28  Ehrlich's  Chemotherapy.  { ^  January,  wu™' 
sickness  in  the  tropics.  The  scourge  of  syphilis  is  produced  by  a 
parasite  known  as  the  spirochete,  which  is  closely  allied  to  the  others 
named,  although  it  is  still  undetermined  whether  its  nature  is  animal 
or  vegetable.  As  will  be  seen,  this  particular  disease  has  been  found 
to  be  one  of  the  most  amenable  to  treatment. 
As  a  result  of  his  researches,  Ehrlich  formulated  a  theory  regard- 
ing the  behavior  of  the  cells  of  living  tissue,  or  of  parasites  toward 
foreign  bodies.  He  conceives  them  as  made  up  of  a  central  "  dom- 
inant body,"  which  throws  out  "  sidechains,"  to  which  he  later 
gave  the  name  receptors.  These  are  of  variable  character,  some 
being  nutrient  receptors,  and  others  chemo-receptors,  that  is,  recep- 
tors or  certain  definite  chemical  elements  or  groups  of  elements, 
known  in  chemistry  as  radicals.  In  a  crude  sense,  the  receptors 
may  be  likened  to  locks,  and  the  nutrient  or  chemical  bodies  as  keys, 
each  fitting  a  particular  lock,  as,  for  example,  the  dyestuff  methy- 
lene-blue  already  mentioned.  The  combinations  thus  affected  may 
be  beneficial  to  the  cell,  as  in  the  case  of  the  nutrients,  or  they  may 
result  in  the  poisoning  and  death  of  the  cell,  as  in  the  case  of  the 
methylene-blue  when  brought  into  contact  with  the  type  of  Plas- 
modium referred  to  above,  or  quinine  for  plasmodia  in  general,  a 
specific  remedy  for  malaria  discovered  by  empirical  research. 
Ehrlich  and  his  co-workers,  with  extraordinary  skill  and  industry, 
prepared  several  hundred  dye-stuffs,  studying  the  varying  effects 
of  alterations  in  chemical  structure,  each  new  compound  having  been 
logically  selected  as  the  result  of  laboratory  tests  of  its  parasiticidal 
efficiency.  Of  all  these,  very  few  finally  withstood  severe  tests,  pos- 
sibly not  more  than  ten  in  all,  but  the  fact  was  established  that  it  was 
possible  in  certain  cases  to  sterilize  the  animal  organism  with  respect 
to  parasites,  by  this  means,  without,  at  the  same  time,  poisoning  the 
animal  itself.  They  were  also  able  to  establish  certain  principles  as 
to  the  chemical  structure  of  the  dye-stuffs  most  likely  to  be  effective. 
They  encountered,  however,  many  difficulties.  A  dye  which  would 
attack  and  destroy  a  given  parasite  in  a  particular  animal  would  not 
always  do  so  in  another  species.  Symptoms  of  disease  would  some- 
times recur  after  varying  intervals,  and  the  parasites  would  then 
often  exhibit  peculiar  resistance  to  further  attack. 
While  these  researches  were  still  in  progress,  Uhlenmuth  and 
Salmon  published  an  account  of  instances  of  marked  success  in  the 
destruction  of  the  spirochete  of  syphilis,  and  the  arrest  of  the  disease, 
by  the  use  of  an  arsenical  compound  known  as  atoxyl.  Secondary 
