. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. 2$2 Biology of the Vertebrates The slant at which hairs emerge from the skin varies in such a way that in their direction the hairs taken together form vortices and streams as they lie over the surface of the body (Fig. 199). This is particularly apparent on a horse or short-haired dog. Convergent vortices form around the base of projecting structures, such as horns, the tail, and the umbilical cord. These hair whirlpools persist even after the structure a
. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. 2$2 Biology of the Vertebrates The slant at which hairs emerge from the skin varies in such a way that in their direction the hairs taken together form vortices and streams as they lie over the surface of the body (Fig. 199). This is particularly apparent on a horse or short-haired dog. Convergent vortices form around the base of projecting structures, such as horns, the tail, and the umbilical cord. These hair whirlpools persist even after the structure around which they converge has disappeared, for example in man about the umbilicus, and at the focus in the coccygeal region where the vanished embryonic tail was formerly located (Fig. 200). Perhaps the most familiar instance of divergent whirlpools is on the human scalp at the vertex of the F' ^00 T*~ft f crown5 where the hairs are centrifugally arranged. Other coccygeal hairs on divergent vortices appear in the axillae. The coarse hair a human embryo of the sloth is divergently parted along the midline of suggesting an ances- the beji insteac} 0f cjown the back as in most mammals, tral tail. (After _ . \ Ed^.) This adaptation, as in other divergent streams and whirlpools wherever found, is useful for shedding rain. The unusual arrangement in the case of the sloth is due to the fact that this mammal customarily hangs suspended upside down from the horizontal branches of trees. Although hairs are not arranged in definite patches like the pterylae ol feathers, they do emerge from the skin embryonically in orderly array with reference to each other (Fig. 201). In man they appear in groups of twos, threes, and fours with the largest hairs in the middle of each row, these rows in turn being spaced in such a way as to suggest that each one is homologous with an interscale area. This hypothesis is further borne out by the arrange- ment of hairs in similar groups in other mammalian sk
Size: 1901px × 1315px
Photo credit: © Library Book Collection / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, booksubjectanatomycomparative, booksubjectverte
