SENSE ORGANS OF PIGEON 747 



marked by an open tube — the external auditory meatus ; 

 the aperture of which Ues 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, 

 except that " a dental ridge " (see Mammals) has been 

 seen in some embryos. A well-developed tongue lies on 

 the floor of the mouth ; unimportant in pigeons, but 

 often useful, as in parrots, woodpeckers, and humming- 

 birds. Associated with the tongue are numerous glands. 

 On the roof of the mouth lie the posterior nares, 

 and behind them the single aperture of the Eustachian 

 tubes. The gullet expands into a: thin-walled, bilobed, 

 non-glandular crop, in which the hurriedly swallowed 

 seeds are stored and softened. Especially at the breeding 

 season, the cells lining the crop degenerate, and form 

 " pigeon's milk," which both sexes give to the young birds. 



From the crop the food canal is continued into the 

 glandular part of the stomach (the proventriculus), where 

 gastric juice is secreted from large glands. 



Beneath the proventriculus is the gizzard, in which the 



