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XII.  Observations  on  the  Neck  of  the  Three-toed  Sloth,  Bradypus  tridactylus,  Linn.  By 
Thomas  Bell,  Esq.,  F.R.S.,  L.S.,  G.S.,  Z.S. 
Communicated  August  13,  1833. 
The  laws  which  regulate  the  numerical  variations  in  the  different  systems  of  organs 
in  animals,  are  perhaps  less  defined,  or  at  least  less  understood,  than  those  which  relate 
to  many  other  conditions  of  their  existence.  In  some  cases,  indeed,  these  variations 
appear  to  be  wholly  anomalous  ;  but  in  others  the  normal  number  of  parts  is  so  strictly 
adhered  to,  as  to  be  absolutely  without  any  known  exception  in  a  whole  group. 
Amongst  these,  one  of  the  most  obvious  and  remarkable  is  the  restriction  of  the 
cervical  vertebrce  in  the  whole  of  the  class  of  mammiferous  animals,  to  the  number  seven. 
That  this  number  should  be  found  equally  in  the  short  interval  between  the  cranium 
and  the  thorax,  scarcely  deserving  the  name  of  a  neck,  which  we  see  in  the  Cetacea, 
and  in  the  long  flexile  neck  of  the  Camel  and  the  Giraffe,  is  indeed  a  striking  and  in- 
teresting fact,  and  may  be  viewed  as  an  important  illustration  of  that  law  which  pro- 
vides for  the  most  considerable  variations  in  the  offices  or  functions  of  a  part,  rather  by 
a  modification,  in  form  or  size,  or  even  situation,  of  organs  already  existing  and  es- 
sential to  the  type  of  the  group,  than  by  the  production  of  new  organs  on  the  one  hand, 
or,  on  the  other,  by  the  abstraction  of  any  which  appertain  to  the  normal  form. 
To  this  normal  number,  however,  the  At,  Bradypus  tridactylus,  Linn.,  has  for  many 
years  been  considered  as  an  exception  ;  as  by  the  examination  of  numerous  specimens, 
the  neck  was  found  to  possess  nine  vertebrce,  which  were  all  believed  to  belong  to  the 
cervical  class. 
An  isolated  exception  to  a  rule  so  general,  and  obtaining  in  cases  of  such  diversified 
forms  as  those  to  which  I  have  alluded,  presents  itself  to  the  mind  of  every  one  accus- 
tomed to  look  at  the  general  harmony  of  the  established  laws  of  formation,  as  a  vio- 
lation of  that  unity  of  design  which  constitutes  one  of  the  most  interesting  objects  of 
our  investigation,  especially  as  the  exception  itself  is  abrupt  and  sudden,  and  without 
any  of  those  intermediate  gradations  of  structure  by  which  the  mind  is  prepared,  as  it 
were,  for  considerable  diversities  of  form,  and  which  so  generally  soften  the  transitions 
which  the  different  offices  of  the  same  organ  in  different  groups  may  render  necessary. 
It  was  from  this  consideration,  rather  than  as  merely  correcting  a  generally  received 
error,  that  I  found,  with  feelings  of  no  ordinary  satisfaction,  that  in  truth  this  nume- 
rical law  is  not  departed  from  in  the  present  instance,  and  that  the  animal  in  question 
forms  no  such  exception  to  the  general  rule  as  had  been  asserted ;  the  two  vertebrcs 
which  have  hitherto  been  considered  as  the  eighth  and  ninth  cervical,  being  in  fact  the 
