The  chief  center  on  either  side  of  the  midline  of  the  medulla  is  con-
nected with  the  motor  neurons  of  both  sides  of  the  spinal  cord,  as  is
proved  by  the  follo\ving  experiment.  When  the  central  end  of  the  vagus
nerve  is  stimulated,  the  respiratory  center  becomes  excited  and  the  respi-
rations more  pronounced,  the  participation  of  the  muscles  on  both  sides
of  the  body  being  equal  in  extent.  If  IIOAV  we  bisect  the  medulla  down  the


Fig.    118. — Diagram   to   show  cuts   required  for   isolation   of   the   phrenic   center.
midline  and  repeat  the  stimulation  of  one  vagus,  the  muscles  on  both  sides
will  still  participate  in  the  increased  respiration,  which  they  will  likewise  do
if  the  cervical  cord  is  bisected  or  hemisected  but  the  medulla  left  intact
(Fig.  119).  The  simplest  interpretation  of  these  results  is  that  commis-
sural  fibers  connect  both  halves  of  the  respiratory  center  in  the  medulla
and  that  each  half  is  also  connected  with  the  motor  neurons  of  both  sides
of  the  spinal  cord.  Often,  especially  in  young  animals,  a  hemisection  of
the  cord  causes  cessation  of  the  movements  of  the  diaphragm  on  the  same
side;  but  this  paralyzed  side  at  once  begins  to  contract  again  when  the
phrenic  of  the  opposite  side  is  cut,  probably  because  the  respiratory
impulse  descending  from  the  chief  center,  on  finding  its  way  along  the
motor  center  of  the  same  side  of  the  cord  blocked,  is  forced  to  follow  the
crossed  path.  The  crossing  in  the  cord  is  believed  to  take  place  at  the
same  level  as  that  at  which  the  subsidiary  center  is  located  (W.  T.
Porter12).
