OXYGEN-SUPPLY

426.  Respiration.  —  This  has  already  been  defined  as
including  the  functions  of  obtaining  oxygen  and  eliminating
carbon  dioxide.  In  some  lower  animals  (e.gf.,  earthworm)
the  skin  is  the  respiratory  organ ;  in  fishes  and  others  there
are  gills;  amphibians  breathe  with  both  skin  and  lungs;
but  in  the  vertebrates  higher  than  the  amphibians  lungs
are  the  sole  respiratory  organs.  In  all  these  cases  the  mem-
branes which  take  up  oxygen  also  give  out  or  excrete  carbon
dioxide.  For  greater  convenience  in  study,  we  shall  in  this
lesson  confine  our  attention  to  the  supplying  of  oxygen  to
cells  in  the  human  body;  and  deal  with  the  excretion  of
carbon  dioxide  in  the  next  lesson.
The  respiratory  organs  consist  of  nasal  passages,  pharynx,
larynx,  trachea  (windpipe),  bronchi  (right  and  left  branches
of  the  trachea),  bronchial  tubes  (branches  of  bronchi),  air-
chambers  at  ends  of  smallest  bronchial  tubes,  diaphragm,
and  wall  of  the  thorax-  or  chest-cavity.
426.  Respiratory  Passages.  —  We  commonly  think  of
the  nose  as  an  organ  for  the  sense  of  smell ;  but  the  fact  is
that  only  a  limited  amount  of  epithelium  (lining  membrane)
in  some  of  the  upper  nasal  cavities  has  nerve-endings  con-
nected with  the  olfactory  part  of  the  brain,  and  hence  has
the  power  of  perceiving  odors.    Most  of  the  cavities  in  the


HUMAN  STRUCTURE  AND  LIFE-ACTIVITIES       508

nose  are  respiratory,  and  serve  the  purpose  of  warming  the
inrushing  air  by  contact  with  the  warm  membranes;  and
also  the  membranes  and  hairs  in  the  nose  collect  much  of
the  dust  from  the  air  which  enters.
The  passages  connecting  those  of  the  nose  with  the  pharynx
are  called  post-nasal.  They  lie  back  of  the  soft  palate.
