ALIMENTAR Y S YSTEM. 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 nervous 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 less 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, 



44 



