22 PHYSIOLOGY tious. matter, in the vulgar sense of these words, but of kinds of motion. In the above brief sketch we have dealt chiefly with such well-known physiological actions as secretion, muscular contractions, and nervous impulses. But we must not hide from ourselves the fact that these grosser activities do not comprise the whole life of the tissues. Even in the simple tissues, and more especially in the highly-developed nervous tissues, there are finer actions which the conception out lined above wholly fails to cover. Finer Two sets of vital phenomena have hitherto baffled in activities quirers, the phenomena of spontaneous activity, rhythmic x) " or other, and the phenomena of " inhibition." All attempts to explain what actually takes place in the inner working of the tissues concerned when impulses passing down the pneumogastric nerve stop the heart from beating, or in the many other analogous instances of the arrest of activity through activity, have signally failed; the superficial re semblance to the physical "interference of waves" breaks down upon examination, as indeed do all other hypotheses which have as yet been brought forward. And we are wholly in the dark as to why one piece of protoplasm or muscular fibre or nervous tissue remains quiescent till stirred by some stimulus, while another piece explodes into activity at rhythmic intervals. We may frame ana logies and may liken the phenomena to those of a constant force rhythmically overcoming a constant resistance, but such analogies bring us very little nearer to understanding what the molecules of the part are doing at and between the repeated moments of activity. Bering s Further, if the ingenious speculations of Hering, that specula- specific colour-sensations are due to the relation of assimi lation (anabolism) to dissimilation (katabolism) of proto plasmic visual substances in the retina or in the brain, should finally pass from the condition of speculation to that of demonstrated truth, we should be brought face to face with the fact that the mere act of building up or the mere act of breaking down affects the condition of protoplasm in other ways than the one which we have hitherto considered, viz., that the building up provides energy to be set free and the breaking down lets the energy forth. In Hering s conception the mere condition of the protoplasm, whether it is largely built up or largely broken down, produces effects which result in a particular state of consciousness. Now, whatever views we may take of consciousness, we must suppose that an affection of con sciousness is dependent on a change in some material. But in the case of colour-sensations that material cannot be the visual substance itself, but some other substance. That is to say, according to Hering s views, the mere con dition of the visual substance as distinct from a change in that condition determines the changes in the other sub stance which is the basis of consciousness. So that, if Hering s conception be a true one (and the arguments in favour of it, if not wholly conclusive, are at least serious), we are led to entertain the idea that, in addition to the rough propagation of explosive decompositions, there are continually passing from protoplasm to protoplasm delicate touches compared with which the nervous impulses which with such difficulty the galvanometer makes known to us are gross and coarse shocks. And it is at least possible, if not probable (indeed present investigations seem rapidly tending in this direction), that an extension of Hering s view, with such modifications as future inquiry may render necessary, to other processes than visual sensations, more especially to the inner working of the central nervous sys tem, may not only carry us a long way on towards under standing inhibition and spontaneous activity but may lay the foundation of a new molecular physiology. This, how ever, is speculative and dangerous ground. But it seemed desirable to touch upon it since it illustrates a possible or probable new departure. What we have said of it and of the more manageable molecular problems of physiology will perhaps show that, vast and intricate as is the maze before the physiologist of to-day, he has in his hand a clue which promises, at least, to lead him far on through it. Space forbids our entering upon a discussion concerning Metho the methods of physiology ; but, accepting the truth of of I 11 ) the preceding discussion as to the nature of physiological sl< problems, the means of solving these problems speak for themselves. From the earliest times the methods of physiological inquiry have belonged to one of two categories : they have been anatomical or experimental. And the same distinc tion holds good to-day, though both methods are often joined together in one inquiry, and indeed at times may be said to merge the one into the other. By the anatom ical method the observer ascertains the gross outlines, the minute structure, and if necessary the physical characters and the chemical composition of an organism or part of an organism ; and by comparison of these with those of differ ent organisms, or of the same organism placed by nature that is, not by himself in different circumstances, he draws conclusions as to the actions taking place in it while it was alive. In early times the comparison of gross struc tures gave important results, but they have now been to a great extent exhausted ; and the most valuable conclusions reached at the present day by the anatomical method are those arrived at by histological investigation of minute structures and by chemical analysis. The marks of this method are that on the one hand it deals for the most part with things which are no longer alive, and hence must necessarily fail to make touch with the inner workings of which we have spoken above, and on the other hand in its comparison of organisms under different conditions it has to wait till Providence brings about what it requires, and has to be satisfied with such differences as the chapter of accidents provides. In the experimental method the observer places the organism or part of the organism under conditions of his own choosing, and applies to the organism under those conditions the same analysis as in the former methods. He ascertains changes in the gross features, minute structure, physical characters, and chemical com position, as before. So that in reality the two methods are in part identical, and differ chiefly by the fact that in the latter the observer chooses the conditions in which to place the organism. But an important corollary follows, viz., that by choosing his own conditions the observer is able to bring his analysis to bear on an organism or part of an organism while still alive. The history of physiology, especially in recent times, shows that this method is the one not only of the greatest fertility but one becoming more and more essential as inquiry is pushed deeper and deeper into the more abstruse parts of physiology. If there be any truth in the sketch given above of the modern tendencies of molecular physio logy, it will be clear to every mind that the experimental method alone can in the future give adequate results. It might indeed be urged that when molecular physics has advanced far enough the molecular problems of physiology will be interpreted by its light without recourse to experi ment. It will be a long waiting till that comes. Mean while, all the power over not only the body but, what is more important, the mind of man which the physiology of the future unmistakably promises must lie unused. Nor is it simply a matter of waiting, for it is at least within the range of possibility that when the molecular problems of physiology are fairly grasped conclusions may be reached which will throw back a light on the molecular processes
of inanimate masses, revealing features of what we call