PIGMENTAEY  SYSTEM  OF  THE  FEOG. 
641 
nature,  while  we  have  reason  to  think  that  the  same  thing  holds  regarding  the  arteries 
in  the  frog’s  web  *.  Such  a view  is  in  accordance  with  the  results  of  recent  anatomical 
discovery,  which  has  revealed  nerve-cells  in  many  parts  where  their  occurrence  had  not 
pre\iously  been  conjectured.  But  in  the  absence  of  more  positive  evidence,  we  must  be 
careful  not  to  trust  too  much  to  analogy  on  such  a point ; for  it  by  no  means  neces- 
sarily follows,  that,  even  if  muscular  fibre-cells  are  incapable  of  acting  in  mutual  har- 
mony without  the  aid  of  the  nervous  system,  the  same  must  be  the  case  with  pigment- 
cells,  which,  it  is  to  be  remarked,  resemble  ganglion  corpuscles  in  being  connected 
together  by  anastomosing  offsets.  The  nerve-cells,  if  such  be  really  the  means  by  which 
the  harmonious  actions  of  the  pigment-cells  in  an  amputated  limb  are  induced,  must  be 
disseminated  among  the  tissues  of  the  web  itself;  for  both^os^  mortem  concentration  and 
secondary  diffusion  occur  m a piece  of  a web  cut  out  and  placed  in  a drop  of  water  on  a 
plate  of  glass.  This  was  ascertained  on  the  4th  of  September,  1858,  in  the  case  already 
alluded  to  as  an  instance  of  rapid  occnrrence  of  concentration.  About  half  an  hour  after 
removal  from  the  body,  the  pigment,  previously  reticular,  was  in  the  dotted  state,  and 
three  hours  later  it  was  found  to  be  again  stellate. 
The  case  of  the  pigment-cells  is  analogous  to  that  of  the  arteries  in  this  respect,  that, 
so  long  as  cu’culation  is  going  on,  they  are  generally  in  entire  subjection  to  the  central 
ganglia,  and  act  only  when  stimulated  by  their  influence.  But  as,  in  the  arteries,  it 
appears  to  be  by  the  independent  action  of  the  local  nerves  that  a contraction  caused  by 
direct  irritation  spreads  to  a considerable  distance  from  the  part  operated  on,  so  it  is 
probably  by  local  means  that  the  pallor  induced  by  pinching  the  web  affects  a circle  of 
surrounding  tissue.  If  this  be  true,  the  case  of  direct  irritation  will  be  an  exception  to 
the  general  rule,  that,  while  cnculation  continues  healthy,  concentration  always  implies 
the  operation  of  the  central  organs  of  the  nervous  system. 
Comparing  the  changes  in  the  pigment  in  an  amputated  limb  with  those  which  take 
place  under  similar  circnmstances  in  the  arteries f,  it  appears  that  the  first  effect  of 
removal  from  the  influence  of  the  nervous  centres  in  the  head  and  trunk  is  arterial  relax- 
ation and  pigmentary  diffusion,  followed  in  a variable  time  by  contraction  of  the  vessels 
and  concentration  of  the  dark  molecules,  giving  place  again  to  relaxation  and  diffusion, 
after  which  succeed  irregular  alternations  of  contraction  and  dilatation  in  the  one  case, 
and  of  concentration  and  diffusion  in  the  other.  Here,  though  the  vascular  and  pig- 
mentary changes  do  not  at  all  correspond  with  one  another  in  point  of  time,  yet  there 
is  an  evident  parallel  between  them ; and,  admitting  that  in  each  case  the  variations 
are  the  result  of  alternate  action  and  inaction  of  the  appropriate  local  nervous  system,  it 
is  evident  that  concentration  of  the  pigment  corresponds  to  contraction  of  the  muscular 
fibres  of  the  arteries ; these  being  both  the  results  of  nervous  action,  while  diffusion  of 
the  pigment,  like  arterial  relaxation,  takes  place  when  the  nerves  cease  to  operate.  It 
* See  the  preceding  paper  on  the  parts  of  the  nervous  system  which  regulate  the  contractions  of  the 
arteries,  page  619. 
t See  the  preceding  paper  before  referred  to,  page  618. 
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