called  antiperistaltic  waves.  The  effect  of  the  movements  is  to  knead
and  mix  the  intestinal  contents,  and  thus  encourage  the  absorption  of
water  from  them.  The  resulting  more  solid  portions  then  collect  toward
the  splenic  flexure,  and  become  separated  from  the  remaining  more  fluid
portion  by  transverse  waves  of  constriction,  which  develop  into  peri-
staltic waves  carrying  the  harder  masses  into  the  distal  portions  of  the
colon,  where  they  collect  chiefly  in  the  sigmoid  flexure.  The  descending
colon  itself  is  never  distended  with  contents  and  merely  serves  as  a  tube
for  transferring  the  masses  from  the  transverse  colon  to  the  sigmoid
flexure.  The  time  taken  for  a  capsule  of  bismuth  to  reach  the  various
parts  of  the  large  intestine  is  shown  in  Pig.  165.
After  a  certain  mass  has  collected  in  the  sigmoid  flexure  and  rectum,
the  increasing  distention  causes  a  reflex  evacuation  of  this  portion  of  the


470  DIGESTION     •

gut  through  centers  located  in  the  spinal  cord.  The  impulses  from  these
centers,  besides  contracting  the  rectum,  etc.,  also  coordinate  the  contrac-
tion of  the  abdominal  muscles  and  the  relaxation  of  the  sphincter  ani
so  as  to  bring  about  the  act  of  defecation.  By  the  skiagraphic  method  it
has  been  found  that  the  pelvic  colon  gradually  becomes  filled  with  feces
from  below  upward,  and  that  the  rectum  remains  empty  until  just  before
defecation.
