'^jSor"1'}  External  Preparations.  277 
them  usually  to  be  spread  on  cloth  and  applied  as  dressing.  They 
are,  therefore,  in  the  epidermatic  group  and  comprise  protective, 
cooling,  emollient  and  vesicating  effects.  The  cantharidal  cerate  is 
the  most  important,  and  when  spread  on  adhesive  plaster  in  such 
thickness  as  to  obliterate  the  yellow  color  of  the  plaster  cloth,  when 
viewed  through  the  light,  it  is  the  most  effective  cantharidal  vesicant 
to  be  had. 
PLASTERS. 
This  most  ancient  form  of  medication  has  largely  fallen  into  dis- 
repute during  recent  years,  owing  partly  to  their  ineligibility  and 
uncertainty  of  action,  and  partly  to  the  advent  of  more  efficacious 
and  more  quickly  responsive  methods  of  treatment.  The  advent  of 
the  rubber  plasters  has  also  caused  the  lay  public,  which  believes  in 
"  anything  that  will  stick,"  to  appropriate  the  plasters  as  its  own 
sovereign  remedy  for  all  kinds  of  aches  and  pains. 
Nevertheless,  the  plasters  serve  a  useful  purpose  owing  to  the 
great  range  of  their  application,  which  may  be  summarized  in  the 
following  groups: 
(1)  Epidermatic :  Supportive,  protective,  antiseptic,  counter-irri- 
tant, vesicant.    Vehicle:  Rubber  or  any  suitable  adhesive. 
(2)  Endermatic:  Anodyne,  astringent,  alterative,  resolvent,  seda- 
tive, stimulant.  Vehicle :  Oleates  or  lead  plaster,  sometimes  with 
resins  or  gum-resins. 
(3)  Diadermatic :  For  constitutional  or  systemic  effects.  Vehi- 
cle :  Lanolin  or  plaster-mull. 
The  vehicle  for  the  commercial  rubber  plasters  is  chiefly  india- 
rubber,  or  caoutchouc,  with  sufficient  orris  powder  and  sometimes 
gum-resins  to  give  it  the  proper  consistence  and  retain  its  adhesive 
properties.  For  epidermatic  purposes  this  serves  admirably  as  a 
convenient  and  fairly  stable  adhesive;  but  since  it  lacks  penetrative 
properties  in  any  considerable  degree,  it  should  not  be  used  for 
endermatic  effects,  such  as  are  sought  from  drugs  like  belladonna. 
The  suggestion  to  increase  the  efficacy  of  rubber  belladonna  plaster 
by  the  addition  of  boric  acid  would  seem  also  to  support  the  view 
that  such  anhydrophile  mixtures  do  not  penetrate  into  the  skin,  as 
has  also  been  clinically  demonstrated. 
For  endermatic  effects  the  oleates,  as  represented  by  the  well- 
known  lead  plaster,  are  the  best  vehicle,  since  they  penetrate  into 
