616 The Animal Kingdom 



addition, they form excellent insulating materials for the homoiothermic 

 condition characteristic of the birds. 



Nearly everything characteristic of the members of this class is 

 related to their ability to fly. Thus the anterior pair of appendages is 

 modified into wings. The bones of the wing are homologous with those 

 of the mammalian front limb, but are highly modified for their special 

 function. The posterior pair of appendages is variously modified, de- 

 pending upon the habits of the bird; some are adapted for perching, 

 some for swimming, and still others for running. 



The fully ossified skeleton is very strong though extremely light. 

 This combination is made possible by the fact that the skeleton is very 

 compact and many of the bones are hollow. The skull, too, is very com- 

 pact, and the bill is formed of a projection from the skull covered with 

 a light horny sheath. There are no teeth in modern birds. Like the 

 reptiles, the skull has but a single occipital condyle. The pelvis is fused 

 to a large section of the vertebral column and opens ventrally. There 

 is a large sternum, often with a keel, to which the muscles of the wings 

 are attached. 



The internal anatomy gives further evidence of the remarkable ad- 

 justment of these animals to flight. The viscera are very compactly ar- 

 ranged in the lower part of the abdomen. The female in most cases 

 has but a single ovary and oviduct. There is no bladder, and feces are 

 not retained within the lower intestine. All of these adjustments re- 

 duce the weight. 



The compact lungs are attached to the ribs. They are connected 

 with air sacs which fill all the spaces between the visceral organs and 

 extend into some of the bones. The air sacs facilitate the rapid ex- 

 change of gases with the blood. 



The adaptation to flight has also necessitated certain radical physi- 

 ological adjustments. Most important of these is the maintenance 

 of a constant body temperature. This condition has freed birds from 

 the necessity of living only in areas with a favorable temperature. It 

 has enabled them to invade the colder regions of the earth. To main- 

 tain this high body temperature, they have a very high metabolic rate, 

 a rapid heart beat, and an efficient respiratory exchange mechanism. 

 The heart has four chambers, with a complete separation of oxygenated 

 and unoxygenated blood. The systemic arches are reduced to the right 

 half of the fourth arch. 



The high metabolic rate and activity have necessitated a greater 

 consumption of food by the birds. Related to this constant search for 



