634 
I^IE.  J.  LISTEE  OX  THE  CrTAXEOHS 
consistent  with  the  appearances  described,  is  that  of  a mutual  repulsion  on  the  part  of 
the  pigment-granules,  induced  by  some  agency  strongest  at  the  centre  of  the  cell  and 
feeble  in  the  remotest  branches  of  the  offsets. 
On  the  27th  of  October,  1857,  I was  observing  a cell  in  which  post  mortem  concen- 
tration had  occurred,  the  pigment  being  in  the  angular  condition.  At  one  of  the  angles 
movements  of  the  granules  were  going  on,  of  which  I will  content  myself  with  giving  two 
examples.  At  one  time  a number  of  molecules  started  off  together  with  great  rapidity 
from  the  black  mass,  but  stopped  after  having  proceeded  a certain  distance ; some  of 
them  remaining  in  their  new  position,  while  others  returned  at  various  rates  towards 
the  centre.  At  another  time  an  individual  granule  moved  slowly  away  for  a little  space, 
and  then  came  back  by  a circuitous  route  to  a different  part  of  the  mass  from  that 
which  it  had  left.  What  I then  saw  has  led  me  to  believe  that  the  movements  of  the 
pigment-molecules  are  of  a complex  character  that  will  perhaps  never  be  fully  explained. 
In  the  mean  time  it  is  clear  that  concentration  and  diffusion  are  both  active  rital  func- 
tions, and  that  both  imply  peculiar  relations  of  the  centre  of  each  cell  to  the  pigment- 
molecules,  as  distinguished  from  the  fluid  in  which  they  are  suspended. 
These  conclusions  invest  the  pigmentary  changes  with  deep  physiological  interest. 
In  the  movements  of  the  granules  towards  and  from  the  centres  of  their  containing 
cells,  we  now  have  ocular  demonstration  that  a particular  kind  of  material  may  have 
impressed  upon  it  by  vital  action,  independently  of  muscular  contraction  or  ciliary 
motion,  tendencies  to  rush  energetically  to  or  from  certain  fixed  points  in  the  tissues, 
through  distances  equal  to  nearly  twice  the  thickness  of  a ’\illus  of  the  human  intestine, 
and  several  times  greater  than  the  average  breadth  of  a human  capillary  interspace. 
Whether  we  be  able  to  explain  the  means  by  which  such  results  are  accomplished  or 
not,  it  is  obvious  that  forces  of  similar  powers  and  range  of  operation,  if  suitably 
modified  according  to  the  chcumstances  of  each  case,  would  be  more  than  adequate  to 
cause  the  passage  of  particles  of  fat  from  the  cavity  of  the  intestine  mto  the  central 
lacteals  of  the  villi,  or  the  transit  of  the  material  requhed  for  a particular  secretion  or 
act  of  nutrition  out  of  a capillary  into  a neighbouring  gland  cell  or  other  portion  of 
tissue ; and,  again,  for  the  discharge  of  an  elaborated  product  of  secretion  into  a duct,  or 
the  return  of  waste  matter  into  the  blood-vessels  or  lymphatics.  We  thus  obtain  a basis 
of  fact  for  what  has  hitherto  been  merely  conjectural,  in  the  explanation  of  the  processes 
of  absorption,  secretion  and  nutrition  generally. 
The  functions  of  the  pigment-cells  are  under  the  control  of  the  nervous  system  *. 
as  is  evident  from  the  effects  produced  on  the  coloui’  of  the  skin  by  a struggle  on  the 
part  of  the  animal. 
Much  attention  has  been  devoted  by  von  Wittich  to  the  inquuy,  by  what  ganglionic 
centres  this  control  is  exercised.  He  found  that  division  of  the  sciatic  nerve  in  the 
* The  part  of  the  paper  devoted  to  this  branch  of  the  subject  has  been  entirely  rewritten ; and  the  dates 
in  the  text  imply  that  most  of  the  observations  •with  reference  to  it  have  been  made  since  the  reading  of  the 
manuscript  before  the  Society. 
