6o8 



BIRDS. 



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, 

 the vascular, nutritive pecten projects. Birds have remark- 

 able powers of optic accommodation. 



The Alimentary System. 



The jaws are ensheathed in horn. 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 

 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. While it remains within 

 the crop, the food is softened a 

 little. Especially at the breeding 

 season the cells lining the crop 

 undergo a strange degeneration, 

 forming " pigeon's milk," which 

 both males and females 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. 



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 cuticle, and within the cavity are small 

 stones which the bird has swallowed. The pyloric opening 

 from the gizzard into the duodenum, is very near the open- 

 ing from the proventriculus into the gizzard. 



In the fold of the long duodenum lies the pancreas with 

 three ducts, and into the same region open two bile ducts 



cd. 



ud.. 



pet.. 



FIG. 214. Diagrammatic 

 section of cloaca of Male 

 Bird. (After GADOW.) 



cd, Upper region of cloaca into 

 which rectum opens ; ud, median 

 region into which ureters (u) and 

 vas deferens (vd) open ; pd, pos- 

 terior region into which Bursa 

 Fabricii (B.F) opens. 



