. Outlines of zoology. 9.—Vertical section of thepineal eye in an embryo of Spheno-don.—After Dendy. E,, Epidermis ; dermis; i., lens ; /. W.^inner wall of the eye ; O. W. outer wall ofthe eye ; i.^.iV., parietal nerve; ,parietal stalk; C, cartilage. of the parietal organ) receives anerve from a parietal centrenear the base, but independentof the epiphysis; this nerve istransitory in Anguis, more orless persistent in Iguana. AboveReptiles the pineal stalk is relativelyshort, and its terminal portion isglandular. Among mammals theepiphysis is absent in the dugongand some Cetaceans; the
. Outlines of zoology. 9.—Vertical section of thepineal eye in an embryo of Spheno-don.—After Dendy. E,, Epidermis ; dermis; i., lens ; /. W.^inner wall of the eye ; O. W. outer wall ofthe eye ; i.^.iV., parietal nerve; ,parietal stalk; C, cartilage. of the parietal organ) receives anerve from a parietal centrenear the base, but independentof the epiphysis; this nerve istransitory in Anguis, more orless persistent in Iguana. AboveReptiles the pineal stalk is relativelyshort, and its terminal portion isglandular. Among mammals theepiphysis is absent in the dugongand some Cetaceans; the pinealbody is absent in Dasypus and thedolphin. The significance of the pinealbody is uncertain. According tosome, its primitive function is thatof an unpaired, median, upward-looking eye—a function retainedonly in the Reptiles mentionedabove, the organ having elsewhereundergone (independent) degenera-tion. It may be, however, that theoptic function is not primitive, butthe result of a secondary Fig. 260.—Diagram of the partsof the brain in Vertebrates.—After Gaskell. Cerebral hemispheres; ,choroid plexus; , optic thai-ami; , optic lobes; cb., cere-bellum; , choroid plexus;, medulla oblongata ; ,spinal cord. THE BRAIN 487 The second primary vesicle of the brain forms the thirdregion, that of the optic lobes (mesencephalon or mid-brain)in the adult brain. The floor and lateral walls form thethickened crura cerebri; the roof becomes the two optic lobes,which are hollow in almost all Vertebrates. In Mammalsa transverse furrow divides each optic lobe into two (corporaquadrigemina). The cavity of the vesicle becomes muchcontracted, and forms the narrow iter or aqueduct of Sylvius,a canal connecting the third ventricle with the fourth. The third primary vesicle gives rise to the metencephalon,or hind-brain, or region of the cerebellum, and to themyelencephalon, or after-brain, or region of the medullaoblongata. In the metence
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Keywords: ., bookcentury1900, bookdecade1910, bookidcu3192, booksubjectzoology
