696 
ME.  J.  LISTEE  ON  THE  EAELT  STAGES  OE  ESTIA3IMATI0N. 
vents  the  occurrence  of  ])Ost  mortem  concentration,  implying  that  the  powers  of  the 
chromatophorous  cells  are  completely  suspended  by  it  within  a few  minutes  at  most  of 
its  first  acting  on  the  part ; yet,  however  long  the  tissues  thus  paralysed  are  kept  sub- 
jected to  its  infiuence,  they  remain  without  any  sign  of  action.  They  will,  however, 
recover  speedily  and  completely  if  soon  taken  out  and  exposed  to  the  air,  so  that  the 
irritating  gas  may  be  dissipated ; whereas  if  retained  for  several  hours  in  the  aerated 
water,  they  may,  indeed,  have  their  powers  restored  to  a ceidain  extent  on  removal  from 
it,  but  exhibit  only  very  feeble  action.  Such  facts  as  these  prove  conclusively  that  the 
tissues  may  have  their  functional  activity  impaired  without  loss  of  vitahty  by  the  direct 
action  of  irritants,  independently  of  any  stimulating  effects  which  may  be  at  first  pro- 
duced by  them ; and  also  that  the  influence  thus  exerted  is  of  an  injurious  tendency. 
The  imperceptible  transition  from  suspension  of  function  to  loss  of  vitaht)'  displayed 
by  the  long-continued  operation  of  carbonic  acid  upon  the  pigment-cells  is  also  well 
illustrated  by  some  of  the  experiments  upon  the  cilia,  especially  those  with  heat  and 
ammonia,  which,  unless  employed  with  extreme  caution,  not  only  permanently  arrested 
the  vibratile  filaments,  but  reduced  the  epithelium-cells  to  a condition  in  which  they 
were  amenable  to  the  ordinary  laws  of  chemical  affinity.  All  irritants  appear  to  be 
agents  which,  if  operating  with  sufficient  energy,  completely  destroy  wtahty,  probably 
by  inducing,  through  chemical  or  physical  action,  an  irreparable  derangement  of  the 
molecular  constitution  of  the  tissues.  Their  essential  property,  however,  is  that  of 
causing,  when  applied  somewhat  more  mildly,  a minor  degree  of  disturbance  or  disorder 
in  the  component  textures  of  the  body,  which  are  rendered  for  the  time  being  unfit  for 
discharging  their  wonted  functions  *,  though  afterwards,  by  \ii’tue  of  their  innate  powers, 
capable  of  spontaneous  recovery,  the  rapidity  and  completeness  of  which  bears  an 
inverse  ratio  to  the  intensity  and  duration  of  the  previous  mitation.  Lastly,  these 
same  noxious  agents,  if  in  a still  more  gentle  form,  operate,  upon  some  of  the  tissues  at 
least,  as  stimulants,  rousing  them  to  increased  exertion  of  their  wtal  functions.  How 
this  effect  is  brought  about  must,  I believe,  be  only  matter  of  uncertain  speculation,  so 
long  as  the  real  nature  of  life  in  the  animal  frame  remains,  as  it  probably  ever  uill 
remain  to  our  finite  capacities,  an  impenetrable  mystery. 
It  is  an  interesting  circumstance  that,  in  the  experiments  mth  warm  water,  the  ciha. 
after  recovering  from  the  state  of  quiescence,  moved  for  a while  more  briskly  than  they 
did  immediately  before  the  application  was  made.  This  increased  action  cannot  be 
attributed,  like  the  primary  acceleration  resulting  Lorn  very  gentle  warmth,  to  a mild 
operation  of  the  irritant ; for  the  epithelium-cells  must  have  been  completely  cooled 
down  before  it  commenced.  It  must  therefore  be  regarded  as  a true  reaction  on  the 
part  of  the  tissue,  whether  dependent  on  accumulation  of  Htal  energy  during  the  period 
of  suspended  function,  or  excited,  as  by  an  irritant,  by  the  state  of  disorder  which  the 
warm  water  had  induced,  seepas  uncertain. 
* The  word  “ irritant  ” is  etymologically  ill  adapted  to  express  the  possession  of  this  property  ; hut  as  it 
is  universally  employed  in  professional  nomenclature,  it  is  perhaps  best  to  continue  to  use  it. 
