IN  THE  PERENNIBRANCHIATE  BATRACHIA. 
.  217 
In  Amphiuma  and  Menopoma  the  contracted  membranous  part  of  the  aorta  intervening 
between  the  ventricle  and  bulb,  is  proportionately  longer  than  in  the  Siren,  but  has  the 
same  spiral  twist.  The  bulb  itself,  on  the  other  hand,  is  much  shorter,  and  broader : 
the  ventricle  in  both  these  genera  is  also  shorter  in  proportion  to  its  breadth,  and  in 
Menopoma  approximates  to  the  triangular  form  which  characterizes  the  ventricle  iu 
Osseous  Fishes.  This  genus  also  presents  an  affinity  to  Cartilaginous  Fishes,  in  having 
two  rows  of  semilunar  valves  in  the  bulbus  arteriosus,  three  in  each  row  ;  but  neither 
in  Menopoma  nor  Amphiuma  is  there  any  fleshy  cylindrical  body  in  the  bulb, — this  ad- 
ditional valve  being  unnecessary  from  the  free  passage  which  the  undivided  branchial 
arteries  afford  to  the  blood  in  these  genera.  In  Amphiuma  the  pulmonary  arteries  are 
given  off"  from  the  end  of  the  bulb  of  the  branchial  aorta  :  in  Menopoma  they  are  formed 
by  the  union  of  two  twigs  given  off  respectively  by  the  first  and  second  branchial  arte- 
ries near  their  origin.  In  the  Siren  the  pulmonary  arteries  are  branches  of  the  lower 
branchial  vein.  It  is  worthy  of  observation,  that  in  both  Amphiuma  and  Menopoma 
the  pulmonary  artery  supplies  other  parts  besides  the  lungs,  branches  being  sent  off 
from  it  to  the  cesophagus :  but  I  have  not  been  able  to  trace  any  ramifications  to  the 
skin,  as  has  been  observed  by  Dr.  Davy  to  be  the  case  in  the  Toad. 
The  presence  of  two  auricles  in  the  heart  of  the  Eepiiles  douteux  now  renders  appli- 
cable to  the  whole  class  of  Reptiles  the  phrase  "  Cor  uniloculare  biauritum  " ;  and  forms 
an  additional  argument  for  retaining  as  an  Order  of  that  Class  the  Amphibia  of  Latreille. 
But  besides  the  zoological  application  of  the  preceding  anatomical  facts,  they  are  in- 
teresting also  in  a  physiological  point  of  view. 
From  the  impediments  which  frequently  occur  to  a  free  and  regular  circulation  of 
blood  in  these  cold-blooded  and  slow-breathing  animals,  the  venous  side  of  the  heart  is 
subject  to  great  distension ;  hence  the  large  size  of  the  auricles,  and  of  the  sinus  which 
receives  the  systemic  veins,  and  the  perfect  development  of  the  intervening  pair  of 
valves,  of  which  the  Eustachian  valve  in  the  Mammiferous  heart  still  presents  a  rudi- 
ment. Had  the  pulmonary  veins  terminated  along  with  the  systemic  in  the  same 
cavity,  their  orifices  would  have  been  subjected  to  the  pressure  of  the  accumulated 
contents  of  that  cavity,  and  there  would  have  been  a  disproportionate  obstacle  to  the 
passage  of  the  aerated  blood  into  the  ventricle.  This  is  obviated  by  providing  the  pul- 
monary veins  with  a  distinct  receptacle,  which  is  equally  ready  with  the  right  auricle 
to  render  its  contents  into  the  ventricle  during  the  diastole  of  that  cavity. 
In  considering  the  heart  with  reference  to  the  breathing  organ  in  other  classes,  we 
find  that  at  its  first  appearance  as  a  distinct  mechanical  and  muscular  organ  its  energies 
are  expended  on  the  systemic  circulation,  and  that  the  respiratory  apparatus  is  placed 
at  the  termination  of  the  circle  ;  the  venous  blood,  prior  to  re-entering  the  heart,  being 
either  diffused  in  extensive  and  irregular  sinuses,  over  v^\\o?,e  parietes  air  is  distributed 
by  minutely  ramified  trachea,  as  in  Insects  ;  or  passing  from  venous  trunks  to  branches 
indefinitely  ramified  upon  a  more  concentrated  respiratory  organ,  as  in  Crustaceans  and 
