IN  THE  PERENNIBRANCHIATE  BATRACHIA. 
219 
the  higher  Reptilia  the  retarding  channels  are  reduced  to  two,  while  in  the  more  ener- 
getic warm-blooded  classes  the  blood  is  distributed  over  all  the  frame  by  branches  of 
a  single  continuous  vessel.  A  corresponding  gradation  may  be  traced  in  the  place  of 
origin  of  the  pulmonary  arteries,  and  the  consequent  impulse  received  by  them  from 
the  contractions  of  the  heart.  In  the  Siren  the  pulmonary  arteries  come  off  at  the  end 
of  the  branchial  circulation.  In  Menopoma  they  come  off  from  a  corresponding  situation, 
but  receive  more  of  the  heart's  impulse,  from  its  not  being  previously  expended  on  sub- 
divisions of  the  branchial  vessels.  In  Amphiuma  the  pulmonary  and  branchial  arteries 
rise  together  from  the  end  of  the  aortic  bulb.  In  the  higher  orders  of  Rej)tiles  the  pul- 
monary arteries  proceed  by  a  common  trunk  from  the  ventricle  itself :  and  lastly,  in  the 
warm-blooded  classes  they  have  a  ventricle  expressly  appropriated  to  accelerate  the 
circulation  through  them. 
In  the  Vertebrate,  as  in  the  Molluscous  division  of  the  Animal  Kingdom,  the  Muscular 
and  Perceptive  energies  rise  in  proportion  to  the  perfection  of  the  Respiratory  and  San- 
guiferous systems. 
PLATE  XXXI. 
Fig.  1 .  The  heart  in  situ  of  Siren  lacertina. 
a.  The  termination  of  the  inferior  vena  cava, 
h,  b.  The  two  superior  venae  cava. 
c,  c.  The  venous  sinus. 
d,  d.  The  right  or  systemic  auricle. 
e,  e.  The  left  or  pulmonic  auricle. 
/.  The  ventricle. 
g.  The  elongated  bulbus  arteriosus. 
h.  The  branchial  arteries.    These  are  divided  on  the  right  side,  and  the 
branchial  arches  turned  outwards,  to  show 
i.  The  descending  aorta,  formed  by  the  union  of  the  trunks  of  the  bran- 
chial veins. 
k,  k.  The  pulmonary  arteries. 
I,  I.  The  commencement  of  the  lungs,  laid  open  to  show  their  cellular  struc- 
ture, and 
m,  m.  The  pulmonary  veins. 
n.  The  (esophagus. 
0,0.  The  external  branchice. 
Fig.  2.  The  heart       pericardium  of  the  Siren,  showing  the  structure  of  the  auricles. 
The  same  letters  denote  the  same  parts  as  in  the  preceding  figure.  The 
whole  extent  of  the  pericardium  is  here  shown.  The  bristles  marked  e"  e" 
are  passed  through  the  pulmonary  veins  along  the  common  trunk  m'  into 
VOL.  I.  2  G 
