Section through the lower part of the mid-brain of a monkey, stained with iron haemotoxylin and neutral red. Above is seen the aqueduct (1). Below the aqueduct on either side of the mid-line is the nucleus of origin of the trochlear nerve (2). It is composed of large motor cells embedded in a plexus of myelinated fibres. Each trochlear nucleus rests on the dorsal aspect of the medial longitudinal bundle (3). The photograph is at a magnification of X 45.

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