ALIMENTARY SYSTEM. 



607 



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 remark- 

 able 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 

 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 

 the tongue there are numerous glands. 

 On the roof of the mouth lie the pos- 

 terior 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 con- 

 tinued into the glandular part of the stomach (the pro- 

 ventriculus), where gastric juice is secreted from large 

 glands. 



Beneath the proventriculus is the gizzard, in which the 

 food is ground. The walls are very muscular, the fibres 

 radiating from two tendinous discs ; the internal surface is 

 lined by a hard horny epithelium ; and within the cavity are 

 small stones which the bird has swallowed. In hawks and 

 fish-eating birds the gizzard region is, naturally enough, 

 fairly soft. The pyloric opening, from the gizzard into the 

 duodenum, is very near the cardiac opening from the 

 proventriculus into the gizzard. 



ucL 



pd.. 



Fig. 267. — Diagram- 

 matic section of clo- 

 aca of male bird. — 

 After Gadow. 



cd., Upper region of clo- 

 aca into which rectum 

 opens; ud., median re- 

 gion into which ureter 

 («.) and vas deferens 

 (W.) open from each 

 sidej^SaT., posterior re- 

 gion into which bursa 

 Fabricii (B.F.) opens. 



