. Birds: the elements of ornithology . we find a foot with a minimumpower of grasping, and a reduced or elevated hallux and an elon-gated tarsus. In the most opposite type—that of the perchers —the insessorial type—there is a long, well-developed hallux,and the other digits are cleft to their bases; while, on the con-trary, in the natatorial type—the Swimmers—we find thefeet webbed more or less completely or else lobate. Ptebtlosis. As before said * the feathers of birds do not usually growall over the body, but along certain definite tracts, each of whichis termed a pteryla. These are separat
. Birds: the elements of ornithology . we find a foot with a minimumpower of grasping, and a reduced or elevated hallux and an elon-gated tarsus. In the most opposite type—that of the perchers —the insessorial type—there is a long, well-developed hallux,and the other digits are cleft to their bases; while, on the con-trary, in the natatorial type—the Swimmers—we find thefeet webbed more or less completely or else lobate. Ptebtlosis. As before said * the feathers of birds do not usually growall over the body, but along certain definite tracts, each of whichis termed a pteryla. These are separated by spaces whereinfeathers are not implanted, or apteria. A. few birds, however, have feathers implanted all over thebody. Amongst these are the Toucan, the Penguin, and theOstrich. The definite pterylse which have been defined arethe foUovring:— The dorsal or spinal pteryla, which extends from the napeof the neck to the tail. Its shape varies greatly in different * See ante, p 141, THE EXTEENAl STETJCTTIEB OF BIBDS. 165. Ptbeyl* op D0K8AL AND Ventral Surfaces of the Swipt,(^After Nitzsch.)Fig. 146. Ventral aspect. 147. Dorsal aspect. I. Dorsal or spinal pteryla. 2. Humeral pteryla. 3. Femoral Cranial pteryla. 5. Alar pteryla. 6. Caudal pteryla. 7. Cruralpteryla. 8. Ventral pteryla. 166 ELEMENTS OF OENITHOLOGfr, kinds of birds, sending out branches, bifurcating or sufferinginterruption, as the case may be. Fig. 147 (i). The ventral pteryla extends from the throat to the rootof the tail, and may vary in disposition, as already stated, withrespect to the dorsal pteryla. Fig. 146 (8). The cranial pteryla, or tract of the head, more or lessextensively invests the head and becomes continuous with thedorsal pteryla. (4). The caudal pteryla, or tail-tract, comprises both the dorsaland ventral surface of the tail, wherein the rectrices and upperand lower coverts are inserted. (6). The hjimeral pterylae, or shoulder-tracts, are constant butsmall. One cross
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