Thus  the  scirrhous  cancer  of  the  breast,  though  limited  for  a  time  to
the  mammary  gland,  at  length  extends  beyond  it,  and  gradually  occu-
pies every  surrounding  part  alike  :  thus  the  epithelial  cancer  extends
from  the  integument  of  the  lip  to  its  muscles,  glands,  and  aU  deeper
tissues,  and  thence  to  the  gum  and  jaw ;  and  thus  the  medullary  cancer
grows  into  and  through  the  walls  of  bloodvessels  and  other  canals,  and
extends,  among  their  contents,  along  their  cavities.  Such  reckless
growth  (if  it  may  be  so  called)  is  scarcely  known  except  in  cancers.
They  supply,  also,  the  iastances  of  most  rapid  increase ;  but  although
they  do  this  frequently  enough  to  make  rapid  growth  one  of  the  diag-
nostic signs  of  cancers,  yet  the  cases  are  far  from  rare  in  which  the
growth  is  very  slow.  Pew  diseases  are  more  variable  than  cancers  are
in  this  respect.     (Compare  pp.  618,  682,  736.)

Two  things  administer  to  the  growth  of  a  cancer  ;  namely  (1)  the
continued  formation  of  the  specific  material  in  the  blood  ;  and  (2)  the
inherent  power  in  the  cancer,  as  a  living  part,  to  assimilate  to  itself
the  common  or  indiiferent  materials  of  the  blood.  The  first  of  these
maintains  and  augments,  as  it  originated,  the  growth ;  the  second
efiPects  an  independent  increase,  like  that  of  a  non-cancerous  tumour.
In  ordinary  cases,  both  these  conditions  are  engaged  in  the  growth
of  cancer  ;  but,  if  the  first  fail,  the  second  may  suffice.  The  canceroua
diathesis  may  cease,  or  be  exhausted  for  a  time,  or  sometimes  even
permanently ;  cancer-material,  we  may  suppose,  is  no  lopger  formed  in
the  blood  ;  yet  the  cancer  may  subsist  and  increase  by  its  own  power.
