]\IE.  J.  L.  CLAEKE  ON  THE  INTIMATE  STEUCTUEE  OF  THE  BEAIN. 
239 
Eeceived  October  22,  1857. 
(17.)  In  Mammalia,  the  decussating  fibres  are  much  less  numerous  than  in  Man. 
Generally,  I have  found  their  number  to  be  in  the  direct  ratio  of  the  p-ominence,  rather 
than  the  IreadtJi^  of  the  pyramids.  They  proceed  more  from  the  dee])  strata  of  the 
lateral  columns,  and  less  from  the  superficial  and  anterior ; so  that  the  whole  decussa- 
tion, as  will  be  seen  fni’ther  on,  resembles  rather  the  upper  portion  of  that  in  Man 
(compare  fig.  12  -with  figs.  13,  15,  16  and  17).  In  the  Cat  it  is  stronger  than  in  the 
Dog,  Sheep  or  Ox,  and  a transparent  section  through  it  presents  a very  beautiful 
appearance.  A transverse  section  of  the  medulla  oblongata  of  the  Tiger  bears  in  every 
respect  a most  striking  resemblance  to  that  of  the  Cat:  the  decussating  fibres  are 
numerous,  and  the  anterior  pyramids  are  consequently  prominent  and  deep,  so  that 
relatively  to  the  size  of  the  cerebrum^  they  are  decidedly  larger  than  in  Man.  In  the 
Guinea  Pig  also  the  decussation  is  well  marked,  and  the  fibres  it  derives  from  the 
posterior  columns  and  the  grey  substance  may  be  very  distinctly  seen  (see  fig.  17, 
Plate  XIII.). 
(18.)  Anatomists  are  divided  in  opinion  with  regard  to  the  existence  of  the  anterior 
pyramids  in  Birds,  Eeptiles  and  Fishes.  Like  Willis,  Haller,  Vicq  DAzyr,  Desmoulins, 
Leuret  and  others,  I have  never  seen  on  the  surface  of  the  medulla  in  birds  any  distinct 
pyramidal  columns  resembling  those  of  the  higher  animals ; but  in  a transparent  trans- 
verse section  through  the  point  (i),  fig.  2,  Plate  XII.,  at  the  level  of  the  hypoglossal  nerves, 
a decided  decussation  maybe  obseiwed  (see  fig.  18,  Plate  XIII.):  its  fibres,  however,  are 
not  much  more  numerous  than  those  of  the  anterior  commissure  of  the  spinal  chord,  and, 
like  them,  are  scattered  and  lost  in  the  substance  of  the  anterior  columns.  Although, 
therefore,  it  cannot  be  said  that  distinct  pyramidal  columns  exist  in  birds,  yet  Leuret 
was  mistaken,  when,  in  reference  to  the  fibres  of  their  spinal  chord,  he  observed, 
“ D’abord  ces  fibres  ne  se  croisent  pas  dans  la  moelle  allongee,  ou  Ton  chercherait  vaine- 
ment  une  disposition  analogue  a celle  des  corps  pyramidaux*.” 
(19.)  In  Fishes,  there  appears  to  be  a slight  decussation  of  fibres  proceeding  from  the 
posterior  portions  of  the  medulla,  but  it  is  not  very  cUstinct. 
(20.)  The  posterior  columns  in  Mammalia  and  in  Birds  are  broad,  but  shallower  than 
in  Man  (compare  a,  in  figs.  10  and  11,  with  a in  figs.  13,  14,  15,  16,  17  and  18).  In 
the  Dog,  the  posterior  pyramids  have  a peculiar  shape,  each  arising  from  the  bottom  of 
the  fissure  as  a thin  lamina,  which  widens  towards  the  surface  in  the  form  of  a Avedge 
(see  fig.  16  b). 
(21.)  As  the  fibres  leave  the  lateral  columns  to  decussate  into  the  anterior  pyramids, 
the  posterior  cornua  sink,  as  it  Avere,  forwards,  and  therefore  extend  more  and  more 
towards  the  sides  of  the  medulla,  where  their  terminal  tufts,  gradually  increasing  in 
bulk,  and  reaching  the  surface,  form  the  columns  called  the  grey  tubercles  of  Rolando 
(see  fig.  19).  At  the  same  time,  on  each  side  of,  and  close  to,  the  posterior  median 
* Anatomie  Comparee  du  Systrine  Nerveus,  tom.  1®'',  p.  296. 
