S2 POPULAR SCIENCE MONTHLY. 



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? M'hat 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 super- 

 ficial 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 muscTilar 

 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 vigor- 

 ously followed. \\Tienever 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 clianges can also be dem- 

 onstrated. It is obvious that the study of such phenomena, and espe- 

 cially 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 tlie 

 most important of recent contributions to the subject is an elaborate 

 investigation of the electrical changes which accompany muscular con- 

 traction by Sir Jolm Burdon Sanderson. By photographing the move- 

 ments of the mercuiT in a capillar}' electrometer connected witJi the 



