216 
MR.  R.  OWEN  ON  THE  STRUCTURE  OF  THE  HEART 
the  branchial  divisions  of  the  vena  cava  in  the  Cephalopods,  which  also  in  that  class  are 
contained  within  a  large  pericardium,  and  serve,  besides  other  uses,  as  reservoirs  of 
blood  to  the  branchial  ventricles. 
The  auricles  are  placed  on  the  dorsal  aspect  of  the  ventricle,  and  more  to  the  left 
than  to  the  right  side  :  they  are  not,  however,  so  absolutely  to  the  left  side  as  Meckel 
describes^,  but,  when  fully  distended,  advance  forwards,  as  in  Pipa,  on  both  sides  the 
ventricle  and  hulhus  arteriosus,  so  as  almost  to  encompass  and  conceal  those  parts.  The 
size  of  the  combined  auricles  is  thus  very  considerable  in  proportion  to  that  of  the  ven- 
tricle. In  Amphiuma  and  Menopoma  the  auricles  are  proportionately  smaller,  and  situated 
more  completely  to  the  left  of  the  ventricle.  The  margin  of  the  auricle  in  Amphiuma  is 
but  slightly  notched  as  compared  with  that  of  the  Siren  ;  in  Menopoma  it  is  almost  entire. 
The  ventricle  in  Sir.  lacertina  is  of  an  oblong-oval  and  slightly  flattened  form,  similar 
to  that  of  Ophidia :  a  slight  notch  or  tendency  to  a  division  is  observable  at  the  apex, 
which  lodges  a  branch  of  the  coronary  vein  ^,  which  is  continued  from  this  end  of  the 
ventricle  into  the  inferior  cava.  The  opposite  end  of  the  ventricle  projects  a  little  be- 
yond the  origin  of  the  artery.  The  serous  investment  of  the  ventricle,  besides  being 
continued  along  the  hulhus  arteriosus  to  the  anterior  end  of  the  pericardium,  is  reflected 
also  from  the  lower  third  of  the  dorsal  margin  of  the  ventricle  upon  the  venous  sinus ; 
and  between  these  two  layers  the  coronary  vein  is  continued  to  the  inferior  cava,  as  in 
the  Crocodile  and  some  Chelonia. 
The  parietes  of  the  ventricle  (which  measured  ^Vths  of  an  inch  in  length  and  -Vths 
of  an  inch  in  breadth,)  are  about  -rVth  of  an  inch  in  thickness,  and  of  a  loose  fasciculate 
structure.  The  most  interesting  appearance  within  the  cavity  is  a  rudimentary  septum^ 
extending  from  the  apex  half  way  towards  the  base  of  the  ventricle,  and  terminating  in 
a  concave  edge  directed  towards  the  orifice  of  the  artery.  The  whole  inner  surface  is 
reticulated  by  the  decussating  carnece  columnce. 
The  valvular  structure  at  the  orifices  of  the  auricles  was  not  very  conspicuous  in  the 
specimen  examined.  A  slight  membranous  production  extended  from  either  side  the 
bar  or  septum  which  separated  the  orifices,  and  this  septum  was  attached  by  a  fleshy 
column  to  the  parietes  of  the  ventricle. 
The  artery  comes  off"  about  a  line  above  the  auriculo-ventricular  apertures.  It  makes 
a  half  spiral  curve,  and  then  dilates  into  an  elongated  muscular  bulb,  which  extends 
straight  forwards  to  the  anterior  end  of  the  pericardium,  and  there  emerging,  divides  at 
once  into  the  six  branchial  arteries,  three  on  each  side.  There  are  two  valves,  one  large 
and  one  small,  at  the  origin  of  the  artery ;  but  the  latter  is  a  mere  ridge.  At  the  com- 
mencement of  the  bulb  there  are  two  similar  but  smaller  valves.  The  bulb  itself  is 
almost  wholly  occupied  by  a  cylindrical  fleshy  valvular  body,  attached  posteriorly,  and 
marked  anteriorly  with  grooves  which  lead  to  the  several  arteries  given  off  above  :  the 
form  of  the  canal,  as  seen  on  a  transverse  section,  is,  at  this  part,  crescentic. 
'  Loc.  cit.,  p.  216.  a  n.  Fig.  2.  ^  /'.  Fig.  3. 
