FUNCTIONS OF THE NERVE CELL 567 



portant role than the nerve cells in the discharge of central functions. Al- 

 though our information is not yet definite enough to warrant taking such a 

 position, it would be well if we had some designation for those constituents 

 of the nervous system discharging these central functions, which would not 

 prejudice either view. For simplicity's sake, we shall retain the old name, 

 expressly remarking that the observations brought together in. this book and 

 the conclusions deducible therefrom are on the whole but little affected by the 

 newer ideas, as to the structure of the nervous system. Those ideas will only 

 become significant when it has been definitely proved that the functions 

 hitherto ascribed to the nerve cell are exercised in greater or less part by 

 extracellular structures. 



A. THE NUTRITIVE FUNCTIONS OF NERVE CELLS 



In 1852 Waller, Sr., found when he cut the posterior roots of the second 

 cervical nerve between the spinal ganglia and the spinal cord and killed the 

 animal (cat or dog) some time later, that the peripheral end of the root still 

 connected with the ganglion remained normal, while the central end and its 

 continuation into the spinal cord degenerated. When he cut the nerve periph- 

 erally to the ganglion, the peripheral end degenerated, while the central end 

 and its continuation into the spinal cord remained normal. Finally, it was 

 shown that after cutting the anterior root the peripheral end of the efferent 

 fibers degenerated while the central end remained normal. 



Before Waller, Tiirck had found that a partially transverse section of the 

 spinal cord produced degeneration above and below the section, and that this 

 degeneration did not follow the same columns continuously. 



Thus it was demonstrated that a nerve fiber maintains its normal condi- 

 tion only so long as its connection with the nerve cell is preserved. These 

 facts have been robbed of much that was originally strange about them by 

 the newer conceptions of the nerve fiber as a mere process of the nerve cell ; 

 for it is perfectly evident that a process must degenerate when its connection 

 with the cell body is lost. 



Wallerian degeneration has been of very great value in tracing out the nerve 

 paths in the central nervous system (cf. later), in determining nerve roots, and 

 in isolating physiologically the different kinds of fibers belonging to a given 

 nerve trunk. The latter is possible because the different kinds of fibers do not 

 degenerate at the same rate after section. 



But we find it necessary to-day to amend the law of Waller somewhat. It 

 turns out that both the stump of the nerve fiber left in connection with the 

 nerve cell and the cell itself undergo secondary changes after section of a nerve. 

 The motor cells and those of the spinal ganglia appear to behave somewhat 

 differently in this respect. The former exhibit certain characteristic alterations 

 of structure within twenty-four to forty-eight hours after the section, and within 

 fifteen to twenty days many of them have gone to pieces. The remainder, even 

 though the end of the nerve may not have healed at all, become from this time 

 on the seat of regenerative changes and gradually recover their normal proper- 

 ties. The same course of events is witnessed in the efferent sympathetic nerves. 

 When the cervical sympathetic is cut, certain cells in the anterior horn of the 

 same side become atrophic; and after section of the fibers coming from the 



