648 BIRDS. 
GENERAL CHARACTERS OF BIRDS 
Warm-blooded, oviparous, feathered bipeds. 
The fore-limbs are modified as wings, generally capable of 
fight ; the neck ts long and the tail is short, except in the 
extinct Archeeopteryx. 
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 ts 
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 ts 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 ts 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 
ts never more than a very rudimentary pinna outside the 
external auditory meatus ; the connection between tympanum 
and inner ear ts 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 pecten. 
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 ts little or no marrow. 
The curvature of the vertebral centra, especially in the cervical 
vegion, viewed from in front, is typically concave from side 
to side, and convex from above downwards (heterocelous), 
but other shapes occur, e.g. opisthocelous in thoracic region 
of gulls and penguins. The cervical vertebra have small ribs, 
Jused in most cases with the transverse processes. The 
thoracic vertebra tend to fuse; and numerous vertebra (one 
to three dorsals, all the lumbars, and some caudals) fuse with 
the two or three true sacrals. The terminal vertebre 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 
