The  lenticular  epithelium  consists  of  a  single  layer  of  cells  which
covers  the  entire  anterior  convexity  of  the  lens,  extending  as  far  back  as
its  equator.  The  height  of  these  cells  varies  with  the  age  of  the  indi-
vidual. In  fetal  life  they  are  distinctly  columnar,  in  youth  short  colum-
nar or  cuboidal,  in  adult  life  low  cuboidal  or  flattened.  Toward  the  mar-
gin of  the  lens  the  epithelial  cells  become  progressively  lengthened,  and
at  its  equator  are  transformed  directly  into  the  fibers  of  the  lenticular
substance.  This  definitive  structure  of  the  lens  recalls  its  manner  of
development  from  the  original  lens  vesicle;  the  hollow  vesicle  becomes
solid  by  the  elongation  of  the  cells  of  the  posterior  wall.
The  substantia  lentis  is,  therefore,  the  product  of  the  epithelium
of  the  lens,  whose  cells  become  greatly  elongated  to  form  slender  hexag-
onal prisms,  known  as  the  lens  fibers.  When  it  is  first  formed  each
prism  exhibits  a  nucleus  which  persists  for  some  time,  but  gradually
disappears  as  in  the  process  of  growth  the  older  fibers  become  farther
and  farther  removed  from  their  source  of  nutrition,  the  lymph  and  the


THE  OCULAE  CONTENTS


667


aqueous  humor  in  which  the  surface  of  the  lens  is  bathed.  This  change
is  accompanied  by  a  hardening  or  cornification  and  slight  shrinkage
of  the  lens  fibers,  so  that  those  prisms  which  come  to  occupy  the  center
of  the  lens  form  a  dense,  hard  mass  of  non-nucleated  fibrous  cells  with
faintly  serrated  margins;  the  peripheral  fibers  retain  their  smooth  edges
and  their  nuclei,  and  form  a  protoplasmic  mass  of  much  softer  con-
sistency. The  hardened  central  mass  is
the  so-called  nucleus  of  the  lens.  Any
opacity  of  the  lens  or  its  capsule  is  known
as  a  cataract.
