area, which was formerly 'accessory' in the control of the arm movements, evidently was accessory still in the control of the facial movements, as soon as the juncture of the new nerve-tract was complete; it is possible that the continued exercise of its functions by electrical and volitional stimulus developed the required variety and differentiation of function necessary for facial control. How the cortical center for the N". accessorius knew (sic) that it was called upon to come to the rescue and improve its discerning qualities, as a part of a more complex and intelligent motor system, may remain for us an unanswerable question.
In connection with this hypothesis we may perhaps help ourselves out with another. The fitting of hitherto unused nerve elements with the medullary sheaths necessary for their employment in voluntary motor functions would seem not to be an improbable assumption in the present case. The researches of Ballana, Stewart and others have shown that the regeneration of fibers in cut nerves is not, as was formerly supposed, effected by the growth of the proximal extremities of the axis-cylinders, but by axis-cylinders shot out from logitudinal cells which appear in the distal segment itself. Thus chains of cells are formed which fuse together and become invested with medullary sheaths. Flechsig has also shown that in the human infant at birth, while all the fibers of the spinal cord except those of the pyramidal tracts, which are used especially as conductors of voluntarily initiated impulses, have become myelinated, the vast multitude of fibers in the brain have not become so. According to Professor Sherrington, all this suggests a conclusion which has other evidence in its favor, namely, that a nerve-fiber is not a single nerve-cell process, but a series or chain of nerve-cells forming a functional continuum. The reason, then, why regenerated nerve-fibers do not attain maturity, and so perform their appropriate functions, unless they become united with the central end of some nerve, is that only by this union can they get an opportunity of actually performing these functions. That seems to amount to saying that the call upon them to perform unaccustomed work causes them to fit themselves for this work.
It is not, therefore, too violent an assumption to suppose that, in such a case of recovery of voluntary and emotional control of paralyzed muscles by anastomosis as I have narrated, a new cerebral apparatus of control may be called into use by the process of myelinating the necessary nerve-elements. Such a process might be relied upon either to equip the cortical center of the accessory nerve for its new and more varied functions of control, or to prepare new paths of connection between this center and that which had formerly exercised exclusive control of the muscles of the face through the facial nerve. In a word, tin-building process in the brain, finding much of the
