777 K OCULO-MOTOK 



961 



tween tin- roote of the small wing of the sphenoid and the body of tl Kit bone, and it lain 



relat ion i ii 'low ;i nil laterally with the ophthalmic artery. 



The intra-cranial portion, which is fn>m ten to twelve millimetres long, runs 

 backwards and medianwards. beneath the posterior part of the olfactory tract, and 

 above the ophthalmic artery, the inner bonier of the internal carotid artery, and the 



diaphragms M-lla\ 



Central Connections. The central connections of the fibres of the optic nerve have been 

 considered with the optic cliiusnia and the optic tract (see p. S~i >. 



The Sheaths of the Optic Nerve. The optic nerve receives a sheath from 



each of the membranes of the brain, and prolongations of the subdural and sub- 

 arachnoid cavities also pass outwards along it to the posterior part of the sclera 

 (fig. 658). 



THE THIRD PAIR THE OCULO-MOTOR NERVES 



The third cranial nerve or oculo-motor is a purely motor nerve. Each supplies 

 seven muscles c< mnected with the eye, two of which, the sphincter of the iris and ciliary 

 muscle, are within the bulbus oculi. The remaining five are in the orbital cavity, 

 and four of them the superior, inferior, and internal recti and the inferior oblique 

 are attached to the bulbus, while the fifth, the levator palpebra) superioris, is inserted 

 into the upper eyelid. 



The fibres < if each t hird nerve spring from their nucleus of origin situated in the grey 

 substance of the floor of the aqua-ductus cerebri in the region of the superior quadri- 

 geminatebody (fig. 611). The cells of this nucleus are divided into t wo main groups, a 

 superior and an inferior (fig. 612). The superior group includes two nuclei, a medial 

 and a lateral. The latter, besides being lateral, is also somewhat dorsal to the former. 

 The inferior group has been divided into six secondary nuclei, according to the eye- 

 muscles the cells of each group innervate. Two of the six lie lateral to the others and 

 somewhat dorsally. and of the remaining four, which are placed more medially, one 

 lies in the mid-line (nucleus medialis) 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 thev are thought to innervate are shown in the following diagram 

 devised by Starr: 



Mesial Plane. 



61 



