As  it  is  not,  then,  the  air  that  sounds  on  percussion,  it  must  be
the  solids  ;  and  observe  how  the  solid  construction  of  the  chest
favours  its  vibrations.  Here  are  a  set  of  membranes,  thin  muscles,
and  integuments,  strained  on  an  elastic  framework  of  bone  and  car-
tilage, and  free  to  vibrate  so  long  as  the  organs  within  do  not  check
their  motions.  Let  us  find  something  familiar  to  illustrate  this  ;
my  hat  will  do  very  well.  The  crown  of  this  hat  is  free  to  vibrate,
and  gives  a  good  hollow  sound  when  struck.  I  put  into  it  this
handkerchief,  which  when  loose,  may  serve  to  represent  the  air-
filled  tissue  of  the  lung.  It  still  sounds  well,  because  the  handker-
chief does  not  check  the  vibrations.  But  when  I  bring  my  hand
in  contact  with  the  crown  inside,  and  then  strike  on  the  outside,
you  perceive  the  sound  is  rendered  short  and  dull,  because  the  vibra-
tions are  stopped  by  the  solid  within.  Suppose  I  were  to  fill  my
hat  with  water  (for  I  am  not  going  to  do  it  ;  I  wont  try  its  water-
proof so  far)  ;  this,  too,  would  check  the  vibrations,  and  you  would
have  no  sound  on  percussion  but  a  short  dead  tap.
