liUW THE ANIMAL I.IVES Zlo 



411. In tlio main, the air is ohangt'd in 

 bivatliinc: as follows : ,, , 



Itxuiim Sitrogen diuxid 

 Iiispirinl. or brenthed-in nir contains . . JO. 81 79. l.") .04 



Kxpirttl, or breiitht-ii-out iiir I'ontnins ItiJUT 79.557 4.38 



111 «'VtM\v 100 parts, air Inscs, l»y Ix'iii^' ltr»'atlif(l, 

 about 4 parts of oxygen, ami gains alntut 4 

 parts of carbon dioxid. 



41"J. In Itroathing, tho air is also cliargt'd 

 with watt'i' \a]«»r and with small (|uantities of 

 ammonia and marsh gas. It also n'('«Mves a 

 volatile organic matter, which may be fcetid, 

 and when eondenstMl in water soon develops a 

 putrid odor. 



41.'>. In the breathing process, the blood and 

 tiie air arc brought into the closest ]>ossiblc 

 contact. ( )ii('-c.-llfd animals breathe thi«>iiLch 

 the entire surface, lishe.^ througii gills waved in 

 th»» water, from whi<'h tliey abstract oxygen, 

 frogs tiirough tho walls of a simple air-sac, 

 in which the blood-vessels circulate. In warm- 

 blooded animals, this sac <»r lung is divided 

 throughout into myriads of mimitf air-sa<'s or 

 colls, varyinix from yir to -yV <'I an inch in 

 <liameter. The walls aro .so thin that the blood 

 llowing through their capillary vessels is con- 

 stantly expos«'d, on two sides, to tho air with 

 whicii they are filled. The membrane consti- 

 tuting the walls of those sacs is so exceedingly 



o 



