642 
IVIE.  J.  LISTEE  OJf  THE  CrTAXEOES 
will  be  remembered  that  a similar  conclusion  was  derived  from  the  study  of  the  influence 
exerted  upon  the  pigment-cells  by  the  central  ganglia.  Hence  it  appears  that  the  ten- 
dency to  diffusion  of  the  pigment-molecules  is  in  constant  operation  in  the  cells,  but 
kept  in  check  by  an  antagonistic  concentrating  agency  var^'ing  in  obedience  to  nervous 
influence. 
It  is  an  interesting  cfrcumstance,  that  two  functions  seemingly  so  totally  distinct  as 
muscular  contraction  and  pigmentary  concentration,  should  both  be  thrown  into  a state 
of  activity  in  consequence  of  arrest  of  the  circulation.  It  is  to  be  remembered,  however, 
that  there  is  no  evidence  that  either  the  involuntary  muscular  fibre  or  the  pigmentarr 
tissue  is  directly  infiuenced  by  the  cessation  of  the  flow  of  blood,  the  effect  being  appa- 
rently produced  through  the  medium  of  the  local  nervous  system.  This  we  know  with 
certainty  in  the  case  of  the  post  mortem  movements  of  the  intestines ; and  we  have  seen 
reason  to  think  it  likely  that  the  same  is  true  regarding  the  contractions  of  the  arteries 
after  death,  and  the  concentration  of  the  pigment  under  similar  circumstances.  It  is  a 
curious  question  how  the  arrest  of  the  circulation  causes  these  actions  of  the  local  nerves. 
The  idea  suggested  by  the  facts  is  that  the  tissues  begin  to  suffer  fr-om  the  want  of  fresh 
supplies  of  the  vital  fluid,  and  resent  the  injury,  as  it  were,  by  a struggle. 
Rich  in  results  of  general  physiological  interest  as  the  study  of  the  pigmentary  system 
of  the  frog  has  proved,  it  has  yielded  fruit  of  not  less  value  in  a pathological  point  of 
view.  Indeed,  what  induced  me  to  investigate  the  functions  of  concentration  and  diffu- 
sion, was  the  unexpected  light  thrown  upon  the  natui’e  of  inflammation  by  the  effects 
produced  by  irritants  upon  those  processes.  For  information  on  this  subject  I beg  to 
refer  the  reader  to  the  next  paper  in  these  Transactions. 
The  pigmentary  system  also  promises  to  render  good  seinice  in  toxological  inquiry. 
Hitherto,  in  experiments  performed  upon  animals  with  that  object,  attention  has  been 
directed  chiefly,  if  not  exclusively,  to  the  effects  produced  upon  the  actions  of  the  neiTous 
centres,  the  nerves  and  the  muscles.  In  the  pigment-cells  we  have  a form  of  tissue  with 
entirely  new  functions,  which,  though  apparently  allied  to  the  most  recondite  processes 
of  the  animal  economy,  yet  produce  very  obvious  effects,  and  thus  afford  great  facilities 
for  ascertaining  whether  or  not  they  have  been  destroyed  by  any  poison  that  may  have 
been  administered. 
An  experiment  of  this  kind  which  I once  performed,  though  mth  a different  object, 
may  be  mentioned  by  way  of  example.  Being  desirous  of  confii-ming  the  conclusion  to 
which  I had  been  led  by  experiments  above  related,  viz.  that  diffusion  always  tends  to 
take  place  when  the  inffuence  of  the  nerves  is  withdrawn  from  the  pigment-cells,  it 
occurred  to  me  that  the  Urari  poison  might  be  brought  into  requisition  for  that  piu’pose: 
for  it  has  been  shown  by  Professor  Kollikee  of  Wurzburg,  that  this  substance  para- 
lyses in  the  first  instance  the  extremities  of  the  motor  nerves  mthout  affecting  the  con- 
tractility of  the  muscular  tissue ; and  supposing  the  nerves  concerned  in  regulating  the 
pigmentaiy  changes  to  be  similarly  deprived  of  their  powers,  while  the  pigment-cells 
themselves  remained  intact,  diffusion  should  take  place  after  exhibition  of  the  drug,  pro- 
