116 
MR.  T.  BELL  ON  THE  NECK  OF  THE  THREE-TOED  SLOTH. 
that  these  vertebra  are  dorsal  instead  of  cervical,  does  not  in  the  least  affect  the  question 
of  their  office.  The  object  of  the  increased  number  of  vertebra  in  the  neck  is  evidently 
to  allow  of  a  more  extensive  rotation  of  the  head ;  for  as  each  of  the  bones  turns  to  a 
small  extent  upon  the  succeeding  one,  it  is  clear  that  the  degree  of  rotation  of  the  ex- 
treme point  will  be  in  proportion  to  the  number  of  moveable  pieces  in  the  whole  series. 
When  the  habits  of  this  extraordinary  animal  are  considered,  hanging,  as  it  does,  sus- 
pended from  the  under  surface  of  boughs  with  the  back  downwards,  it  is  obvious  that 
the  only  means  by  which  it  could  look  down  towards  the  ground  must  be  by  rotation 
of  the  neck ;  and  as  it  was  necessary,  in  order  to  effect  this  without  diminishing  the 
firmness  of  the  cervical  portion  of  the  vertebral  column,  to  add  certain  moveable  points 
to  the  number  possessed  by  the  rest  of  the  class,  the  necessary  additional  motion  w^as 
acquired  by  modifying  the  two  superior  dorsal  vertebra,  and  giving  them  the  office  of 
cervical,  rather  than  by  infringing  a  rule  which  is  thus  preserved  entire,  without  a 
single  known  exception. 
PLATE  XVII. 
Fig.  1 .  The  two  last  cervical  and  four  first  dorsal  vertebra  of  Bradypus  tridactylus. 
a.  the  first  rudimentary  rib  ;  b.  the  second  rudimentary  rib. 
Figg,  2  and  3.  The  two  rudimentary  ribs  enlarged  to  three  times  their  natural  mag- 
nitude. 
