120        MR.  R.  OWEN  ON  THE  ANATOMY  OF  THE  CONCAVE  HORNBILL. 
quadrato-maxillary  articulation ;  and  would  at  all  times,  when  not  drawn  over  the 
centre  of  motion  by  the  backward  movement  of  the  lower  jaw,  contribute  materially  to 
support  that  part,  and  relieve  the  temporal  muscle.  Another  strong  ligament  is  destined 
to  prevent  dislocation  backwards  ;  its  origin  from  the  zygoma  is  concealed  by  the  pre- 
ceding ;  it  passes  directly  backwards  to  the  posterior  part  of  the  condyle,  or  articular 
depression,  of  the  lower  jaw. 
On  contemplating  the  maxillary  muscles  in  situ,  the  first  impression  that  arises  is 
surprise  at  the  disproportion  they  seem  to  bear  to  the  vast  apparatus  they  have  to  move  ; 
but  this  disproportion  is  more  apparent  than  real.  The  jaws,  notwithstanding  their  mag- 
nitude, are  rendered  extremely  light  by  the  air-cells  which  are  continued  into  them ; 
their  bony  parietes  do  not  exceed  a  line  in  thickness,  except  at  the  points,  and  the 
delicate  columns  by  which  these  parietes  are  supported  are  themselves  hollow.  The 
disposition  of  these  columns  is  remarkably  beautiful ;  they  are  always  perpendicular, 
or  nearly  so,  to  the  part  of  the  outer  parietes  in  which  they  are  implanted ;  and  at  the 
expanded  base  of  the  mandibles  they  radiate  from  a  central  cylinder,  which  is  formed 
by  a  delicate  osseous  net-work ;  and  thus  while  the  requisite  strength  is  gained,  light- 
ness is  combined  with  magnitude.  With  respect  to  the  attachments  of  the  muscles, 
we  may  conclude  that  any  apparent  disadvantage  in  their  insertion  is  sufficiently  com- 
pensated by  the  superior  energy  of  contraction  with  which  the  muscular  fibres  of  birds 
are  endowed. 
The  bones  and  muscles  of  the  cervical  region  of  the  spine  have,  however,  an  obvious 
adaptation  in  their  development  to  the  bulk  of  the  entire  head. 
The  apparatus  for  flight^  is  more  perfect  than  in  the  Toucan.  The  clavicles,  which 
are  separate  in  that  bird,  are  here  joined,  forming  a  complete  furculum,  which,  how- 
ever, is  slender,  being  about  a  line  in  thickness  at  the  junction.  The  sternum  has  two 
notches  posteriorly,  one  on  either  side  the  keel,  as  in  the  CorvidcB,  but  they  are  shal- 
lower than  even  in  that  tribe :  the  greatest  depth  of  its  keel  is  an  inch ;  the  inferior 
margin  of  this  part  forms  an  almost  straight  line,  and  is  not  expanded  laterally. 
I  have  already  alluded  to  the  extension  of  the  air-cells  among  the  soft  parts ;  and  on 
an  examination  of  the  skeleton  it  appears  that  every  bone,  from  the  mandibles  to  the 
last  of  the  coccygeal  vertehrcB,  from  the  clavicles  and  scapulae  to  the  last  phalanx  of  the 
wings,  from  the  femora  to  the  last  joint  of  the  toes,  and  even  every  rib,  is  permeated 
by  air. 
On  comparing  the  anatomy  of  the  Hornhill  with  that  of  the  Toucan,  we  find  a  close 
resemblance  in  the  structure  of  those  organs  which  relate  to  the  assimilation  of  nutri- 
ment. In  both  birds  a  simple  gullet  corresponds  in  its  diameter  with  the  capacity  of 
the  beak ;  the  proventriculus  and  gizzard  in  both  equally  manifest  their  adaptation  to 
an  omnivorous  diet,  the  latter  being  neither  so  membranous  as  in  the  carnivorous  birds, 
nor  so  muscular  as  in  the  granivorous.    The  short  but  ample  intestines  deprived  of 
'  The  noise  of  this  action  is  heard  from  a  considerable  distance. 
