CHAPTER XIII 

 AVES 



The characteristics of this familiar class are more definite and un- 

 varying than those of almost any other class in the animal kingdom. 

 Birds are known by their feathers, which are possessed by no other ani- 

 mals. They are warm-blooded, in which they differ from all preceding 

 forms, their blood being several degrees warmer than that of man and 

 other mammals. 



In almost all living species the anterior appendages are developed as 

 wings and are used in locomotion. To support the large pectoral 

 muscles used in flight the sternum is usually prolonged into a ventrally 

 projecting keel or carina. 



In all living birds teeth are absent. The olfactory organs are usu- 

 ally poorly developed while the eye is unusually large and perfect and 

 contains a pecten. The cervical vertebrae are usually numerous while 

 the caudal are few and are usually fused to form a pygostyle around 

 which the tail feathers are attached. The coracoid is usually large and 

 pillar like; the clavicles are united to form the familiar "wish bone;" 

 and the scapula is long and sabre-shaped. There are numerous other 

 internal peculiarities that might be mentioned, such as the absence of 

 the left instead of the right (as in mammals) aortic arch and of the right 

 ovary and oviduct; the frequent enlargement of the gullet to form a 

 crop or craw; the division of the stomach, in some forms, into the glan- 

 dular part or proventriculus and the hard, muscular gizzard; the pres- 

 ence of air spaces in the bones and in various parts of the body that are 

 connected with the lungs; the presence of a peculiar voice apparatus, 

 the syrinx, situated at or near the point of division of the trachea into 

 the bronchi; the four-chambered heart containing oval, nucleated, red- 

 blood corpuscles. 



Birds are all oviparous; the egg is large in proportion to the bird 

 that produces it and consists of a large amount of yolk and albumen in- 

 closed in a hard, calcareous shell. The newly hatched young are either 



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