The  brain  of  the  lower  animals  is  undeveloped  compared
to  thiit  ni  man,  and,  in  accordance  with  the  general  fact  that
the  more  highly  a  portion  of  the  nervous  mechanism  is
organiztid  the  more  powerfully  is  it  influenced  by  drugs,  it
follows  th'tt  drugs  acting  on  the  brain  and  cord  are  more


DBUOa  ACTOIO  ON  THE  NERV0C8  8T8TEM


39


proQd  to  affect  the  cord  in  veteritiaiy  patients,  while  impres-
Bing  the  brain  more  potently  in  man.  For  tliis  reason  we
notioe  iu  the  horse  that  the  primury  period  of  excitability
(aometimes  seen  in  man)  following  the  ndministratinn  of
morphine  is  much  prolonged  and  not  infrequently  com-
pletely obliterates  the  Bomnifacient  action  of  the  drug  in
this  animaL

We   shall  be   content  to  classify  drugs  acting  on  the
nervons  system  according  lo  their  most  pronounced  action

tiioilerat**  doses.
(a)  Ctrchral  Exciiaids.
Camphor  Qoinima

Caffeine  Cocaine

Uses. — These  drags  are  rarely  used  simply  to  excite  the
brain,  bntfonother  purposes.    Camphor,  caffeine  and  quinine
are  employed  to  generally  excite  the  nervous  system  in
^^.depreesed  conditions.     Caffeine  is  ayaluable  antidote  to  the
^Hkpressiug  cersbral  action  of  opinm  in  poisoning.
H         (b)  Cerehrul   DfpresamilH. —It   is   fortunate    that   drugs
VpTogreaflively  paralyzing  the  functions  of  the  brain  follow
the  so-called  law  of  dissolution — 1>.,  paralyze  the  various
■  functions  of  the  brain  in  the  inverse  order  of  their  evolu-
|ti.>uary  development.    The  centres  last  to  be  acquired  are
[the  first  to  be  paralyzed  (cerebral  centres) ;  while  those  of
Uent  origin  (the  respiratory,  vagna  and  vasomotor  centres)
last  to  Bncouinb  to  the  action  of  cerebral  depressants.
