838 



THE NERVOUS SYSTEM 



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

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

 apparatus: — two intrinsic, the sphincter iridis and the ciliary muscle, and five 

 extrinsic. Of the latter, the levator palpebrse superioris is of the upper eyelid, 

 while the remaining four, the superior, medial, 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 trochlear nerve. 



Practically continuous with that of the trochlear 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 Ues in the ventral part of the central grey 

 substance, and is very intimately associated with the medial longitudinal fasciculus. Its thickest 



Fig. 662. — Transverse Section Through Level of Superior Quadrigeminatb Bodies. 



Stratum zonale of thalamus 



Stratum zonale 



i Nucleus of superior colliculus 



Epiphysis (pineal 

 "~~" body) 



___ Central grey 

 "~~~ substance 



Medial genicu- 

 late body 



Lateral genicu- _ ^ 

 late body 



>*"_■«._ Optic-acoustic 

 reflex path 



Aquseductus 



cerebri 

 Nucleus of mes- 



j %, p. encephalic (de- 



\5d-V '^ A"^<>ni,\hl I Tvl-O :-v>, scending)root of 



masticator 

 nerve 

 Nucleus of 

 oculo-motor 

 nerve 



Thalamo-olivary ,.»-* 

 tract 



Red nucleu 



Medial longitu- 

 dinal fasciculus 



Interpeduncu- 

 lar fossa 



Cerebral _^ _ ^J 



peduncle 



Fila of oculomotor nerve 



Substantia nigra 



portion is beneath the summit of the superior quadrigeminate body. The root-fibres leave the 

 nucleus from its ventral side and 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 whicli blend to form the trunk of the nerve in the oculomotor 

 sulcus of the cerebral peduncles. Those bundles which arise from the more lateral portion of 

 the nucleus course in a series of curves through and around 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 lateralward 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 U> the opposite medial 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 dividtnl them into eight cell-groups. The nucleus may be more easily con- 

 sidered as composed of an inferior and a superior medial group. The inferior group consists of 

 a long lateral portion continuous with the nucleus of the trochlear nerve below, and a smaller 

 medial portion, situated in the medial plane and continuous across the mid-line with its fellow 

 of the ofjpositc side. The superior medial group consists of cells of smaller size than the 

 inferior, and is known as the nucleus of Edingcr and Wcstphal. It is believed to give origin to 



