. The biology of birds. Birds. 64 THE BIOLOGY OF BIRDS entire cells—including the nuclei—of an epidermic thickening changed into horny fibres, running longitudinally, are transversely, and perpendicularly. In the formation of the ordinary horny covering of the beak, it is only the mantle of the epidermic cells that is changed into the horny fibre, and more or less thin plates result. The egg-tooth and a corresponding rudiment on the tip of the mandible appear earlier than the horny bill, and, as has been stated, their mode of cornification is different. It may be, Rosenstadt suggests, that the
. The biology of birds. Birds. 64 THE BIOLOGY OF BIRDS entire cells—including the nuclei—of an epidermic thickening changed into horny fibres, running longitudinally, are transversely, and perpendicularly. In the formation of the ordinary horny covering of the beak, it is only the mantle of the epidermic cells that is changed into the horny fibre, and more or less thin plates result. The egg-tooth and a corresponding rudiment on the tip of the mandible appear earlier than the horny bill, and, as has been stated, their mode of cornification is different. It may be, Rosenstadt suggests, that they represent an old-fashioned armature of the jaws, older than the modern horny Fig. 14.—Side view of bird's skull. From a specimen, par., parietal , supra-occipital; sq., squamosal; Q., quadrate ; fr., frontal; as., ali-' sphenoid ; os., orbitosphenoid ; , optic foramen ; me., mesethmoid ; L., lachrymal; na., nasal; pmx., premaxilla ; no., nostril; , nasal pro- cess of premaxilla ; , orbital process of frontal; j., jugal; pal., pala- tine ; PT., pterygoid; , quadrato-jugal; art., articular of the lower jaw or mandible (mn). (c) The complex lower jaw is made up of six bones on each side, thus revealing affinity with reptiles and marked contrast to mammals (where the mandible is simple), but from the present point of view it is more interesting to notice its loose articulation with the movable quadrate, for this increases the gape—an important point when the food has to be swallowed without chewing and without too many questions in regard to size. With the absence of any mastication we may associate the slenderness of the infra-. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Thomson, J. Arthur (John Arthur), 1861-1933. New York : Macmillan
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Keywords: ., bookcentury1900, bookdecade1920, booksubjectbirds, bookyear1923
