MOTORES OCULORUM. 437 



of the thalamus opticus and from the nates. Proceeding from this 

 origin it winds around the cms cerebri as a flattened band, under the 

 name of tractus opticus, and joins with its fellow in front of the tuber 

 cinereum to form the optic commissure (chiasma). The tractus opticus 

 is united with the crus cerebri and tuber cinereum, and is covered 

 in by the pia mater ; the commissure is also connected with the tuber 

 cinereum, from which it receives fibres, and the nerve beyond the 

 commissure diverges from its fellow, becomes rounded in form, and 

 is enclosed in a sheath derived from the arachnoid. In passing 

 through the optic foramen the optic nerve receives a sheath from the 

 dura mater, which splits at this point into two layers ; one, which 

 becomes the periosteum of the orbit ; the other, the one in question, 

 which forms the sheath for the nerve, and is lost in the sclerotic coat 

 of the eyeball. After a short course within the orbit the optic nerve 

 pierces the sclerotic and choroid coats and expands into the nervous 

 membrane of the eyeball, the retina. Near the globe, the nerve 

 is pierced by a small artery, the arteria centralis retinae, which runs 

 through the central axis of the nerve and reaches the internal surface 

 of the retina, to which it distributes branches. 



The commissure rests upon the processus olivaris of the sphenoid 

 bone ; it is bounded by the lamina cinerea of the corpus callosum in 

 front, by the substantia perforata on each side, and by the tuber 

 cinereum behind. Within the commissure the innermost fibres of 

 the optic nerves cross each other to pass to opposite eyes, while the 

 outer fibres continue their course uninterruptedly to the eye of the 

 corresponding side. The neurilemma of the commissure, as well as 

 that of the nerves, is formed by the pia mater. 



THIRD PAIR. MOTORES OCULORUM. The motor oculi, a nerve 

 of moderate size, arises from the inner side of the crus crebri, close to 

 the pons Varolii, and passes forward between the posterior cerebral 

 and superior cerebellar artery. It pierces the dura mater imme- 

 diately in front of the posterior clinoid process ; descends obliquely 



chium anterius of the corpora quadrigemina. a. Their brachium posterius. b. 

 The origin of the fourth nerve, which may be seen descending over the crus 

 cerebri. c. The processus e cerebello ad testem, or superior peduncle of the 

 cerebellum, d. The band of fibres termed laqueus, the superior division of the 

 fasciculus olivaris crossing the superior peduncle of the cerebellum to enter the 

 corpora quadrigemina. Through the small triangular space in front of this 

 band, crossed by the fourth nerve, some of the fibres of the superior peduncle 

 of the cerebellum may be seen. e. The superior portion of the crus cerebri, 

 termed tegmentum. /. Its inferior portion, g. The third nerve, h. The 

 pons Varolii. i. The crus cerebelli, or middle peduncle of the cerebellum, k. 

 The inferior peduncle derived from the corpus restiforme. The mass lying in 

 the angular interval upon these is the superior peduncle. /. The fifth nerve 

 issuing from between the transverse fasciculi of the pons Varolii. m. The sixth 

 nerve, n. The seventh nerve; the inferior and smaller cord is the facial 

 nerve, the superior and larger the auditory, o. The corpus olivare crossed in- 

 feriorly by the superficial arciform fibres, p. The corpus pyramidale. </. The 

 median posterior fasciculi of the medulla oblongata. r. The corpus restiforme. 

 s. The spinal cord, t. The fourth ventricle. 



