A L THE NT A R Y S YSTEM. 



641 



and this sheath takes the place of teeth, and is sometimes 

 ridged, as in ducks. It is interesting to notice that this 

 homy 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 detected in some 

 embryos. A narrow tongue lies in the floor of the mouth ; 

 it is unimportant in the pigeon, but is often useful, as in 

 parrots, woodpeckers, and humming-birds. Associated with 



olf. 



FIG. 318. Brain of pigeon. 



(i) Dorsal, (2) ventral, and (3) side view, olf., Olfactory lobes ; 

 c., cerebral hemispheres; ol., optic lobes; c/>., cerebellum; 

 /;/<?., medulla oblongata. 



the tongue there 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, slightly bilobed, non-glandular crop, in which 

 the hurriedly swallowed seeds are stored and softened a 

 little. Especially at the breeding season, the cells lining 

 the crop degenerate, 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 



4 1 



