UREA  RELATIVE  TO  ANIMAL  PHYSIOLOGY. 
435 
Although  at  present  we  can  only  consider  the  first  step  as 
having  been  taken,  I  believe  that  I  have  obtained  some  results 
which,  while  they  remove  previous  doubt  and  present  the  subject 
under  new  aspects,  may  perhaps  serve  as  the  basis  of  further 
research. 
Among  these  results  are  the  following  : 
1.  Urea  is  unquestionably,  under  all  circumstances,  the  mea- 
sure of  the  metamorphosis  of  nitrogenous  constituents  of  the 
organism.  It  never  originates  from  a  direct  metamorphosis  of 
the  albumen  of  the  blood  and  vascular  system.  It  is  formed  in 
the  blood  only  from  gelatin,  and  this  perhaps  never  enters  the 
blood  unaltered  in  the  normal  conditions  of  life.  The  urea  in 
this  case  is  not  a  product  of  the  metamorphosis  of  solid  portions 
of  the  organism. 
2.  But  although  urea  always  originates  in  this  manner  from 
the  metamorphosis  in  the  organs,  still,  the  quantity  and  quality 
of  the  food  exercise  a  far  greater  influence  upon  the  production 
of  urea  and  the  general  metamorphosis  than  could  hitherto  have 
been  supposed.  It  is  indeed  true  that  urea  is  formed  and  ex- 
creted under  a  total  deprivation  of  food ;  but  the  per  centage  of 
nitrogen  in  the  food  exercises  so  great  an  influence  upon  the 
quantity,  that  when,  for  example,  the  dog  on  which  I  made  my 
observations  consumed  in  twenty-four  hours  4000  grms.  of  cow 
flesh  without  fat  or  bone,  he  excreted  in  the  same  time  190  grms. 
of  urea,  while  with  500  grms.  of  potato  and  250  grms  of  fat  the 
quantity  excreted  was  only  6  or  8  grms. 
Food  destitute  of  nitrogen,  such  as  fat,  under  all  circumstances 
produces  a  limitation  of  the  metamorphosis  of  the  nitrogenous 
portions  of  the  organism.  At  the  same  time  there  is  in  most  in- 
stances, caeteris  paribus,  a  diminution  in  the  quantity  of  urea  ex- 
creted, but  not  always.  When  the  food  consists  solely  of  fat 
both  consequences  obtain ;  the  excretion  of  urea  as  well  as  the 
metamorphosis  is  diminished.  The  same  is  the  case  with  a  very 
full  flesh  diet.  With  a  flesh  diet  merely  sufficient  for  maintain- 
ing the  weight  of  the  body  fat  limits  the  metamorphosis,  but  the 
quantity  of  urea  excreted  is  not  necessarily  diminished  at  the 
same  time ;  it  may  indeed  become  greater  than  that  excreted 
when  the  same  quantity  of  flesh  is  consumed  without  fat,  in  ac- 
cordance with  a  law  stated  subsequently. 
