932 



THE NERVOUS SYSTEM 



tinuous with the corresponding group of the opposite side and is common to the oculo-motor 

 nerves of both sides. 



It has been found, by the study of diseased conditions and by experiments with animals, 

 that the centres of innervation of the eye-muscles supplied by the nerve correspond to the above 

 divisions of both the superior and inferior group of cells into a medial and lateral series. The 

 relative position of the divisions of each group and the muscles they are thought to innervate 

 are shown in the following diagram devised by Starr: — 



As they leave their nucleus of origin in the mid-brain, the fibres of the oculo- 

 motor nerve form a series of fasciculi, which curve ventrally around and through 

 the red nucleus and the medial part of the substantia nigra, to the oculo-motor 

 sulcus on the medial surface of the cerebral peduncle, where they emerge in from 

 six to fifteen small bundles which pierce the pia mater and collect into the trunk 

 of the nerve. Immediately after its formation along the oculo-motor sulcus, the 

 trunk of the nerve passes between the posterior cerebral and the superior cere- 

 bellar arteries, and, running downward, forward, and laterally in the posterior 

 part of the cisterna basalis, it crosses the anterior part of the attached border of the 

 tentorium cerebelli at the side of the dorsum sellse, and, piercing the arachnoid 

 and the inner layer of the dura mater, it enters the wall of the cavernous sinus 

 al)out midwa}^ between the anterior and posterior clinoid processes. Immediately 

 after its entry into the wall of the sinus it lies at a higher level than the trochlear 

 nerve, but the latter soon crosses on its lateral side and gets above it, and directly 

 afterward the oculo-motor nerve divides into a smaller superior and a larger 

 inferior branch (fig. 734). Before its division communications join it from the 

 cavernous plexus of the sympathetic about the internal carotid artery, and from 

 the ophthalmic division of the trigeminus. Both branches proceed forward, and 

 the nasal branch of the trigeminus, which has passed upward, on the lateral side of 

 the inferior branch of the oculomotor lies between them. At the anterior end of 

 the cavernous sinus the two branches pass through the superior orbital (sphe- 

 noidal) fissure, between the heads of the lateral rectus muscle, and enter the or- 

 bital cavity. In the orbit, the superior branch lies between the superior rectus and 

 the optic nerve; it supplies the; sup(>rior rectus and then turns round the medial 

 border of that muscle and terminates in the levator palpebrte superioris. The 

 inferior branch runs forward, beneath the optic nerve, and divides into three 

 branches which supply the inferior and medial recti and the inferior oblique. 



The branch to the inferior oblicjue muscle is connected with the ciliary ganglion 

 by a short thick offset, tiie short root of the ciliary ganglion, by mediation of the 

 sympathetic neurones of wliicii the oculo-motor nerve scnids impulses to tlie ciliary 

 muscl(! and the sphincter muscle of the iris. The inferior brancli also gives some 

 small twigs to th(! inferior rectus. IMie branches of th(! o(;ulo-motor nerve, which 

 supf)!}^ the recti mus(-les, enter the musch's on their ocuhir surfaces, but the branch 

 to the inferior obliciuc muscle enters the posterior l)ortler of thjit muscle. 



