The  Midbrain.  The  general  arrangement  of  these  parts
has  been  already  described  (Fig.  83).  Cross-sections  show
(Fig.  176)  the  aqueduct  of  Sylvius,  S,  traversing  the  mid-
brain near  its  upper  part  and  surrounded  by  a  thin  layer  of
gray  matter,  in  close  connection  with  which  are  the  origins
of  the  third  and  fourth  cranial  nerves,  iv,  and  of  part  of  the
fifth.  The  crura  cerebri  form  the  main  mass  of  the  midbrain.
Each  is  divided  by  gray  matter  {locus  niger,  Ln)  into  a  ven-
tral portion  {pes  or  crusta,  P),  which  forms  the  semicylin-
drical  portion  of  the  cms  seen  on  the  base  of  the  brain  and  a
dorsal  portion,  the  tegmentum,  Tg.  The  pes  consists  mainly
of  the  fibres  of  the  pyramidal  tract,  Py,  but  some  fibres  of  the
fillet,  /,  also  run  forward  in  it,  as  do  fibres, /r  and  oc,  connecting


THE  PHTSIOLOQT  OF  THE  BBAIN.  617

it  with  the  frontal  and  occipital  lobes  of  the  cerebral  hemisphere.
The  tegmentum  contains  gray  masses  and  many  transverse
and  longitudinal  fibres.  Many  of  the  fibres,  cp,  come  from  the
anterior  peduncle  of  the  cerebellum ;  these  cross  in  the  pos-
terior part  of  the  tegmentum ;  most  of  them  end  in  a  large
mass  of  gray  matter  in  the  front  of  the  tegmentum  named  the
red  nucleus  j  others  run  forward  to  the  optic  thalamus  direct.
