596 BIRDS. 



General Characters. — Th£ fore-limbs are generally 

 modified as wings capable of flight ; the neck is long, and the 

 tail is short except in tlu extinct Saururce. 



The epidermic exoskeleton is represented by feathers, with 

 sometimes a few scales ; there are no scutes. 

 ■ Almost the only skin gland is an oil or preen gland at the 

 root of the tail. 



The pectoral muscles used in flight are generally large ; in 

 many there is a muscular gizzard ; the diaphragm is only 

 hinted at. 



In the brain, the predominance of the basal parts of cerebrum 

 and cerebellum has resulted in displacing the optic lobes to the 

 sides. 



The nostrils are often overhung by a sensitize cere ; there is 

 no external ear ; the connection between tympanum and 

 intier ear is by mea?is of a columella ; the eyeball is strengthened 

 by sclerotic ossicles, there is a well-developed third eyelid and 

 a large nutritive pecten. 



There are no epiphyses in connection with the bones, many 

 of ivhich contain prolongations of the air sacs connected with 

 the lungs, and are in the adult without marrow. The curva- 

 ture of the vertebral centra, viewed from in front, is concave 

 from side to side, and convex fro?n above downwards. The 

 cervical vertebrcE have small ribs. A large number of vertebm 

 (one to three dorsals, all the lumbars, and some caudals) fuse 

 with the two or three true sacrals. The terminal vertebrce 

 fuse in a ploughshare bone. 



Most of the bones of the skull fuse, the sutures being obliter- 

 ated. Only the lower jaw, the quadrate, the colutnella, a?id 

 hyoid are always movable, but the pterygoids usually articulate 

 freely with the basi-sphenoid, the lachr\-mals may remain free, 

 and there may be a joint in the beak at the end of the pre- 

 fnaxillcB. There is but one condyle. A membrane bone called 

 the basi-temporal covers the basi-sphenoid. There is an inter- 

 orbital septum formed from presphenoid and mesethmoid. The 

 otic bones fuse with adjacent bones and with o?ie another 

 about the sa>?ie time. In modern birds there are no teeth, but 

 the jaws are covered by horny sheaths. The pre maxi lice are 

 large, and form most of the beak. The lower jaw consists on 

 each side of flve membrane bones and a cartilage bone — the 

 articular — ivhich ivorks on the quadrate. Many of the skull 



