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The pattern of the
ventricular system is established early. The prosencephalon separates into 2
expanding
telencephalic vesicles (lateral
ventricles) and a slit-like
diencephalic vesicle (3rd ventricle). Thickening of its walls narrows the
lumen of this segment to a vertical slit, the 3rd ventricle. The lumina of the
lateral and 3 ventricles connect just behind the
lamina terminalis via the paired
foramina of Monro. Expansion of the
mesencephalon narrows its vesicle forming the cerebral aqueduct, which
connects the 3rd and 4th ventricle of the
rhombencephalon. The rhombencephalic roof is thin and covers the 4th
ventricle which forms a shallow, diamond-shaped depression. The expansion of the
cerebral hemispheres affects the shape of the lateral ventricles which become
“C”-shaped.
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