666 BIRDS. 



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 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 breed- 

 ing 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 

 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, 

 soft. The pyloric opening, from the gizzard into the 

 duodenum, is very near the cardiac opening from the 

 proventriculus into the gizzard. 



In the fold of the long duodenum lies the pancreas with 

 three ducts, whose number points to the triple origin of 

 the pancreatic rudiment in the embryo. Into the same 

 region open two bile-ducts from the two-lobed liver, which 

 is without a gall-bladder in the common pigeon, though 

 this is present in some birds, and even in some species of 

 pigeon. 



The small intestine is long ; the large intestine very short, not 

 more than a rectum two inches in length. At the junction of the 

 small and the large intestine there are two short caeca. In some birds, 

 e.g. the fowl, these are of considerable length ; in the ostrich they are 

 very long ; there are three in many ducks and birds of prey ; there is 

 only one in some fish-eating birds; in hornbills, parakeets, etc., they 

 are absent. 



The cloaca has three divisions (see Fig. 369), an upper part into 

 which the rectum opens, a median part into which the ureters and the 

 genital ducts open, and a posterior region (proctodseum), opening into 



