UREA  RELATIVE  TO  ANIMAL  PHYSIOLOGY. 
437 
which  a  certain  portion  experiences  a  further  change — into  car- 
bonate of  ammonia — proportionately  greater  when  the  quantity 
of  urea  is  large  than  when  it  is  small.  The  presence  of  fat  in 
the  food  appears  under  certain  circumstances  to  prevent  or  limit 
this  further  alteration  of  urea.  It  is  owing  to  this  influence, 
that  although  fat,  as  already  remarked,  limits  the  metamorphosis, 
and  consequently  the  formation  of  urea  upon  the  whole,  still 
under  a  diet  consisting  of  flesh  and  fat,  the  quantity  of  urea  ex- 
creted may  become  greater  than  when  the  same  quantity  of  flesh 
is  taken  without  fat,  because  the  nitrogen  of  the  metamorphosed 
tissues  remains  in  the  form  of  urea.  I  am  of  opinion  that  fat 
exerts  this  influence  by  virtue  of  its  connexion  with  the  process 
of  respiration.  Lastly,  water  exercises  an  influence  upon  the 
deficiency  of  nitrogen  appearing  as  urea.    Thus,  for  instance  : 
4.  The  quantities  of  water  and  urea  always  bear  a  very  con- 
stant relation  to  each  other.  No  other  constituent  of  the  urine 
has  so  decided  an  influence  upon  its  density  as  urea.  Dense 
urine  always  contains  much  urea ;  specifically  light  urine  is 
always  poor  in  urea.  Nevertheless,  the  quantity  of  urea  excreted 
upon  the  whole  within  a  given  period,  is  related  in  the  most  inti- 
mate manner  with  the  quantity  of  water,  and  cseteris  'paribus  a 
large  quantity  of  urine  carries  off  more  urea  than  a  small  quan- 
tity passed  in  the  same  time,  although  its  specific  gravity  may 
fall  considerably  at  the  same  time. 
This  influence  of  water  may  be  owing  to  several  circumstances 
— an  increased  facility  in  the  solution  and  extraction  of  urea ; 
perhaps  also  to  an  increased  facility  in  the  formation  of  urea. 
But  it  is  moreover  quite  certain  that  water  has  an  influence  upon 
the  quantity  of  urea  in  so  far  as  the  time  and  rapidity  with  which 
the  urine  is  evacuated  depend  upon  its  greater  or  less  quantity. 
In  the  presence  of  much  water  the  urea  formed  is  very  rapidly 
separated  from  the  blood  and  from  the  organism.  There  is  not 
much  time  then  for  any  further  alteration  of  the  urea,  and  con- 
sequently its  quantity  is  greater  while  the  quantity  of  nitrogen 
not  in  the  form  of  urea  becomes  less.  Hence  it  is  more  particu- 
larly explicable  why  with  different  quantities  of  nitrogenous  food 
(flesh  ;)  with  little  there  is  a  comparatively  and  even  absolutely 
great  deficiency  of  nitrogen  in  the  state  of  urea,  and  with  much 
flesh,  on  the  contrary,  little  deficit.    For  in  the  former  case  the 
