The oculomotor nerve arises in the nucleus nervi oculomotorii, which lies in the region of the superior colliculus, ventral to the aquaeductus Sylvii, within the floor of the central gray substance (Figs. 88 and 89). The nucleus consists of a medially placed medial nucleus and a pair of large-celled lateral nuclei. The nerve conveys fibres which originate within the me dial nucleus and the lateral nucleus of the same and, in part, of the opposite side. The fibres pass ventrally in laterally convex curves and emerge from the brain-stem along the sulcus nervi oculomotorii on the medial surface of the pedunculus cerebri. The voluntary innervation of the nucleus proceeds, as in the case of all the motor cere bral nerves, from the cerebral cortex. The entire path of conduction includes: a. The central neuroneŚcerebral cortex to nucleus; b. The peripheral neurone Ś nucleus, peripheral nerve, muscle.
It must be pointed out, however, that the course of the central path is not yet known; probably the path is composed of several neurones. Likewise, uncertainty exists regarding the location of the cortical centres, which have been variously assumed as lying within the gyrus angularis, the occipital lobe or the frontal lobe. The centre for voluntary eye-movements, however, is quite
generally regarded as including the posterior part of the second or middle frontal convolution. Investigations have shown, that the entire oculomotor nucleus is made up of certain groups of cells, of which a particular group always gives origin to the fibres for a particular muscle. Con cerning these special subdivisions of the nucleus, however, we shall not enter more fully, for the reason that these relations, as yet, have been by no means definitely established. Fig. 162 represents the individual cell-groups, according to the investigations of Bernheimer on monkeys. In the middle is the medial chief nucleus, on either side the lateral chief nucleus with its various subdivisions, and, medial from the latter, the small lateral nucleus, which is also known as the nucleus of