Section through the lower end of the cerebral aqueduct (1) of the mid-brain in a monkey. In the roof of the aqueduct is seen the decussation of the trochlear nerve. Close to this on either side are seen some of the large round cells of the mesencephalic nucleus of the trigeminal nerve (3), and still more laterally is seen the mesencephalic root of the nerve (5). Below the aqueduct on either side of the mid-line is the medial longitudinal bundle (4). The section is stained with iron haematoxylin and neutral red, and the photograph is at a magnification of X 33. 

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