SIS 



THE NERVOUS SYSTEM 



in the superior portion of the anterior medullary velum. In 'this it meets and under- 

 goes a total decussation with the root of its fellow nerve, and then emerges at the 

 inner margin of the superior cerebellar peduncle of the opposite side. Having 

 emerged, it then passes ventrally around the cerebral peduncle, and thence pursues 

 its course to the superior oblique muscle of the eye. 



The oculomotor or third nerve, like the trochlear, is purely motor. It is the 

 largest of the eye-muscle nerves. 1 1 supplies in all seven muscles of the optic appara- 

 tus: two intrinsic, the sphincter iridis and the ciliary muscle, and five extrinsic. 

 Of the latter, the levator palpebnv superioris is of the upper eyelid, while the re- 

 maining four, the superior, internal, and inferior recti and the obliquus inferior, 

 are attached to the bulb of the eye. As is to be expected, its nucleus of origin is 

 larger and much more complicated than that of the troehlear nerve. Practically 

 continuous with that of the troehlear below, the nucleus is 5 or 6 mm. in length and 

 extends anteriorly a short distance beyond the bounds of the mesencephalon into 

 the grey substance by the side of the third ventricle. It lies in the ventral part of 

 the central grey substance, and is very intimately associated with the medial lon- 

 gitudinal fasciculus. Its thickest portion is beneath the summit of the superior 

 quadrigeminate body. The root-fibres leave the nucleus from its ventral side and 



FIG. 610. DIAGRAMS SHOWING THE COURSE OF ORIGIN OF THE TROCHLEAR NERVES. (Stilling.) 

 The upper figure shows roughly the entire central course of the fourth nerves ; the lower rep- 

 resents their region of exit in transverse section. 



IV 



AQU^DUCTUS CEREBRl 



..." OF 

 TROCHLEAR NERVE 



MEDIAL LONGITUDINAL 

 FASCICULI X 



RAPIIE 



A Q r.v.D i < 7 r.v < -F.REBRI 



111!. 1 1 '/HUM 



i'i>.\.ir.\i-ri\-u.v 



LATERAL l.KMMXCUX 



collect into bundles which pass through the medial longitudinal fasciculus and 

 course ventrally to the mesial portion of the substantia nigra, where they emerge in 

 from six to fifteen rootlets which blend to form the trunk of the nerve in the oculo- 

 motor sulcus. Those bundles which arise from the more lateral portion of the nucleus 

 course in a series of curves through the substance of the red nucleus below and, in the 

 substantia nigra, join those which pursue the more direct course. The trunk thus 

 assembled passes outwards around the mesial border of the cerebral peduncle. 



A portion of the fibres of the oculomotor nerve upon leaving the nucleus decussate 

 in the tegmentum immediately below and pass into the nerve of the opposite side, 

 in which they are believed to be distributed to the opposite internal rectus muscle. 

 The cells of the nucleus have been variously grouped and subdivided with reference 

 to the different muscles supplied by the nerve. Perlia has divided them into eight 

 cell-groups. The nucleus may be more easily considered as composed cf an inferior 

 and a superior group. The inferior group consists of a long lateral portion continuous 

 with the nucleus of the troehlear nerve below, and a smaller medial portion, situated 

 in the medial plane and continuous across the mid-line with its fellow of the opposite 

 side. The superior group consists of cells of smaller size than the inferior, and is 

 known as the nucleus of Edinger and Westphal. It is believed to give origin to the 



