Section through the mid-brain of a monkey, stained with iron haemotoxylin and iron alum, showing the oculo-motor nucleus. The somatic element of this nucleus (2) consists of large motor cells forming compact groups on either side of the mid-line. Dorsal to the somatic element is the visceral element, consisting of smaller cells and forming the "nucleus of Edinger-Westphal" (3). This nucleus lies close to the floor of the aqueduct (1). On either side of the oculo-motor nucleus is the upper end of the medial longitudinal bundle, (4), penetrated by the root fibres of the oculo-motor nerve which run ventrally in discrete fasciculi (5). Note the red nucleus (6) on either side, the medial part of which is penetrated by the oculo-motor fasciculi.

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