THE OCULOMOTOR NERVE. 1225 



its posterior end the tract exhibits a furrow that indicates a subdivision into a mesial 

 and a lateral root (Fig. 915). The latter, the visual portion of the optic tract, is 

 traceable into the prominent overhanging pulvinar of the optic thalamus, the ill- 

 defined lateral geniculate body and, by means of the superior brachium, into the supe- 

 rior quadrigeminal body. The mesial root, on the other hand, contains the fibres 

 forming Gudden's commissure (page mo) and is related to the distinct median 

 geniculate body and, by the inferior brachium, to the inferior quadrigeminal body. 



Central and Cortical Connections. Arising as axones of the retinal neurones, the optic 

 nerve-fibres are continued backward through the commissure and tract and end in relation with 

 the neurones of the primary centres situated in the pulvinar, the lateral geniculate and the 

 superior quadrigeminal body. It is, however, within the lateral geniculate body that the greater 

 number (80 per cent, according to Monakow) of the visual fibres terminate, relatively few pass- 

 ing to the pulvinar and the superior quadrigeminal body (Spiller). The cortical connections are 

 established by fibres which pass from the cells of these primary centres and, as the optic radia- 

 tion (page 1123), sweep outward and backward into the occipital lobe to end in the cortex of the 

 cuneus in the vicinity of the calcarine fissure. It is probable that a limited number of retinal 

 fibres pass directly to the cerebral cortex without interruption in the primary centres. In addi- 

 tion to the centripetal paths just mentioned, fibres arise from the cortical cells of the cuneus 

 and, sharing the optic radiation, pass as efferent tracts which not only terminate in the lateral 

 geniculate and quadrigeminal bodies, but also establish indirect relations with the nucleus of the 

 oculomotor nerve. The ultimate distribution and influence of the impressions of sight are very 

 complex and far reaching, such impressions being capable of affecting numerous motor and 

 sensory centres. 



The exact path by which pupillary impulses reach the oculomotor nucleus is uncertain and 

 perhaps two-fold. It may be assumed, however, that if they proceed by way of the superior 

 quadrigeminal body, the optic fibres are not directly continued to the nucleus of the third nerve, 

 but end within the superior colliculus, from whose neurones the immediate connecting links pro- 

 ceed to the oculomotor nucleus. Accumulating evidence points to the existence of a more 

 remote special centre for pupillary reflexes within the lower part of the medulla ; in such case the 

 oculomotor nucleus is, perhaps, influenced by impulses which pass from the medullary centre 

 upward by way of the posterior longitudinal fasciculus (Bach). 



Practical Considerations. The cranial nerves of the eye will be discussed 

 in connection with that organ. 



THE OCULOMOTOR NERVE. 



The third or oculomotor nerve (n. oculomotorius), the chief motor nerve of the 

 intrinsic and extrinsic muscles of the eyeball, supplies branches to all the extraocular 

 muscles, with the exception of the external rectus and superior oblique, as well as 

 fibres to the sphincter pupillae and the ciliary muscle within the eyeball. 



Its deep origin is from the ocidomotor nucleus situated medially and deeply 

 within the gray matter of the floor of the Sylvian aqueduct, in close relation with the 

 dorsal surface of the posterior longitudinal fasciculus (Fig. 963). 



The nucleus is from 6-8 mm. in length and extends from opposite the upper end to the 

 caudal pole of the superior quadrigeminal bodies. Below, its posterior end comes almost into 

 contact with the nucleus of the fourth nerve, but is separated from it by a narrow interval. In 

 its entirety the oculomotor nucleus includes a number of more or less distinct cell-groups, 

 which vary in importance as well as in their individual prominence. Of these the most impor- 

 tant and constant are two long columns of cells, the chief nuclei, that extend, one on each side, 

 along the dorsal surface of the posterior longitudinal fasciculi. Each nucleus tapers slightly 

 towards either end and consists of two fairly distinct subdivisions which, from their relative 

 positions, are termed the dorsal and the ventral cell-group. The component nerve-cells include 

 those of large, medium and small size, the large multipolar ones (from .04O-.045 mm. in diam- 

 eter) probably being the elements from which the root-fibres of the third nerve arise. Dislo- 

 cated portions of the chief nucleus are seen as small groups of nerve-cells that lie scattered 

 among or even beneath the fibres of the posterior longitudinal bundle. 



Dorsal to the chief nucleus and partially overlying its postero-median surface is the taper- 

 ing column of small nerve-cells known as the Edinger-Westphal nucleus. This tract, much 

 more bulky above than below (Tsuchida), exhibits a subdivision into a dorso-lateral and a 

 ventro-median portion, which, however, are fused in the superior pole of the nucleus. The 



