I EURO LOGY 





The Optic Tract (Fig. 774), passes backward and outward from the optic chiasma 

 over the tuber cinereum and anterior perforated space to the cerebral peduncle 

 and winds obliquely across its under surface. Its fibers terminate in the lateral 

 geniculate body, the pulvinar and the superior colliculus. It is adherent to the 

 tuber cinereum and the cerebral peduncle as it passes over them. In the region of 

 the lateral geniculate body it splits into two bands. The medial and smaller one is 

 a part of the commissure of Gudden and ends in the medial geniculate body.' 



From its mode of development, and from its structure, the optic nerve must be regarded as a 

 prolongation of the brain substance, rather than as an ordinary cerebrospinal nerve. As it 

 passes from the brain it receives sheaths from the three cerebral membranes, a perineural 

 sheath from the pia mater, an intermediate sheath from the arachnoid, and an outer sheath 

 from the dura mater, which is also connected with the periosteum as it passes through the 

 optic foramen. These sheaths are separated from each other by cavities which communicate 

 with the subdural and subarachnoid cavities respectively. The innermost or perineural sheath 

 sends a process around the arteria centralis retina; into the interior of the nerve, and enters 

 intimately into its structure. 



THE OCULOMOTOR NERVE (N. OCULOMOTORIUS ; THIRD NERVE) 



(Figs. 775, 776, 777). 



The oculomotor nerve supplies somatic motor fibers to all the ocular muscles, 

 except the Obliquus superior and Rectus lateralis; it also supplies through its 

 connections with the ciliary ganglion, sympathetic motor fibers to the Sphincter 

 pupillse and the Ciliaris muscles. 



LEVATOR PALPEBRjC 



RECTUS SUPERIOR 



Ciliary ganglion 



Short ciliary 



? 



RECTUS MEDIALIS 

 RECTUS INFERIOR 

 OBLIQUUS INFERIOR 



FIG. 775. Plan of oculomotor nerve. 



The fibers of the oculomotor nerve arise from a nucleus which lies in the gray 

 substance of the floor of the cerebral aqueduct and extends in front of the aqueduct 

 for a short distance into the floor of the third ventricle. From this nucleus the 

 fibers pass forward through the tegmentum, the red nucleus, and the medial part 

 of the substantia nigra, forming a series of curves with a lateral convexity, and 

 emerge from the oculomotor sulcus on the medial side of the cerebral peduncle. 



The nucleus of the oculomotor nerve does not consist of a continuous column 

 of cells, but is broken up into a number of smaller nuclei, which are arranged in 

 two groups, anterior and posterior. Those of the posterior group are six in number, 

 five of which are symmetrical on the two sides of the middle line, while the sixth 

 is centrally placed and is common to the nerves of both sides. The anterior group 

 consists of two nuclei, an antero-medial and an antero-lateral (Fig. 762). 



The nucleus of the oculomotor nerve, considered from a physiological standpoint, 

 can be subdivided into several smaller groups of cells, each group controlling a 

 particular muscle. 



On emerging from the brain, the nerve is invested with a sheath of pia mater, 

 and enclosed in a prolongation from the arachnoid. It passes between the superior 

 cerebellar and posterior cerebral arteries, and then pierces the dura mater in front 



