A  January  Pi9i4m* }  Eltrlich's  Chemotherapy.  27 
More  than  thirty  years  ago  Ehrlich  began  using  coal-tar  colors  in 
his  physiological  studies,  employing  them  as  stains  for  preparations 
to  be  examined  under  the  miscroscope.  It  is,  of  course,  now  com- 
monly known  that  certain  dye-stuffs  appear  to  have  a  selective 
affinity  for  certain  tissues  of  the  body,  or  for  certain  parasites  when 
residing  within  it,  and  these  stains  are  in  every  day  use  by  the 
pathologist.  But  it  was  not  so  thirty  years  ago,  and  Ehrlich  first 
found  that  a  dye-stuff  known  as  methylene-blue,  and  its  congeners, 
were  the  only  colors  which  would  stain  live  nerve  tissue,  and  drew 
from  this  the  important  inference,  which  is  at  the  basis  of  chemo- 
therapy, that  this  was  because  of  a  particular  receptivity  for  these 
dye-stuffs  on  the  part  of  these  tissues  or  parasites.  It  is  easy  to 
understand  something  of  the  importance  of  this  use  of  these  stains, 
or  dyes,  if  it  is  recalled  that  the  changes  produced  in  the  individual 
cells  or  tissues  by  drugs  are  not  detectable  even  under  the  micro- 
scope in  most  cases,  and  that  it  is  only  through  these  stains  that  a 
knowledge  of  what  has  actually  happened  can  be  even  approximately 
learned. 
Ehrlich  concluded  from  his  observations  that  it  was  probable  that, 
since  these  tissues  and  parasites  possessed  this  receptivity  for  these 
specific  bodies,  there  must  be  some  definite  effect  produced  as  a  re- 
sult of  the  combination,  if  combination  it  were,  and  proceeded  to 
conduct  investigations  in  this  direction.  After  some  time  these 
researches  were  rewarded,  and  in  1890  Ehrlich  and  Lappmann  pub- 
lished a  paper  on  the  pain-relieving  properties  of  methylene-blue, 
and,  later,  Ehrlich  and  Guttmann  found  that  the  same  dye  was  fatal 
to  one  type  of  the  plasmodium,  the  parasite  which  causes  malaria. 
As  the  latter  field  of  investigation,  that  of  the  effect  upon  parasites, 
appeared  very  promising,  they  turned  their  attention  to  a  particular 
class  of  parasites  known  as  trypanosomes,  because  these  could  be 
more  easily  studied  by  the  inoculation  of  mice. 
The  disease-producing  parasites  are  sometimes  of  vegetable 
origin,  as  the  bacteria,  and  sometimes  of  animal  nature,  as  the  pro- 
tozoa. The  trypanosomes  are  worm-like  bodies,  somewhat  larger 
than  bacteria,  belonging  to  the  animal  class,  and  the  diseases  which 
they  produce  prevail  most  generally  in  tropical  countries.  Of  these 
diseases,  surra,  most  generally  known  in  India  among  cattle,  dogs 
and  camels;  nagana  (tsetse-fly  disease),  known  in  Africa  among 
animals  in  general;  and  mal  de  cadaras,  known  in  South  America 
among  horses,  are  typical,  while  man  is  also  attacked  by  the  sleeping 
