﻿STRUCTURE 
  OF 
  THE 
  INTESTINE. 
  217 
  

  

  collect 
  around 
  the 
  nucleus, 
  Fig. 
  43, 
  and 
  as 
  they 
  accumulate 
  they 
  extend 
  

   through 
  all 
  the 
  rest 
  of 
  the 
  cell, 
  except 
  the 
  upper 
  part 
  underneath 
  the 
  

   cuticula, 
  which 
  portion 
  always 
  remains 
  clear, 
  as 
  is 
  seen 
  in 
  Fig. 
  44. 
  Viewed 
  

   from 
  above 
  the 
  epithelial 
  cells 
  appear 
  as 
  polygonal 
  pigmented 
  fields, 
  

   each 
  separated 
  from 
  its 
  neighbors 
  by 
  a 
  clear 
  line. 
  Posteriorly 
  the 
  cells 
  

   become 
  less 
  and 
  less 
  pigmented, 
  and 
  pass 
  by 
  gradual 
  changes 
  into 
  the 
  

   epithelium 
  of 
  the 
  ileum. 
  

  

  Ileum. 
  — 
  The 
  ileum 
  is 
  traversed 
  by 
  six 
  longitudinal 
  folds, 
  with 
  inter- 
  

   vening 
  furrows. 
  Outside 
  each 
  furrow 
  is 
  a 
  longitudinal 
  muscular 
  band. 
  

   Viewed 
  from 
  the 
  inner 
  surface, 
  the 
  epithelium 
  is 
  seen 
  to 
  have 
  an 
  unusual 
  

   character. 
  The 
  cells 
  in 
  the 
  middle 
  of 
  each 
  of 
  the 
  flat 
  folds 
  are 
  quite 
  large 
  

   (Fig. 
  50 
  A), 
  polygonal 
  in 
  outline, 
  with 
  large, 
  round, 
  granular 
  nuclei, 
  which 
  

   stain 
  very 
  darkly 
  with 
  haematoxiline. 
  Toward 
  the 
  furrows 
  the 
  cells 
  be- 
  

   come 
  very 
  much 
  smaller, 
  those 
  at 
  the 
  edge 
  of 
  the 
  furrow 
  being 
  not 
  more 
  

   than 
  one-sixth 
  the 
  size 
  of 
  those 
  in 
  the 
  middle 
  (Fig. 
  50 
  B). 
  Underneath 
  

   the 
  furrow, 
  the 
  longitudinal 
  muscles 
  (Fig. 
  50 
  L) 
  are 
  seen 
  shining 
  through. 
  

  

  A 
  transverse 
  section 
  (Fig. 
  51) 
  shows 
  that 
  the 
  walls 
  are 
  double 
  ; 
  the 
  

   inner 
  leaf 
  is 
  composed 
  of 
  epithelium, 
  Ep., 
  and 
  connective 
  tissue, 
  the 
  outer 
  

   leaf, 
  of 
  the 
  circular 
  muscles, 
  muc. 
  C. 
  The 
  furrows 
  are 
  indicated 
  by 
  the 
  

   six 
  bands 
  of 
  longitudinal 
  muscles, 
  L 
  L. 
  It 
  is 
  only 
  opposite 
  these 
  bands 
  

   that 
  the 
  two 
  leaves 
  are 
  united, 
  as 
  is 
  shown 
  more 
  plainly 
  in 
  Fig. 
  52. 
  

   The 
  epithelium, 
  Ep., 
  rests 
  directly 
  upon 
  and 
  is 
  intimately 
  united 
  with 
  the 
  

   connective 
  tissue, 
  so 
  forming 
  a 
  single 
  leaf, 
  which 
  then 
  bends 
  down, 
  mak- 
  

   ing 
  a 
  furrow, 
  F, 
  opposite 
  the 
  longitudinal 
  muscle, 
  L 
  L, 
  where 
  it 
  is 
  united 
  

   with 
  the 
  circular 
  muscular 
  layer, 
  muc 
  C. 
  The 
  consequence 
  of 
  this 
  ar- 
  

   rangement 
  is 
  that 
  underneath 
  each 
  fold 
  there 
  is 
  a 
  very 
  large 
  longitudi- 
  

   nal 
  cavity 
  between 
  the 
  propria 
  and 
  the 
  muscularis. 
  

  

  The 
  cuticula 
  (Fig. 
  52), 
  cu., 
  is 
  thin, 
  but 
  probably 
  chitinous 
  ; 
  it 
  resembles 
  

   that 
  on 
  the 
  gastro-ileal 
  folds, 
  except 
  that 
  there 
  are 
  no 
  spines, 
  but 
  it 
  is 
  

   not 
  in 
  the 
  least 
  like 
  the 
  ventricular 
  cuticula. 
  It 
  extends 
  equally 
  over 
  

   the 
  folds 
  and 
  the 
  furrows. 
  

  

  The 
  epithelium 
  has 
  round 
  nuclei 
  ; 
  the 
  size 
  both 
  of 
  the 
  cells 
  and 
  of 
  the 
  

   nuclei 
  diminishes 
  rapidly 
  towards 
  the 
  bottom 
  of 
  the 
  furrows 
  (Fig. 
  52), 
  F. 
  

   The 
  bases 
  of 
  the 
  cells 
  are 
  somewhat 
  dome-shaped. 
  The 
  nuclei 
  are 
  sur- 
  

   charged 
  with 
  granules, 
  and 
  have 
  a 
  less 
  distinct 
  outline 
  than 
  the 
  nuclei 
  

   from 
  other 
  parts 
  of 
  the 
  body. 
  

  

  The 
  circular 
  muscles 
  are 
  moderately 
  developed. 
  Each 
  longitudinal 
  

   muscular 
  band 
  consists 
  of 
  10 
  to 
  15 
  single 
  bundles. 
  The 
  fibres 
  are 
  

   striped. 
  

  

  Colon. 
  — 
  In 
  the 
  colon 
  the 
  six 
  longitudinal 
  folds 
  of 
  the 
  ileum 
  are 
  con- 
  

   tinued, 
  but 
  their 
  surface, 
  instead 
  of 
  being 
  smooth 
  as 
  in 
  the 
  ileum, 
  is 
  

   thrown 
  up 
  in 
  numerous 
  irregular 
  curved 
  and 
  zigzag 
  secondary 
  folds, 
  as 
  

   is 
  imperfectly 
  indicated 
  in 
  Fig. 
  2, 
  col. 
  The 
  cells 
  of 
  the 
  epithelium 
  are 
  

   of 
  uniform 
  size, 
  and 
  contain, 
  especially 
  at 
  the 
  summits 
  of 
  the 
  secondary 
  

   folds, 
  pigment 
  granules 
  like 
  those 
  in 
  the 
  cells 
  of 
  the 
  gastro-ileal 
  valve. 
  

   The 
  epithelium 
  is 
  covered 
  by 
  a 
  highly 
  refringent 
  cuticula 
  without 
  spines, 
  

  

  