﻿12 
  GEOrFllEY 
  SMITH. 
  

  

  which 
  serves 
  to 
  prove, 
  at 
  any 
  rate, 
  that 
  these 
  early 
  stages 
  are 
  

   absolutely 
  necessary 
  for 
  the 
  interpretation 
  of 
  the 
  later. 
  

  

  The 
  following 
  essay 
  will 
  be 
  divided 
  into 
  three 
  parts 
  : 
  — 
  (1) 
  

   anatomical, 
  in 
  which 
  certain 
  new 
  details 
  are 
  described, 
  and 
  

   an 
  adequate 
  account 
  of 
  the 
  disposition 
  of 
  the 
  chorda 
  tympani 
  

   is 
  given 
  for 
  the 
  first 
  time; 
  (2) 
  erabryological; 
  in 
  which 
  special 
  

   attention 
  is 
  paid 
  to 
  the 
  derivation 
  and 
  homology 
  of 
  the 
  stapes 
  

   or 
  proximal 
  portion 
  of 
  the 
  columella 
  (an 
  homology 
  which 
  

   constitutes 
  the 
  crux 
  of 
  the 
  Sauropsidan 
  middle 
  eai-) 
  ; 
  and 
  

   finally 
  (3) 
  a 
  summary 
  with 
  some 
  general 
  conclusions. 
  

  

  I 
  am 
  much 
  indebted 
  to 
  Mr. 
  Jenkinson, 
  Lecturer 
  in 
  Em- 
  

   bryology 
  in 
  the 
  University 
  Museum, 
  for 
  his 
  advice 
  and 
  a 
  

   great 
  deal 
  of 
  material. 
  

  

  ]. 
  Anatomy. 
  

  

  The 
  Columella 
  (Fig. 
  1) 
  — 
  Anatomically 
  the 
  columella 
  of 
  

   birds 
  is 
  composed 
  of 
  two 
  pieces, 
  an 
  inner 
  ossified 
  piece, 
  the 
  

   stapes, 
  apposed 
  to 
  the 
  fenestra 
  ovalis, 
  and 
  an 
  outer 
  cartila- 
  

   ginous 
  piece, 
  the 
  extra-columella, 
  united 
  to 
  the 
  stapes 
  proxi- 
  

   mally, 
  and 
  attached 
  distally 
  to 
  the 
  tympanic 
  membrane. 
  

   There 
  is 
  no 
  real 
  joint 
  between 
  the 
  stapes 
  and 
  extra-columella, 
  

   but 
  great 
  flexibility 
  exists 
  between 
  the 
  two, 
  owing 
  to 
  the 
  

   pliability 
  of 
  the 
  cartilaginous 
  neck 
  which 
  unites 
  them. 
  The 
  

   extra-columella 
  may 
  be 
  described 
  as 
  consisting 
  of 
  three 
  

   pieces, 
  supra-, 
  extra-, 
  and 
  infra-stapedial, 
  all 
  perfectly 
  

   continuous. 
  The 
  disposition 
  of 
  these 
  parts 
  is 
  shown 
  in 
  

   fig. 
  1, 
  which 
  represents 
  the 
  left 
  columella 
  of 
  Gallus, 
  viewed 
  

   from 
  within 
  the 
  tympanum. 
  

  

  The 
  columella 
  is 
  supplied 
  with 
  a 
  single 
  muscle, 
  the 
  tensor 
  

   tympani, 
  which 
  is 
  attached 
  to 
  the 
  iufra-stapedial, 
  and 
  

   to 
  the 
  edge 
  of 
  the 
  tympanic 
  membrane, 
  between 
  the 
  infra- 
  

   and 
  extra-stapedial 
  cartilages. 
  The 
  muscle 
  passes 
  out 
  of 
  the 
  

   ear 
  by 
  a 
  large 
  foramen 
  close 
  to 
  the 
  stylo-mastoid 
  foramen, 
  

   curls 
  round 
  on 
  to 
  the 
  back 
  of 
  the 
  skull, 
  and 
  is 
  broadly 
  

   attached 
  to 
  the 
  basi-occipital 
  bone 
  in 
  a 
  shallow 
  groove 
  which 
  

   slopes 
  nearly 
  to 
  the 
  occipital 
  condyle. 
  

  

  