27)  the  essential  contractile  part  of  the  tissue  consists  of  a
complex  body  containing  proteid,  carbohydrate  and  fatty
residues;  and  that  during  muscular  work  this  is  broken  up,
yielding  proteids,  carbon  dioxide,  sarcolactic  acid,  and  prob-
ably other  things.
During  life  and  for  a  certain  time  after  general  death  the
muscles  are  soft,  translucent,  extensible  and  elastic,  and
neutral  or  feebly  alkaline  in  reaction;  after  a  period  which  in
warm-blooded  animals  is  brief  (varying  from  a  few  minutes
to  three  or  four  hours)  they  gradually  become  harder,  more
opaque,  less  extensible  and  less  elastic,  and  distinctly  acid  in
reaction.  The  result  of  these  changes  is  the  well-known
cadaveric  rigidity  or  rigor  mortis.  The  rigid  condition  lasts
for  a  day  or  longer  and  then  it  gradually  and  finally  disappears
and  more  marked  decomposition  changes  commence.  Until
a  short  time  before  the  commencement  of  rigor  the  muscles
remain  contractile  and  can  be  thrown  into  activity  by  various


124  THE  HUMAN  BODY.
excitants,  as  electric  shocks  j  that  is  to  say,  although  the  body-
in  general  is  dead  and  the  beat  of  the  heart  and  the  flow  of
blood  have  ceased,  yet  the  muscles  retain  their  vitality  for  a
while.  This  is  especially  the  case  with  the  muscles  of  cold-
blooded animals,  as  frogs  and  turtles,  the  muscles  of  which
may,  especially  if  kept  cool,  retain  their  living  properties  for
several  hours  after  removal  from  the  body  of  the  animal.
