system that serves to join the spinal cord to the brain. We are further aware that those centers are in touch with all parts of the body by afferent nerves, along which impulses are continually streaming to the centers. It is thoroughly established that the activity with which the centers discharge impulses along the efferent nerves to the heart and the vessels is modified by the arrival of afferent impulses. And it is fairly well understood how, by the action of this craftily balanced apparatus of nerve-fibers and nerve-centers, the supply of blood to the various tissues is adjusted to their ever-changing needs. But when we ask ourselves what happens in one of the nerve-cells which compose the nervous centers when it discharges an impulse? what that impulse which flies at the rate of a hundred miles an hour along the nerve-fiber really is? what is the precise nature of the actions which it arouses or represses in the muscular fibers of the heart or of the arteries when, in the twinkling of an eye, it impinges upon them? we have to answer that we do not know. We are in exactly the same position with regard to the voluntary contraction of the striped or skeletal muscles by means of which the ordinary movements of the body are executed. The nerve cells in which the impulses originate have been located with considerable precision in the so-called motor region of the brain, which comprises the middle portion of the superficial gray matter of each hemisphere. The tracts of nerve-fibers along which those impulses pass to the muscles have been mapped out. The influence of temperature, tension and other conditions on the muscular contraction has been investigated in great detail. But we are again almost completely in the dark as to the actual nature and course of the events that take place within the envelopes of the nerve-cell, the nerve-fiber and the muscular fiber when a muscle contracts in obedience to the will.
One or two promising clues there are, and these are being vigorously followed. Whenever a nerve or a muscle (or, indeed, for that part, a gland, although the phenomena are best seen in muscle and nerve) enters into a condition of physiological activity, an electrical change is set up in the excited part. In muscle, although not as yet in nerve, certain chemical, thermal and optical changes can also be demonstrated. It is obvious that the study of such phenomena, and especially their quantitative study, under as many different conditions as possible, is essential to the solution of our problem. Accordingly, data of this kind, which, it may be hoped, will some day become the basis of a great generalization, are being diligently gathered. Among the most important of recent contributions to the subject is an elaborate investigation of the electrical changes which accompany muscular contraction by Sir John Burdon Sanderson. By photographing the movements of the mercury in a capillary electrometer connected with the
