The  raised  threshold  of  sensory  stimulation  is  no  doubt  an  effect  of  the
low  blood  pressure.  It  has  been  shown,  for  example,  by  E.  L.  Porter36
that  when  the  arterial  blood  pressure  is  maintained  at  a  uniform  level,
the  threshold  stimulus  for  spinal  cord  reflexes  remains  practically  uni-
form, but  becomes  promptly  increased  when  the  arterial  blood  pressure
is  made  to  fall.  Why  a  lower  blood  pressure  should  have  this  effect  is,
however,  difficult  to  understand  in  the  light  of  the  researches  of  Stewart
and  his  coworkers,  who,  as  remarked  above,  found  that  the  cells  of  the
central  nervous  system  may  endure  total  anemia  for  many  minutes  and
still  recover  their  physiologic  condition.  It  may  be,  however,  that  the
low  blood  pressure  affects  the  conductivity  of  the  synapsis.
The  muscular  weakness  is  probably  also  dependent  on  low  blood
pressure,  for  it  has  been  found  in  animals  that,  when  the  arterial  blood
pressure  is  lowered  to  about  90  mm.  Hg,  the  muscles  contract  much  less
efficiently  than  ordinarily.  The  fall  in  body  temperature  is  dependent
upon  the  muscular  inefficiency.
In  conclusion,  it  should  be  pointed  out  that  W.  T.  Porter,  in  the  inves-
tigation of  acute  shock  met  with  at  the  front,  has  found  that,  in  many
cases  at  least,  the  circulatory  disturbance  is  due  to  a  condition  of  fat
embolism.  The  fat  is  derived  from  the  marrow  of  long  bones,  such  as
the  femur,  by  injuries  which  smash  the  bones.  Porter's  observations
are  at  least  very  suggestive.
CIRCULATION  REFERENCES

(Monographs)

Wiggers,  C.  J. :     The  Circulation  in  Health  and  Disease,  Philadelphia,  1915.
Mackenzie,  J. :     Diseases  of  the  Heart,  Oxford  Medical  Publishers,  ed.  2,  1910.
Lewis,   Thomas:  *  Mechanism   of    the   Heart   Beat,    1911,   Shaw   &   Son,    Fetter   Lane,
London.
