3 2 4 POPULAR SCIENCE MONTHLY. 



area, which was formerly ' accessory ' in the control of the arm move- 

 ments, evidently was accessory still in the control of the facial move- 

 ments, 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 differentia- 

 tion 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 un- 

 accustomed 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 con- 

 nection 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 



