626 BJKDS. 



flight; the neck is long; and the tail is short, except in the 

 extinct Archceopteryx. 



The epidermic exoskeleton is represented by the feathers, 

 which are 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 ; there is no diaphragm 

 comparable to that of mammals. 



In the brain, which fills the large cranial cavity, the 

 predominance of the basal parts of cerebrum and cerebellum 

 has resulted in displacing the optic lobes to the sides. The 

 spinal cord is at an angle to the medulla oblongata, not in a 

 line with it as in lower Vertebrates. 



The nostrils are often overhung by a sensitive cere ; there 

 is never more than a rudimentary pinna outside the external 

 auditory meatus ; the connection between tympanum and 

 inner ear is by means of a columella ; the eyeball is strengthened 

 b\ 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 which contain prolongations of the air-sacs connected with 

 the lungs. When a long bone contains an air-sac there is 

 little or no marrow. The curvature of the vertebral centra, 

 especially in the cervical region, viewed from in front, is 

 concave from side to side, and convex from above downwards 

 (heteroccclous]. 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 

 lumbar s, and some caudals] fuse with the two or three true 

 sacrals. The terminal vertebrce. usually fuse to form a 

 ploughshare bone. 



In most birds the bones of the brain-case fuse very early, 

 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 premaxillce. There is but one 

 condyle. A membrane bone called the basitemporal covers 



