by the kidneys is kept equally distinct. Moreover, the intermediate changes being performed in different organs still further simplifies investigation of the vital process; for the physical effects are also more easily studied when exaggerated in a particular part of the animal. The electrical changes in a single cell might long have remained unsuspected had we not been able to observe those in a muscle with the galvanometer.
Now, while the cells which make up the body of man differ very greatly owing to the different tasks which they have to perform in obtaining food and getting rid of refuse, they all require very much the same fuel to enable them to live, and having got it, they all treat it in very much the same way; therefore our first business is to consider what the body wants, and what it does with it. Afterwards we can try to find out how it gets it, and where.
