ME.  J.  LISTEE  ON  THE  EAELT  STAOES  OF  INFLAMMATION. 
689 
nized  in  consequence  of  the  changes  of  cahbre  which  they  produce  in  the  vessels; 
and  the  manner  in  which  the  arteries  are  affected  in  a congested  part  of  the  web  indi- 
cates that  the  muscular,  like  the  pigmentary  tissue,  has  its  functional  activity  impaired 
by  a certain  amount  of  irritation.  Thus  I have  repeatedly  been  struck  with  the  fact, 
and  noted  it  before  I knew  its  significance,  that  an  artery  running  through  a limited 
area  on  which  an  irritant  has  acted,  remains  dilated  in  the  spot,  although  it  may  vary  in 
other  parts  of  its  course.  This  I have  observed  in  one  experiment  with  mustard,  in  one 
with  acetic  acid,  in  two  with  ammonia,  and  in  one  with  heat.  In  the  last-mentioned 
case  the  appearance  was  particularly  striking,  from  the  circumstance  that  two  arteries 
happened  to  pass  through  the  burnt  part,  and  were  constricted  to  absolute  closure  in  the 
rest  of  their  coui’se,  contrasting  strongly  with  their  fully  dilated  state  within  the  area. 
In  the  ammonia  experiments  also  the  artery  concerned  was,  in  the  progress  of  each 
case,  seen  to  be  completely  constricted  beyond  the  congested  area,  though  still  dilated 
within  it.'  The  limitation  of  this  effect  on  the  arteries,  to  the  extent  of  the  part  acted 
on  by  the  irritant,  proves  that  it  is  the  result  of  its  direct  action  on  the  tissues ; differing 
remarkably  in  this  respect  from  the  dilatation  of  the  vessels,  which  is  produced  indi- 
rectly through  the  medium  of  the  nervous  system,  and  affects  a wide  space  round  about 
the  spot  irritated. 
But  with  regard  to  both  the  muscular  fibre-cells  of  the  arteries  and  the  pigment-cells, 
it  may  fairly  be  questioned  whether  the  diminution  of  power  to  act  resides  in  them  or 
La  those  portions  of  their  nerves  which  are  situated  in  the  irritated  region.  The  view 
that  the  nerves  are  paralysed  by  irritants  is  consistent  with  the  benumbing  influence 
well  known  to  be  exerted  upon  the  human  skin  or  mucous  membranes  by  some  of  those 
agents,  e.  g.  mechanical  violence,  the  galvanic  shock,  cold,  and  chloroform.  I have  also 
obser\’ed,  as  before  alluded  to  *,  that  mustard  produces  a similar  result  on  the  cutaneous 
sensory  nerves,  and  hence  it  seems  probable  that  the  same  is  true  of  the  whole  class  of 
irritants.  Again,  the  diffusion  induced  by  mustard,  croton  oil,  and  cantharides  indicates, 
according  to  what  we  have  seen  to  be  its  most  probable  explanation,  that  the  nerves  of 
the  pigment-ceUs  suffer  impairment  of  functional  activity  under  the  action  of  these  three 
substances.  On  the  other  hand,  the  fact  that  diffusion  is  arrested  equally  with  concen- 
tration by  most  irritants,  appears  to  prove  that  the  chromatophorous  cells  are  them- 
selves also  affected  with  loss  of  power ; for,  as  has  been  before  alluded  to,  the  withdrawal 
of  nervous  influence  from  them  in  a healthy  state  of  the  tissues  invariably  gives  rise  to 
diffusion,  and  the  same  result  would  necessarily  follow  the  action  of  an  irritant  which 
merely  paralysed  the  nerves.  I have  also  observed,  on  two  occasions,  after  the  energetic 
operation  of  an  irritant  upon  a part  of  a web  containing  a large  artery  f,  that  drawing 
* See  page  677. 
t The  main  arteries  lying  between  the  layers  of  skin  of  which  the  web  consists,  are  not  so  speedily  acted 
upon  by  irritants  as  the  capillaries  of  the  dermis.  This  is  most  marked  in  large  frogs  with  thick  webs.  In 
one  such  specimen,  a drop  of  chloroform  caused  first  stagnation  and  then  discoloration  from  chemical  action 
on  the  blood  in  the  capillaries  of  the  dorsal  layer  of  the  web  to  which  it  was  applied,  while  a main  artery 
4x2 
