CHAPTER  XXVI
THE  CONTROL  OF  THE  CIRCULATION  (Cont'd)

THE  NERVE  CONTROL  OF  THE  PERIPHERAL  RESISTANCE

As  already  explained,  the  nerve  control  of  the  peripheral  resistance
takes  place  through  the  action  of  vasoconstrictor  and  vasodilator  nerve
fibers  on  the  musculature  of  the  arteriole  walls.  The  vasoconstrictor
impulses  like  those  in  the  vagus  of  the  heart  are  tonic,  so  that  when  a
nerve  containing  such  fibers  is  cut,  the  corresponding  blood  vessels  un-
dergo dilatation  (see  page  135),  and  when  their  peripheral  ends  are  stim-
ulated artificially,  constriction  occurs.  On  the  other  hand,  the  vasodi-
lator impulses  do.  not  appear,  at  least  under  ordinary  circumstances,  to
be  tonic,  so  that  the  cutting  of  such  fibers  does  not  cause  vasoconstriction ;
their  stimulation,  however,  causes  marked  dilatation.  Vasomotor  fibers
are  contained  in  most  of  the  efferent  (motor)  nerve  trunks,  and  to
detect  their  presence  the  nerve  must  be  either  cut  or  stimulated  and  the
condition  of  the  blood  vessels  of  the  innervated  area  observed.
Methods  for  the  Detection  of  Constriction  or  Dilatation

Several  methods,  varying  with  the  exact  area  under  observation,  can
be  used  for  the  detection  of  vasoconstriction  or  dilatation.  In  many  cases
visual  inspection  is  sufficient,  as  in  the  well-known  experiment  of  Claude
Bernard  on  the  blood  vessels.in  the  ear  of  the  rabbit  (see  Fig.  106).  When
this  is  held  with  a  light  behind  it,  and  the  cervical  sympathetic  of  the
corresponding  side  is  cut,  marked  dilatation  will  become  evident  and
vessels  will  spring  into  view  where  previously  there  were  none  visible.
Visual  inspection  is  usually  also  a  satisfactory  method  of  demonstrat-
ing vasodilatation  or  constriction  in  exposed  glands,  in  mucous  pas-
sages and  in  the  vessels  of  the  skin.
