AmMarch.f903.rm"}       Nature  and  Action  of  Toxins.  103 
classes  are  further  subdivided  according  to  the  structure  upon  which 
the  toxins  act  specifically.  These  subgroups  are  distinguished  by- 
such  self-explanatory  terms  as  leucotoxins,  spermotoxins,  hemo- 
lysins, hemagglutinins,  etc.  A  further  very  important  effect  of 
toxins  consists  in  their  leading  to  the  formation  of  antagonistic  sub- 
stances. 
To  explain  these  effects  of  toxins,  we  must  premise  some  concep- 
tions of  the  chemical  processes  of  life. 
The  molecules  of  the  substance  which  forms  the  chemical  basis 
of  life — the  "  biogen  "  molecules,  as  they  are  called — must  be  struct- 
ures of  the  greatest  complexity,  composed  of  enormous  numbers  of 
atoms.  It  has  been  truly  said  that  the  composition  of  the  biogen  is 
constant  only  in  the  sense  in  which  the  composition  of  a  flame  is 
constant.  Whilst  the  ratio  of  the  atoms  may  remain  unaltered,  the 
individual  atoms  are  ever  changing  their  position  and  relations. 
The  most  characteristic  feature  ot  biogen  is  its  metabolism — the 
tact  that  it  assimilates  molecules  which  are  foreign  to  it.  This  pre- 
supposes that  the  structure  ot  the  biogen  molecule  is  of  such  a 
nature  that  it  can  readily  add  other  molecules.  It  must  have  un- 
saturated or  loosely  saturated  affinities.  In  order  that  two  mole- 
cules can  combine  they  must  have  affinities  for  each  other.  These 
affinities  do  not  reside  in  the  molecule,  but  in  certain  of  its  atoms  or 
groups  of  atoms,  so-called  side-chains,  just  as  in  the  benzol  ring,  sub- 
stitutions do  not  occur  in  the  entire  molecule  but  in  the  H  atoms  or 
other  side -groups.  These  side  -  groups,  corresponding  to  the  H6 
atoms  of  benzol,  or  to  the  CH3  of  toluene,  etc.,  are  called  "  receptors  " 
in  the  case  of  the  biogen.  The  fact  that  the  biogen  molecule  can 
combine  with  almost  innumerable  substances,  proves  that  it  pos- 
sesses a  large  number  and  variety  of  these  side-chains.  The  almost 
infinite  number  of  such  reactions,  which  have  been  demonstrated  or 
forecast,  makes  it  probable  that  the  biogen  molecule  does  not  only 
possess  a  large  number  of  rceptor  side-chains,  but  that  these  are 
capable  of  such  readjustment  that  they  may  develop  affinity  lor 
almost  every  conceivable  reagent.  It  is  plain  that  these  open  side- 
chains  or  receptors — as  we  shall  henceforth  call  them — are  of  the 
utmost  importance  to  the  cell,  since  they  are  prerequisite  to  metabol- 
ism. Their  saturation  by  food-molecules  does  not  lead  to  an 
appreciable  diminution  of  them,  since  the  receptor  food-molecules 
