﻿THE 
  DIGESTIVE 
  CANAL 
  AS 
  A 
  WHOLE. 
  219 
  

  

  cells 
  lying 
  at 
  the 
  bases 
  of 
  the 
  high 
  columnar 
  ciliated 
  epithelium. 
  It 
  is, 
  

   however, 
  uncertain 
  whether 
  these 
  small 
  cells 
  lie 
  between 
  the 
  others, 
  or 
  

   form 
  a 
  sub 
  epithelial 
  endothelium, 
  similar 
  to 
  that 
  described 
  by 
  Debove. 
  313 
  

  

  Underneath 
  the 
  epithelium 
  appear 
  the 
  round 
  nuclei 
  of 
  the 
  tunica 
  

   propria, 
  and 
  the 
  very 
  much 
  elongated 
  nuclei 
  of 
  the 
  tracheal 
  ramifica- 
  

   tions. 
  

  

  In 
  a 
  transverse 
  section, 
  Fig. 
  42, 
  it 
  is 
  seen 
  that 
  each 
  gland 
  is 
  a 
  low 
  flat 
  

   fold 
  of 
  the 
  epithelium 
  ; 
  each 
  fold 
  is 
  separated 
  from 
  its 
  neighbor 
  on 
  

   either 
  side 
  by 
  a 
  deep 
  but 
  narrow 
  furrow, 
  F, 
  F', 
  and 
  is 
  covered 
  internally 
  

   by 
  a 
  cuticula, 
  which 
  is 
  quite 
  resistent, 
  highly 
  refringent, 
  and 
  very 
  

   slightly 
  tinged 
  with 
  yellow. 
  The 
  epithelium, 
  Fig. 
  41, 
  is, 
  as 
  was 
  to 
  be 
  

   expected 
  from 
  the 
  presence 
  of 
  the 
  two 
  sets 
  of 
  nuclei, 
  composed 
  of 
  two 
  

   kinds 
  of 
  cells 
  ; 
  1st, 
  cylindrical 
  cells 
  corresponding 
  to 
  the 
  oval 
  nuclei 
  j 
  

   in 
  sections 
  these 
  nuclei 
  appear 
  round 
  and 
  are 
  seen 
  to 
  lie 
  in 
  the 
  basal 
  por- 
  

   tion 
  of 
  the 
  cells 
  ; 
  2d, 
  cells 
  corresponding 
  to 
  the 
  superficial 
  nuclei 
  ; 
  each 
  

   of 
  these 
  nuclei 
  is 
  surrounded 
  by 
  a 
  clear 
  space, 
  as 
  indicated 
  in 
  Fig. 
  41, 
  

   but 
  this 
  space 
  has 
  not 
  a 
  sharp 
  outline 
  as 
  there 
  represented 
  ; 
  the 
  shape 
  

   of 
  these 
  cells 
  I 
  have 
  been 
  unable 
  to 
  determine. 
  

  

  The 
  epithelium 
  rests 
  upon 
  a 
  layer 
  of 
  connective 
  tissue, 
  in 
  which 
  there 
  

   are 
  round 
  granular 
  nuclei, 
  as 
  before 
  stated. 
  Outside 
  of 
  the 
  connective 
  

   tissue 
  there 
  is 
  a 
  thin 
  layer 
  of 
  circular 
  muscular 
  fibres, 
  Fig. 
  42, 
  muc. 
  

   The 
  tracheae, 
  with 
  their 
  distinctive 
  nuclei, 
  ramify 
  throughout 
  all 
  parts 
  

   of 
  these 
  two 
  layers. 
  Opposite 
  each 
  furrow 
  there 
  is 
  a 
  longitudinal 
  mus- 
  

   cular 
  band, 
  Fig. 
  42, 
  L 
  D 
  ', 
  composed 
  of 
  some 
  twenty 
  or 
  more 
  striated 
  

   bundles. 
  Attached 
  to 
  the 
  outer 
  walls 
  are 
  found 
  large 
  tracheal 
  trunks, 
  

   Tr., 
  and 
  Malpighian 
  vessels, 
  M. 
  v. 
  

  

  At 
  the 
  points 
  where 
  the 
  epithelium 
  of 
  the 
  folds 
  descends 
  to 
  form 
  the 
  

   intervening 
  furrows, 
  there 
  is 
  a 
  little 
  accumulation 
  of 
  pigment 
  granules. 
  

  

  From 
  the 
  above 
  description 
  it 
  will 
  be 
  seen 
  that 
  the 
  rectal 
  folds 
  do 
  

   not 
  offer 
  the 
  least 
  appearance 
  of 
  glandular 
  structure 
  ; 
  neither 
  is 
  any 
  evi- 
  

   dence 
  deducible 
  from 
  their 
  microscopic 
  anatomy 
  to 
  indicate 
  that 
  their 
  

   function 
  is 
  that 
  of 
  absorption. 
  Neither 
  does 
  it 
  appear 
  to 
  me 
  that 
  Chun, 
  

   in 
  his 
  memoir, 
  has 
  elucidated 
  their 
  function 
  in 
  other 
  insects, 
  and 
  the 
  

   opinions 
  he 
  expresses 
  with 
  apparent 
  confidence 
  I 
  cannot 
  regard 
  as 
  any- 
  

   thing 
  more 
  than 
  speculative. 
  

  

  SUMMARY 
  OF 
  OBSERVATIONS 
  ON 
  THE 
  DIGESTIVE 
  CANAL. 
  

  

  If 
  we 
  now 
  glance 
  back 
  at 
  the 
  descriptions 
  above 
  given 
  of 
  the 
  histo- 
  

   logical 
  peculiarities 
  of 
  the 
  various 
  divisions 
  of 
  the 
  digestive 
  canal, 
  

   there 
  are 
  certain 
  general 
  features 
  which 
  deserve 
  especial 
  attention. 
  In 
  

   the 
  first 
  place 
  it 
  will 
  be 
  recognized 
  that 
  the 
  digestive 
  tract 
  is 
  composed 
  

   of 
  three 
  main 
  divisions 
  : 
  1, 
  the 
  oesophagus, 
  crop, 
  and 
  proventriculus 
  ; 
  

   2, 
  the 
  ventricle 
  and 
  diverticula 
  ; 
  3, 
  the 
  ileum, 
  colon, 
  and 
  rectum. 
  

  

  In 
  the 
  first 
  division 
  there 
  are 
  two 
  coats 
  of 
  muscles, 
  an 
  internal 
  longi- 
  

  

  313 
  Debove 
  : 
  M6moire 
  sur 
  la 
  couche 
  endotheliale 
  sous-6pitheliale 
  des 
  membranes 
  muqueuses. 
  Arch, 
  d© 
  

   Physiol., 
  1874, 
  p. 
  19. 
  

  

  