ALIMENTARY SYSTEM OF THE PIGEON. 511 



cervical part of the sympathetic nervous system is double on 

 each side. 



Sense-Organs. — The sense of smell does not seem to be 

 keenly developed in many birds. The nostrils are longi- 

 tudinal slits overhung by the swollen cere. 



The sense of hearing is acute. Externally the ear is 

 marked by an open tube — the external auditory meatus ; 

 the aperture of which is surrounded by a regular circlet of 

 feathers. Within the tube beneath the surface lies the drum 

 or tympanum ; connecting this with the fenestra ovalis of 

 the inner ear lies the well-developed 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 basi-sphenoid bone. As 

 regards the internal ear, it should be noticed that the cochlea, 

 or curved protuberance of the sacculus, which is incipient 

 in Amphibians, and larger in Reptiles, is yet more marked 

 in Birds. 



As to the eye, its protection by an upper, a lower, and a 

 third eyelid or nictitating membrane, is obvious. The front 

 of the sclerotic protrudes in a rounded cone, and is strength- 

 ened by a ring of little bones. Into the vitreous humour, 

 a vascular and doubtless nutritive fold called the pecten pro- 

 jects. This is also seen in some Fishes and Reptiles. 

 Birds have remarkable powers of optic accommodation. 



The Alimentary System, — The jaws are ensheathed in 

 horn. There are no hints of teeth except in the embryos of 

 some parrots. A narrow tongue lies in the floor of the 

 mouth; it is unimportant in the pigeon, but is thick in 

 parrots, and long in woodpeckers and humming-birds. 

 Associated with the tongue there are numerous glands. 

 Into the mouth there open the posterior nares, and the 

 united Eustachian tubes. 



The gullet expands into a thin- walled, slightly bilobed, non- 

 glandular crop, in which are stored the hurriedly swallowed 

 seeds. No great change happens to the food while it remains 

 within the crop, but it is softened a little. At the breeding 

 season the cells lining the crop undergo a strange degenera- 

 tion, and form "pigeon's milk," which both males and 

 females give to the young birds. 



From the crop the food-canal is continued into the 



