Human anatomy, including structure and development and practical considerations . the rhinencephalon, but the lines of itsgrowtii are particularly backward and downward, in coiiseiiuence of which, in addition tothe production of a temporal and the distinctive occipital lobe, the other brain-segmentsbecome gradually covered over and deposed from their original sujierior position tcjward thebasal surface of the brain. This process is already marked during the third nu^nth (Fig. 1031),by the end of which period the pallium covers the diencephalon. By the beginning of the fifthmonth the mid-brain
Human anatomy, including structure and development and practical considerations . the rhinencephalon, but the lines of itsgrowtii are particularly backward and downward, in coiiseiiuence of which, in addition tothe production of a temporal and the distinctive occipital lobe, the other brain-segmentsbecome gradually covered over and deposed from their original sujierior position tcjward thebasal surface of the brain. This process is already marked during the third nu^nth (Fig. 1031),by the end of which period the pallium covers the diencephalon. By the beginning of the fifthmonth the mid-brain is ct)mpletely overlaid, and by the eighth month the entire upper surfaceof the cerebellum is covered. Development of the Sulci and Gyri.—The modelling of the surface ni the cerebral hemi-sphere begins towards the end of the fifth month of fcetal life, by which time the occipital lobeis well formed and the brain-case is separated from the cerebral surface by an intervening layer Fig. 102S. Lalt-riil vtntricle Roof plate of IIT /vfiilriflf, witlichoroid plex\i- Corpus striatm. nuiliiiliualfissiirt- Clioroi<l plexus Hipi)ocami)alfissure 1 Iipi)ocanii)US Choroidal fissures Frontal section of brain of rabbit embryo showing invajjiiiation of mesial wall of hemisphere along hippocampal andchoroidal fissures; thin roof-plate of third ventricle stretches between Ihalanii. ?, 13. of yielding arachnoid tissue, which offers little opposition to the production of the convolutionswhich now follows. Preceding this period, the outer surface of the young hemisphere is quitesmooth, with the exception of the crescentic .Sylvian fossa ( 982) which marks the positionof the later insula. This depression has been described (page 1137) in connection with the pro-duction of the Sylvian fissure. The uncertain creases, the so-called transitory fissures, some-times seen on brains of a much earlier period are without morphological significance and arenow usually regarded as artefacts (Ziehen, Ho
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Keywords: ., bookauthormc, bookcentury1900, bookdecade1910, booksubjectanatomy
