THE TROCHLEAR NERVE IN THE HUMAN EMBRYO AND FOETUS

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Abstract
In the 9 mm. human embryo the metencephalic alar thickenings, situated on each side of the cephalic portion of the 4th ventricle and forming the cerebellar plates, are continued rostrally as the lateral walls of the isthmus rhombencephali, which in turn merge into the basal plate of the mesencephalon at a point termed the intra (basal)-laminar sulcus. The trochlear nucleus is a semi-lunar mass of large neuroblasts lying on the outskirts of the mantle layer of the basal plate of the isthmus which is compressed due to the narrowing of the cavity of the isthmus and, in consequence, lies at the ventral extremity of the lateral isthmic walls. Nerve fibers arise from the cells in the nucleus and turning dorsally enter the lateral isthmic walls and pass to the position where the rostral extremities of the cerebellar plates come together and form the superior medullary velum. This route offers little resistance to the course of the nerve. After the 19 mm. stage, it is also the route followed by the cerebello-mesencephalic fibers. Beyond the 22 mm. stage, the trochlear nucleus becomes oriented in the caudal region of the mesencephalon. This may follow the slower rate of growth of the isthmus as compared with the meteneeph-alon and mesencephalon. The line of demarcation between the alar and basal plates of the metencephalon does not appear to correspond to the inter-laminar sulcus, but is indicated by a cell free zone.