Interstitial  emphysema  is  a  condition  analogous  to  emphysema
of  subcutaneous  tissues ;  there  is  an  accumulation  of  air  in
the  connective-tissue  septa  between  the  lobules  of  the  lung  as
the  result  of  rupture  of  an  air-vesicle.  It  occurs  most  fre-
quently in  the  course  of  whooping-cough  or  broncho-pneu-
monia in  young  children  ;  and  from  excessive  straining-efforts
in  parturition,  defecation,  coughing,  etc.
By  vesicular  emphysema  we  understand  a  dilatation  of  the
air-vesicles  which  occurs  as  the  result  of  a  weakened  condi-
tion of  the  lung-tissue  from  inflammatory  or  degenerative
processes,  lessening  its  ability  to  resist  increased  pressure
within,  from  such  causes  as  chronic  cough,  blowing  of  wind-
instruments,  etc.  The  lungs  are  considerably  larger  than
normal  j  there  is  a  gritty  feeling  on  section,  and  the  dilatation


PASSIVE  HYPER^EMIA.  337

of  the  air-spaces  may  be  so  great  as  to  be  apparent  to  the
naked  eye.
Microscopically  the  air-vesicles  are  found  to  be  rounded  in-
stead  of  polygonal,   and   are   increased   in   diameter.     The

FIG.  147.
Section  from  an  emphysematous  lung  (Ribbert).  The  pulmonary  alveoli  are  en-
larged; their  walls  are  stretched  and  thinned;  atrophied  because  of  repeated
excessive  air-pressure  within  the  alveoli.  In  more  extreme  cases  of  emphysema
the  atrophy  of  the  alveolar  walls  may  lead  to  their  total  destruction  in  places,
so  that  the  cavities  of  neighboring  alveoli  communicate.  (Compare  with  Fig.
149.)

alveolar  walls  are  thinned  (see  Fig.  147)  ;  and  the  bloodvessels,
being  stretched  over  a  larger  surface,  are  narrowed,  the
anaemia  of  the  lung-tissue  thus  produced  further  favoring  the
atrophy  and  degenerative  changes  in  the  alveolar  septa.  As
a  remote  consequence  of  the  increased  pulmonary  blood -pressure
there  is  hypertrophy  of  the  right  heart.
PASSIVE  HYPER^SMIA.
