. Comparative anatomy of vertebrates. Anatomy, Comparative; Vertebrates -- Anatomy. 144 COMPARATIVE MORPHOLOGY OF VERTEBRATES. longata, which also bends in the same direction. The pontal flexure, beneath the cerebellum, bends in the opposite direction and thus tends to counteract the other two. Nuchal and pontal flexures are at best but weakly developed in the ichthyopsida and all are practically obliterated in the adult, but in the amniotes they are increasingly developed and persist through life (fig. 148). The brain, like the spinal cord, is composed of nerve cells (gray matter) and fibres
. Comparative anatomy of vertebrates. Anatomy, Comparative; Vertebrates -- Anatomy. 144 COMPARATIVE MORPHOLOGY OF VERTEBRATES. longata, which also bends in the same direction. The pontal flexure, beneath the cerebellum, bends in the opposite direction and thus tends to counteract the other two. Nuchal and pontal flexures are at best but weakly developed in the ichthyopsida and all are practically obliterated in the adult, but in the amniotes they are increasingly developed and persist through life (fig. 148). The brain, like the spinal cord, is composed of nerve cells (gray matter) and fibres (white matter), but their arrangement is exceedingly complicated and but the slightest outline of their distribution can be attempted here, in connection with the general account of the regions of the FIG. 149.—Cross-section of medulla of Acanthias embryo, 60 mm. long, showing the greatly broadened roof plate and, below, a bit of the meninx of the nervous system, c, cartilage of basal plate; e, ependyma; mp, meninx primitiva; pc, perichondrium (endo- rhachis); r, roof plate. The myelencephalon is most nearly like the spinal cord of any part of the brain. It is triangular hi outline, viewed from above, and is widest anteriorly, due in part to the separation of the side walls by the great development of the roof plate over the fourth ventricle. Blood- vessels press against the roof, carrying parts of it before them into the ventricle, thus forming the chorioid plexus of the fourth ventricle, a means of introducing nourishment into the brain. (Usually in dis- sections this roof is torn away, leaving a triangular or rhomboid opening into the fourth ventricle—fossa rhomboidea). The floor plate in this region is obliterated by the development of numerous nerve centres —'nuclei' or ganglia—in the walls, some closely connected with the. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectanatomy, bookyear1912
