726 PHYLUM CHORDATA : CLASS AVES — BIRDS 



flight ; the neck is long and the tail is shorty except in the 

 extinct Archaeopteryx. 



The epidermic exoskeleton is represented by the feathers, 

 which are usually arranged in definite feather tracts (pterylce), 

 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 dorsally 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 surrounded by a sensitive cere ; there 

 is never more than a very rudimentary pinna outside the 

 external auditory meatus ; 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 and secretory pec ten. 



There are no epiphyses on the bones. Many bones 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 typically concave from side 

 to side, and convex from above downwards {heterocoelous), 

 but other shapes occur, e.g. opisthocoelous in thoracic region 

 of gulls and penguins. The cervical vertebrce have small ribs, 

 fused in most cases with the transverse processes. The 

 thoracic vertebrce tend to fuse ; and numerous vertebrce {one 

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

 the two or three true sacrals. The terminal vertebrce usually 

 fuse as 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 



