All the substances used as food, found in or excreted by the body, are being thoroughly studied; but it should be remembered that this is chemistry, not physiology. Physiology is only concerned with protoplasm, and the physiologist who goes deeply into the chemistry of non-living matter has to discipline his mind against forgetting its ceaseless change, and trying to regard it as though it were constant. The actual chemistry of protoplasm will be a very hard nut to crack, and may defy us until we can depict molecules as well actually as we now can symbolically. Some idea of the difficulty may be formed if we consider that it is impossible to imagine a pure sample. From the restless activity which is the condition of its existence, it is always working changes in its surrounding, always mixed with raw material, and always masked by the products of its own metabolism. Even if we withhold the former, it consumes its own substance until the moment of death. It does not even look homogeneous under the microscope.
Before, however, we can pursue the chemical methods further, it will be necessary to describe the histological. The reader may have already wondered how we managed to find out so much about the cellular structure of the body. It is no easy matter to cut up soft tissue, of the consistency of an unboiled egg, into thin slices which can be examined under the microscope. It is done in the following manner: The bloodvessels of the freshly killed body are injected with a fluid which instantaneously kills and fixes the cells in much the same way as an egg is fixed by being hard-boiled. The natural shape of the cells is thus preserved, and the loss of any of their chemical constituents by putrefaction prevented. The piece of organ is then impregnated with and cast in the middle of a solid block of paraffin wax, which is put into a machine and shaved up into thin slices, about 40,000 to the inch sometimes. One of these shavings is then stuck upon a glass slide, and on the wax being dissolved away with some such substance as benzine, a section of the tissue, about one cell thick, is left on the glass ready for microscopic survey.
To do anything like justice to the histological methods would require a volume in itself. When the sections are fixed upon slides, they are treated with a number of reagents to show their chemical and structural peculiarities. One section is stained specially to show the nucleus; another to show the centrosome; another zymogen granules, etc. And, as all these cannot be shown at their best in one cell, the differently treated sections have to be separately drawn or photographed, and the typical structure compiled from several. By careful staining, the chemical composition of the different parts of the cell is being worked out, and the effects of rest, activity, feeding, and other influences, studied.
Take as an example the effects of a meal. A number of animals of the same litter are fed together out of the same trough. One has been killed before the meal, and the rest are killed at intervals dividing the time which must intervene before their next feeding-time comes round. Series of sections from their organs are prepared, one from each animal being mounted in order upon the same piece of glass, dipped in the same reagents, and examined under the same microscope. From a number of these sections the progressive effects of a meal upon each of the several constituents of the cell are traced out, and some of the chemical processes deduced.
Turning to the physical side of physiology, it is unnecessary here to say more about the means employed for studying the properties of muscle and nerve than that many of the phenomena occur with such extreme rapidity that they can only be perceived by the photographic plate. In the study of the large organs, the physiologist finds a fascinating employment in devising models in which, so far as possible, all the physical conditions are reproduced, and this not only for the benefit of his pupils, but to help himself in perceiving their meaning. Too much reliance must not be placed on these models, of course, but they have added considerably to our knowledge of the eye and throat.
It requires no great imagination to perceive the difficulties which lie in the way of studying the nervous system. Tracing nerve fibres under the microscope through interminable series of sections is a labour which can neither be hurried nor scamped. It is greatly aided by pathological specimens. An animal which has been through life with only one eye will obviously have central organs of vision showing wide contrasts. Those connected with the blind eye will be undeveloped, because never used, while the corresponding lobes of the brain connected with the other eye will show the effects of doing extra work.
Many of the problems which meet the physiologist can only be solved by experimenting upon a live animal, and these experiments form by no means the easiest part of his work. The animal must be kept, so far as possible, under physiological conditions—that is to say, free from pain and fright and unpoisoned by drugs. Thanks, however, to an extensive knowledge and skilful use of anæsthetics, the obstacles to this method of investigation have been overcome, and its results have proved very profitable. The absence of pain is a very important factor in an experiment, and even if the physiologist took the wanton delight in inflicting suffering which the imagination of his enemies attributes to him, he would have to restrain its indulgence in his laboratory, or forego the hope of even moderate success. In this country, moreover, the Government will not allow such experiments without its express permission, and the license is very rightly only granted to men whose researches promise an adequate return, and who are likely to conduct them humanely and successfully.
Physiological research is not a hobby to be lightly taken up. It is not one merry round of exciting tussles with tortured and infuriated cats and dogs; on the contrary, it entails arduous labour and needs infinite patience. The experiments, often tedious in themselves, have to be repeated again and again in as many different ways as possible, until every slight difference in result can be accounted for; and the certainty that both the methods used and the interpretation given will, when published, receive the closest, and not in every case the friendliest, scrutiny by other members of the profession serves as an admirable corrective to jumping at conclusions. It is, however, an occupation of absorbing interest, and the physiologist feels amply repaid if he can think that his labours have added, no matter how little, to that control over Nature which the severe conditions of modern life make every day more pressingly necessary.