594 BIRDS. 



The epidermic exoskeleton is represented by feathers, usually 

 arranged in definite feather tracts (pterylia), with bare 

 patches between, and also by scales on the legs similar to those 

 of reptiles. Almost the only skin gland is an oil or preen 

 gland,, lying dor sally at the root of the tail. 



The pectoral muscles used in flight are generally large ; in 

 many there is a muscular gizzard ; a diaphragm is at most 

 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 sensitive cere , there is 

 no external ear ; the connection between tympanum and inner 

 ear is by means 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 i?i the adult without marrow. The curva- 

 ture of the vertebral centra, especially in the cervical region, 

 viewed from in front, is concave from side to side, and convex 

 from above downwards. The cervical vertebra, have small 

 ribs, which fuse in most cases with the transverse processes. 

 The dorsal vertebra tend to fuse together into an immovable 

 mass ; and a large number of vertebra (one to three dorsals, 

 all the lumbars, and some caudals) fuse with the two or three 

 true sacrals. The terminal vertebra usually fuse to form a 

 ploughshare bone. 



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

 obliterated. Only the lower jaw, the quadrate, the columella, 

 and hyoid are always movable ; but the pterygoids usually 

 articulate freely with the basisphenoid, the lachrymals may 

 remain free, and there may be a joint in the beak at the end 

 of the premaxilla. There is but one condyle. A membrane 

 bone called the basitemporal covers the basisphenoid. There 

 is an interorbital septum formed from presphenoid and meseth- 

 moid. The otic bones fuse with adjacent bones and with one 

 another about the same time. In modern birds there are no 

 teeth, but the jaws are covered by horny sheaths. The pre- 

 maxilla are large, and form most of the beak. The lower 

 jaw consists on each side of five membrane bones and a cartilage 



