1016 
aiiy  information  about  anything  but  the  number  of  the  medullated 
iierve  fibres  inider  difïereiit  circiimslances.  As  far  as  the  post- 
embryoiial  growtli  in  tlie  number  of  [axons  is  eoncerned,  these 
investigations,  wluch  in  themselves  are  in  many  cases  very  fine 
pieces  of  work,  alford  no  information,  but  with  this  method  one 
can,  of  course,  only  obtain  a knowledge  of  the  number  of  medullated 
axons,  and  the  total  i-esult  is  that  they  indicate  a gradual  process 
of  myelinisation,  which  is  even  stated  by  Donaldson  *) : “The  increase 
in  the  number  of  myelinated  fibres  in  the  spinal  root  with  advancing 
age  is  due  mainly  to  [uogressive  myelination.  Both  roots  at  maturity 
still  contain  functional  fibres  without  myelin  sheaths  (Ranson  ’06).” 
During  the  progress  of  the  work  of  counting  the  number  of  nerve 
tibres  in  the  roots  of  the  spinal  nerves  in  the  animals  I investigated, 
it  soon  appeared  that  it  was  impossible  to  get  reliable  niimbers  as 
to  the  conditions  of  the  nerve  fibres  in  specimens  of  different  ages 
by  counting  those  in  the  i'oots  of  a single  or  a small  number  of 
spinal  nerves.  For  fairly  great  displacements  and  individual  varia- 
fions  occur,  and  these  prevent  the  vahies  from  being  as  good  as 
would  be  desirable,  if  only  a small  number  of  nerves  are  taken 
into  consideration.  On  account  of  this  and  also  in  order  to  obtain 
an  insight  into  the  distribiition  of  the  axons  in  the  different  regions 
I have,  in  most  animals,  counted  each  spinal  nerve  on  the  same 
side  and  in  certain  specimens  on  both  sides.  As  I intend  to  give  a 
more  coinfilete  and  detailed  account  of  these  matters  in  a future 
woi'k,  1 rnerely  include  here  sonie  totals  trom  a part  of  the  calcu- 
lations  in  question. 
By  calculating  the  number  of  nerve  tibres  in  the  same  section  of 
the  root  and  by  using  the  method  of  fewest  squares  for  the  values, 
it  has  been  shown  that  the  percentage  of  error  in  the  dorsal  roots 
of  very  young  animals  (from  four  to  ten  days  old)  has  not  exceeded 
± 10,  and  that  for  other  places  it  is,  as  a rule,  about  ± 2.  (In  the 
dorsal  roots  of  the  spinal  nerves  the  nerve  fibres  are  sitiiated  very 
close  together  in  young  specimens,  so  that  one  has  to  work  with 
thin  (5  fi)  sections  and  strong  light  in  order  to  obtain  exact  results). 
The  totals  of  the  dorsal  and  ventral  roots  of  the  spinal  nerves  given 
in  the  tables  below  are  thus  to  be  considered  as  exact  within  the 
limits  of  the  percentages  rnentioned.  That  there  is  thus  a post- 
embryonal  increase  in  the  number  of  nerve  fibres  in  the  dorsal  and 
ventral  roots  is  shown  with  all  the  clearness  that  could  be  desired 
b Donaldson,  H.  H.  The  rat,  reference  tables  and  data  for  the  albino  rat  and 
the  Norway  rat,  1915,  Fbiladelpbia. 
