. Outlines of zoology. Zoology. 6o8 BIRDS. third eyelid or nictitating membrane, is obvious. Tiie 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 IVEammals) 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 parrot
. Outlines of zoology. Zoology. 6o8 BIRDS. third eyelid or nictitating membrane, is obvious. Tiie 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 IVEammals) 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- 1 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 thg 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 pj'loric 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. Fig. section Bird. 214. —Diagrammatic of cloaca of Male (After Gadow.) cd, Upper region of cloaca inlo which rectum opens ; iid, median region into which ureters (?/) and vas deferens {vd) open
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1895
