S'^^G VEKTEP.liATK ANIMALS. 



and there can lie no doubt tlia,t they ai'e in many cases essential to 

 liealth, tlie bird being otlierwise nnaljle to triturate its food projierly. 

 Tlie intestinal C"Uj;il extends from tlie g-izz:ird to the cloaca {d), and 

 is coni]jarativ-e]y sliort. Tlie secretions of tlie liver and pancreas 

 are poured into the eouimencenjent of the snj:dl intestine. The 

 conuueneenient of the large intestine is furnished in most Birds 

 ^Yith two bhnd tubes or " ca-;r,-i " (/). These vary considerably in 

 length in dilVerent ISirds, :ind are sometimes w.-mting ; whilst their 

 exact fnnetion is still cjuestionabli'. The large intestine is seldom 

 UK.ire than a tejitli ]«irt of the length of the Injily, and is genei-ally 

 conducted straight from the cirea to the cloaca. The i-limcn, is a 

 common cavity which in Birds, as in Reptile-s, receives the terndna- 

 tion of the intestine and the ducts of the generative and urinary 

 organs (.7). 



J!r,i//iriitii'ii is effected in liirds more coni|)letely, extensively, and 

 actively tlian in any other class of the Vciichnitd, and, as the result 

 of this, their .aveiag-e temperature is higher tlia,n in any other Verte- 

 brates. This extensive development of the respiratory ]irocess is 

 due to the fict that air is admitted in Birds not only to the lungs, 

 but also to a series of air-receptacles which are scattered through 

 various jiarts of the body, and to the interior of a greater or less 

 nundjer of the bones. The lungs are two in nvunber, of a bright-red 

 cnliiur, and siJongy texture, and they are confined to the back part 

 of the chest. They differ from the lungs of Mammals in not being 

 freely suspeiided in a membranous bag (jilenra), but in lieing 

 fixed to the b.iek wall of the chest. The thoracic and abdominal 

 caA'ities are not sejiarated from one another by a comjilete partition 

 (midritr i<r diaphragm) as in the Mammals, but the connnon thoracico- 

 .abdoniinal ca\ity is subdivided by jneaiis of membra.n<">us ]tartitions 

 info a seri(-s of ca\'ities <-ir sacs, A\"hich are termed the ^' air-i'ece]i- 

 tacles." These aii'-sacs are tilled with air from the lungs, and va.ry 

 coiisidei'.ably ill size in different Birds. 'J1n-y not onlyserve greatly 

 to reduce tlie specilic gra\'ity of the body, Ijut ai.so assist largely in 

 the .aer.-if ion of the blood. < 'oniiei-ted with the air-recejjtacles, and 

 siippleiiieiitiiig their action in both of these respects, is a series of 

 ca\i(ies oeeiipying (he iiifi'iior of a greater or le-<s number of the 

 bones, and also containing air. fii young birds these air-ca-vities in 

 (he bones do not exist, .'ind the bones are siiii|ily lilled with marrow, 

 .•IS in (he ^l;iniiiials. ]ii (he I'eiigiiins, whieli do not (iy, none of 

 the bones coiit.ain air-i'a\'ities or are " |iiiemiiatic " ; anil in the 

 Ostrieli only a few of the bones contain air. In the .Pelican and 

 llannet all (he bones of (he skeledm, except the phalanges of the 

 tors, .-irr pernieati'd by air; and in the llornbill e\en these are 



]>lielini;i( ir. 



