The  cavities  in  the  adult 'cord  present  many
varieties.  The  most  common  form  resembles  in
its  essential  features  the  congenital  disease  which
we  have  just  considered.  Between  the  two  there
are  also  differences,  but  the  resemblance  is  suffi-
ciently close,  and  the  origin  of  the  differences  is  suf-
ficiently clear,  to  make  it  practically  certain,  as
already  mentioned,  that  the  two  forms  are  really
the  same,  and  the  condition  met  with  in  adults  is  the  congenital  condition
persisting  until  later  life.  Of  the  characters  which  this  form  has  in  common
with  the  congenital  disease,  the  most  important  are  the  position  of  the
cavities,  and  the  presence  of  tissue  in  their  vicinity  of  embryonal  or  glioma-
tous  nature.  The  differences  depend  on  an  apparent  increase  of  this  tissue
by  a  process  of  growth,  on  the  greater  damage  to  the  other  parts  of  the  cord
by  the  distention  of  the  cavity,  and  on  the  extent  to  which  the  formation  of
27


yea

hydrocephalus  and  absence  of
cerebellum.  {After  Leyden,  Vir~
chow's  Arch.,  Bd.  68.)  A,  B,  cer-
vical region ;  gelatinous  (embry-
onal) tissue  in  the  posterior  col-
umns encloses  a  medial  cavity,
lined  in  places  with  cylindrical
epithelium,  and  believed  therefore
to  be  the  dilated  central  canal.
C,  lower  dorsal ;  in  each  posterior
column  is  a  mass  of  similar  gela-
tinous substance  (which,  by  a  mis-
take, is  shown  in  C  as  an  open
cavity).  In  D,  lumbar  region,  this
tissue  occupies  only  the  position
of  the  posterior  median  columns.
426  SPINAL   CORD.
new  cavities  or  enlargement  of  old  ones  takes  place  by  a  process  of  disinte-
gration of  the  newly-formed  or  persistent  tissue.
