286 VERTEBRATE ANIMALS. 



and there can be no doubt that they are in many cases essential to 

 health, the bird being otherwise unable to triturate its food properly. 

 The intestinal caual" extends from the gizzai'd to the cloaca (cQ, and 

 is comparatively short. The secretions of the liver and pancreas 

 are poured into the commencement of the small intestine. The 

 commencement of the lai-ge intestine is furnished in most Birds 

 with two blind tubes or " caeca '' (k). These vary considerably in 

 length in different Birds, and are sometimes wanting ; whilst their 

 exact function is still questionable. The large intestine is seldom 

 more than a tenth part of the length of the body, and is generally 

 conducted straight from the Cfeca to the cloaca. The chjaea is a 

 common cavity which in Birds, as in Reptiles, receives the termina- 

 tion of the intestine and the ducts of the generative and ui'inary 

 organs (cl). 



Respiration is effected in Birds more completely, extensively, and 

 actively than in any other class of the Vertebrata, and, as the result 

 of this, their average temperature is higher than in any other Verte- 

 brates. This extensive development of the respiratory process is 

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

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

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

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

 colour, and spongy texture, and they are confined to the back part 

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

 ^eely sus|]ende<l in a membranous bag (plein-d), but in being 

 fixed to tlie Lack wall of the chest. The thoracic and abdominal 

 cavities are not .sepaiated from one another by a complete partition 

 (midriff nv diaphragm) as in the Mammals, but the common thoracico- 

 abdominal cavity is siilidivided by means of membranous partitions 

 into a sei-ies of cavities or sacs, which are termed the " air-recep- 

 tacles." These air-sacs are filled with air from the lungs, and vary 

 considerably in size in different Birds. They not only serve greatly 

 to reduce the sjiecific gravity of the body, but also assist lai-gely in 

 the aeration of the blood. Connected with the air-receptacles, and 

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

 cavities oeeu]j\ing the interior of a greater or less number of the 

 bones, and also containing air. In young birds these air-cavities in 

 the bones do nut exist, and the bones are simply filled with marrow, 

 as in the Mammals. In the Penguins, which do not fl}-, none of 

 the bones contain air-cavities or are " pneumatic " ; and in the 

 r)strich only a few of the bones contain air. In the Pelican and 

 liannet all the hones of the skeleton, except the phalanges of the 

 toes, are jiermeated by air ; and in the Hornbill even these are 

 pneumatic. 



