It has also been found quite recently that the spleen contracts during muscular exercise and shunts considerable concentrated blood, which has been stored here, into the general circulation. This helps in two ways, by increasing the amount of circulating blood and by allowing the blood to carry more oxygen.

Even now, if the exercise is maximal, the contracting muscles will not get enough oxygen, so the body goes into oxygen debt - borrows oxygen, so to speak, on further credit; that is, does work without oxygen - and the blood may get into such an acid condition as is seen in disease just before death. After a bout of violent exercise of only a few minutes' duration, it may take an hour or so to pay back the oxygen debt and restore things to normal. In this example we have followed only briefly the integration necessary to supply sufficient oxygen to the active muscles.

Closely bound up with this integration of the various organs and parts, so that the body can act as a harmonious unit, is the stability and constancy of the organism. Claude Bernard may well be said to have formulated this idea in his theory of the constancy of the composition of the internal environment of the organism, the blood plasma and lymph. It is found that physiological mechanisms exist which tend to maintain the constancy of the organism when its normal state is disturbed. There takes place an increase or decrease of some function in the nature of an adaptation which tends to restore things to normal. The study of such adaptations would seem to be one of the central problems of physiology. Lawrence Henderson has written:"The law of adaptation in organisms, founded upon the fact of survival, seems to be quite as well established as the second law of thermodynamics, and almost equally serviceable."

With this introduction, then, to what physiology is, we may ask what are the present problems which occupy its disciples, and what are its trends. Many problems are old, as old as the science itself. Thus, in the field of the circulation of the blood, many important problems which are being actively attacked were already thoroughly grasped and formulated by William Harvey, the father of scientific physiology. What happens is that with each generation of workers, with new methods and new angles of attack devised, the old problems are reattacked and pushed somewhat further towards a solution. Other questions appear quite new, but in many cases this is due to the fact that it has only been with the marked advance of the sciences, like physics and chemistry, that these problems could be properly formulated. The increasing importance of biochemistry to the study of living processes is quite obvious to anyone who has even superficially scanned this field. Many problems connected with these processes can be formulated, attacked, and solved only by using the technique of chemistry.

The marked advance of biochemistry as a method of analyzing living processes may be granted as one of the present trends of the physiology of to-day. The intimate nature of muscular contraction is much better understood as the result of chemical methods of investigation which have involved a quantitative analysis of what happens to the various compounds present in muscle during and after contraction. The whole story of the regulation of the acidity of the blood and the close relationship of blood and breathing as carried out at the lungs, has depended almost entirely upon biochemical technique for its present better understanding. The study of the glands of internal secretion and the hormones which they elaborate, has been one of the most fascinating stories of the last quarter of a century and has depended, to a large extent, upon chemical methods for its advance.

The results from the application of physical methods to the problems of physiology, although not so spectacular and generally recognized as those of the biochemical school, have nevertheless been of immense theoretical importance, and promise to be even more valuable in the future. The advances in the mechanical aspects of muscular contraction, the investigation of the heat production of isolated muscles under varying conditions, and the application of newer physical methods to the study of the nervous impulse and the electrical change in nerve during activity may serve as a few examples of this type of approach to physiological problems.

A third method of attack, the method of Harvey, is as old as our science itself, and still must hold an important place in the immediate future. This is what we may call the purely physiological method. Careful anatomical considerations, experiments on living animals, deductions from these experiments, more experiments to test these, and so forth, characterize it. Within the past decade or so, our knowledge of the importance of the capillaries in the dynamics of the circulation, of the functions and interrelations of many parts of the brain and central nervous system, and of the relation of the emotions to bodily functions - to mention only a few examples - has been enormously amplified by the use of the purely physiological method.

With the increasing interest, which we have mentioned, in regard to the physiology of the animal as a whole as opposed to a study of its systems and organs separately, a beginning has been made in building up what we may call human physiology, where the experiments have been carried out on man himself. In the fields of metabolism and energy exchange of the body, of circulation, of respiration, of the physiology of muscular exercise, of digestion and of excretion, considerable bodies of facts exist for the human subject, and certain generalizations have been attempted. This trend in physiology is of tremendous importance to medicine, but has been necessarily limited by the lack of methods suitable for use on the human being. As such methods are being constantly sought and found, this side of physiology will probably yield a rich harvest.