ALIMENTARY SYSTEM. 665 
curvature of the brain is well marked in the adult; thus 
the medulla is quite hidden by, and descends almost 
vertically from, the cerebellum. 
There are as usual twelve cranial nerves. 
In connection with the spinal cord, the brachial plexus of nerves 
to the forearm, and the sacral plexus to the leg, should be noticed. 
In the lumbar region the halves of the cords diverge for a short , 
distance, forming a wide space—the rhomboidal sinus—roofed only 
by membrane. The cervical part of the sympathetic nervougy system 
is double on each side. 
Sense organs.—The sense of smell is not well developed 
in Birds. The nostrils are longitudinal slits overhung by 
the swollen, more or Icss tactile, cere. Apart from the 
cere, there is only a diffuse sense of touch, and the sense 
of taste is also slightly developed. 
The sense of hearing is acute. Externally the ear is 
marked by an open tube—the external auditory meatus ; 
the aperture of which lies behind the eye, concealed 
beneath the feathers. Within the tube, a little beneath 
the surface, lies the drum or tympanum; connecting this 
with the fenestra ovalis of the inner ear is the columella; 
the tympanic chamber is continued past the ear as the 
Eustachian tube, which unites with that of the opposite 
side, and opens into the mouth cavity in front of the 
basisphenoid bone. The cochlea, or curved protuberance 
of the sacculus, which is incipient in Amphibians, and 
larger in Reptiles, is yet more marked in Birds. 
The eye has an upper, a lower, and a third eyelid or 
nictitating membrane. The last is frequently twitched 
across the eye, and helps: to keep the front clean; it is 
present in many Reptiles and most Mammals. The 
front of the sclerotic protrudes in a rounded cone, and is 
strengthened by a ring of little bones. Into the vitreous 
humour a vascular pigmented pecten protrudes from the 
‘region of the blind spot where the optic nerve enters. 
Birds have remarkable powers of optic accommodation. . 
Alimentary system.—The jaws are ensheathed in horn, 
and this sheath takes the place of teeth, and is sometimes 
ridged, as in ducks. It is interesting to notice that this 
horny beak was absent in some of the extinct toothed 
birds. In modern birds there are no hints of teeth, 
