The  Chemical  Processes  of  Secretion.  As  above  pointed
out  certain  secretions,  called  transudata,  seem  to  be  products
of  filtration  and  dialysis  alone,  containing  only  such  sub-
stances as  those  which  are  found  in  the  blood-plasma,  more
or  less  altered  in  relative  quantity  by  the  ease  or  difficulty
with  which  they  severally  passed  through  the  layers  met
with  on  their  way  to  the  surface.  But  in  many  cases  the
composition  of  a  secretion  cannot  be  accounted  for  in  this
way  ;  it  contains  some  specific  element,  either  a  substance
which  does  not  exist  in  the  blood  at  all  and  must  therefore
have  been  added  by  the  secreting  membrane,  or  some  body
which,  although  existing  in  the  blood,  does  so  in  such  minute
proportion,  compared  with  that  in  which  it  is  found  in  the
secretion,  that  some  special  activity  of  the  secreting  cells  is
indicated :  some  affinity  in  them  for  these  bodies  by  which
they  actively  pick  them  up.
288  THE  HUMAN  BODY.
