﻿430 
  Prof. 
  M'Intosh 
  on 
  the 
  

  

  the 
  median 
  septum. 
  The 
  histological 
  characters 
  of 
  the 
  lower 
  

   half 
  just 
  described 
  differ 
  from 
  the 
  superior, 
  in 
  so 
  far 
  as 
  the 
  

   whole 
  section 
  is 
  more 
  granular 
  and 
  the 
  striae 
  less 
  distinct 
  ; 
  

   indeed, 
  in 
  most 
  views 
  it 
  presents 
  a 
  glandular 
  appearance 
  

   with 
  numerous 
  concentrically 
  arranged 
  striae. 
  Soft 
  cellular 
  

   substance 
  generally 
  fills 
  up 
  the 
  space 
  in 
  the 
  centre. 
  When 
  

   the 
  crown-like 
  appearance 
  in 
  section 
  has 
  attained 
  full 
  

   development, 
  the 
  muscle 
  in 
  each 
  central 
  space 
  is 
  most 
  

   bulky 
  internally, 
  and 
  the 
  fasciculi 
  from 
  the 
  median 
  septum 
  

   extend 
  further 
  downward, 
  the 
  arrangement 
  having 
  a 
  some- 
  

   what 
  pennate 
  appearance. 
  Before 
  the 
  separation 
  of 
  these 
  

   chambers 
  occurs 
  (i. 
  e. 
  when 
  they 
  have 
  assumed 
  the 
  form 
  of 
  a 
  

   figure 
  of 
  eight) 
  the 
  muscle 
  occupies 
  about 
  three 
  fourths 
  of 
  

   the 
  diminished 
  area, 
  passing 
  from 
  the 
  upper 
  half 
  of 
  the 
  short 
  

   median 
  septum, 
  and 
  extending 
  past 
  the 
  junction 
  of 
  the 
  

   external 
  chamber 
  superiorly. 
  The 
  direction 
  of 
  the 
  muscular 
  

   fasciculi 
  is 
  also 
  more 
  horizontal 
  — 
  except 
  at 
  the 
  outer 
  or 
  

   superior 
  border, 
  — 
  and 
  the 
  cellular 
  or 
  glandular 
  tissue 
  is 
  much 
  

   less. 
  The 
  latter 
  becomes 
  still 
  more 
  diminished, 
  and 
  the 
  

   fasciculi 
  in 
  the 
  nearly 
  circular 
  area 
  are 
  directed 
  downward 
  

   and 
  inward, 
  the 
  most 
  conspicuous 
  occupying 
  the 
  middle. 
  

   Finally 
  the 
  muscle 
  ceases 
  in 
  ordinary 
  sections 
  in 
  a 
  line 
  with 
  

   the 
  tentacles. 
  The 
  median 
  muscles 
  are 
  thus 
  confined 
  

   entirely 
  to 
  the 
  snout, 
  and, 
  from 
  their 
  relations 
  with 
  the 
  

   ehitinous 
  environment, 
  act 
  in 
  various 
  ways 
  on 
  the 
  flattened 
  

   organ. 
  The 
  presence 
  of 
  the 
  cellular 
  or 
  glandular 
  tissue 
  

   within 
  the 
  same 
  sheath 
  and 
  its 
  changes 
  in 
  those 
  bearing 
  the 
  

   lateral 
  organs 
  are 
  also 
  noteworthy. 
  The 
  whole 
  tissues 
  of 
  the 
  

   snout, 
  indeed, 
  sympathize 
  with 
  such 
  changes, 
  for 
  the 
  muscles 
  

   become 
  less 
  firm 
  and 
  more 
  granular, 
  the 
  ehitinous 
  supports 
  

   less 
  distinct, 
  and 
  the 
  blood-vessels, 
  as 
  will 
  be 
  seen 
  afterwards, 
  

   shrink 
  to 
  short 
  trunks 
  about 
  a 
  third 
  their 
  ordinary 
  length. 
  

  

  The 
  lateral 
  muscles 
  of 
  the 
  snout 
  take 
  origin 
  anteriorly 
  a 
  

   little 
  behind 
  the 
  former, 
  gradually 
  widen 
  into 
  broad 
  ribbon- 
  

   shaped 
  bands, 
  then 
  become 
  narrower 
  toward 
  the 
  mouth, 
  and 
  

   are 
  continued 
  backward 
  as 
  the 
  ventral 
  longitudinal 
  muscles 
  

   of 
  the 
  body-wall. 
  Their 
  fibres 
  are 
  chiefly 
  longitudinal 
  in 
  

   direction, 
  and 
  in 
  partial 
  contraction 
  the 
  muscles 
  in 
  the 
  snout 
  

   assume 
  a 
  barred 
  appearance 
  at 
  the 
  outer 
  margins 
  — 
  the 
  bars 
  

   or 
  ridges 
  slanting 
  from 
  without 
  inward 
  and 
  slightly 
  back- 
  

   ward. 
  Careful 
  examination 
  in 
  the 
  fresh 
  specimen 
  also 
  

   reveals 
  a 
  series 
  of 
  very 
  fine 
  transverse 
  fibres, 
  which, 
  from 
  

   the 
  triangular 
  region 
  in 
  front 
  of 
  the 
  mouth 
  to 
  the 
  anterior 
  

   third, 
  slant 
  forward 
  and 
  outward. 
  In 
  general 
  arrangement 
  

   these 
  muscles 
  present 
  in 
  the 
  snout 
  certain 
  well-marked 
  

   differences 
  from 
  their 
  subsequent 
  structure. 
  In 
  front 
  each 
  

  

  