436  UREA  RELATIVE  TO  ANIMAL  PHYSIOLOGY. 
3.  It  has  moreover  been  found,  that  the  quantity  of  nitrogen 
in  the  food  or  portions  of  the  organism  metamorphosed  within  a 
certain  period  never  appears  entirely  as  urea,  but  that  a  certain, 
and  under  some  circumstances  considerable  part,  must  be  ex- 
creted in  another  form.  This  is  likewise  true  in  the  case  of  dogs, 
although  their  urine  does  not  contain  uric  acid,  and  scarcely  a 
trace  of  other  nitrogenous  organic  substances.  Only  very  small 
quantities  of  nitrogen  are  excreted  in  the  faeces,  and  as  this  is 
also  true  with  regard  to  the  lungs  and  skin,  according  to  admi- 
rable researches  of  Regnault  and  Reiset,  it  is  difficult  to  form  a 
correct  opinion  as  to  the  form  in  which  that  part  of  the  nitrogen 
of  metamorphosed  portions  of  the  organism  that  is  not  found  in 
the  urine  is  excreted.  It  is  most  probable  that  this  deficiency 
is  owing  to  a  partial  conversion  of  urea  in  the  blood,  or  perhaps 
even  in  the  bladder,  into  carbonate  of  ammonia,  which  is  excreted 
either  by  the  skin  and  lungs  or  in  the  urine.  However  worthy 
of  confidence  the  observations  of  Regnault  and  Reiset  may  be, 
I  am  still  of  opinion,  that  it  has  not  hitherto  been  possible  to 
continue  them  for  a  sufficiently  long  period,  and  under  the  ne- 
cessary alterations  of  diet  for  determining  with  absolute  certainty, 
whether  or  not  carbonate  of  ammonia  is  excreted  by  the  skin 
and  lungs.  The  presence  of  carbonate  of  ammonia  in  the  urine 
would  be  very  probable,  at  least  when,  even  with  an  exclusively 
flesh  diet,  or  under  a  deprivation  of  food,  it  was  alkaline  while 
quite  fresh  and  effervesced  on  the  addition  of  an  acid. 
The  quantity  of  nitrogen  of  the  metamorphosed  portions  of 
the  organism,  which  does  not  make  its  appearance  as  urea,  is 
upon  the  whole  tolerably  constant  under  very  diverse  circum- 
stances of  diet  and  metamorphosis.  It  was  found  greatest,  both 
relatively  and  absolutely,  under  a  deficient  supply  of  nitrogenous 
lood  (250  grms.  of  flesh.)  It  might  in  this  case  amount  to  more 
than  two-thirds  of  the  total  nitrogen  of  the  metamorphosed  tis- 
sues. With  a  supply  of  nitrogenous  food  adequate  for  maintain- 
ing the  weight  of  the  body  (500  grms.  of  flesh)  it  amounted  to 
one-third.  Under  a  very  full  and  excessive  flesh  diet  it  was 
smaller  absolutely  than  in  the  above  cases,  and  was  consequently 
so  much  reduced,  relatively,  as  to  be  almost  insignificant.  I  re- 
gard these  facts  as  the  strongest  evidence  that  the  original  pro- 
duct of  metamorphosis  of  nitrogenous  tissues  is  solely  urea,  of 
