434 
UREA  RELATIVE  TO  ANIMAL  PHYSIOLOGY. 
For  on  the  one  hand  the  chemical  methods  adopted  for  the  quan- 
titative estimation  of  urea  were  either  liable  to  inaccuracy,  or 
involved  troublesome  and  tedious  operations,  which  were  applica- 
ble only  in  a  few  particular  instances.  On  the  other  hand,  it 
was  certain  that  the  constitution  of  urine  and  the  quantitative 
relation  of  its  several  constituents  are  variably  influenced  by  so 
many  circumstances,  that  a  correct  insight  into  its  qualitative 
importance  and  quantitative  excretion  could  only  be  attained 
from  a  very  great  number  of  observations,  and  when  the  condi- 
tions under  which  they  were  made  were  exceedingly  varied,  and 
at  the  same  time  well  known  and  definite. 
If,  therefore,  the  relation  of  urea  to  the  general  functions  of 
life  are  to  be  more  exactly  investigated,  and  if  the  quantity  in 
which  it  is  excreted  is  to  be  recognized  as  the  measure  of  meta- 
morphosis of  nitrogenous  constituents  of  the  organism,  a  method 
must  be  found  for  its  quantitative  estimation  which  will  be  at 
once  certain,  facile,  and  rapid  in  its  execution. 
Such  a  method  has  been  contrived  by  Professor  Liebig,  which, 
with  a  little  practice,  admits  of  an  estimation  of  urea  being  made 
in  a  quarter  of  an  hour.  I  have  in  this  manner  instituted  a  large 
number  of  experiments  with  human  urine  and  that  of  dogs  and 
rabbits.  The  quantity  of  urea  that  the  dog  formed  under  the 
most  diverse  conditions  of  feeding  was  daily  estimated  during  a 
whole  year,  The  same  was  done  for  a  period  of  five  months  with 
a  rabbit. 
For  the  human  organism  I  have  only  endeavored  to  ascertain 
the  quantitative  relations  of  urea  under  the  normal  circumstances 
of  life  during  long  periods  and  for  individuals  of  different  sex 
and  age.  The  results  which  have  thus  been  obtained  present 
very  considerable  discrepancies  with  the  statements  previously 
made. 
In  the  case  of  the  animals  mentioned,  however,  I  have  more 
especially  convinced  myself  that  the  determining  conditions  for 
the  formation  and  excretion  of  urea  are  far  more  variable  and 
multiform  than  has  hitherto  been  supposed,  and  that  they  are 
influenced  by  circumstances  so  numerous  and  changeable  that 
there  is  still  a  necessity  for  a  much  larger  accumulation  of  accu- 
rate observations  before  the  laws  of  this  excretion  and  its  corre- 
lative phenomena  can  be  definitely  evolved. 
