THE OCULO-MOTOR NERVES 



931 



medial and lower aspect by the central artery of the retina. As it enters the optic foramen to 

 become continuous with the intra-osseous part, it is in close relation with the ligaments of Lock- 

 wood and Zinn (annulus tendineus communis) and with the four recti muscles which arise from 

 them. 



The intra-osseous portion is from six to seven millimetres long. It lies be- 

 tween the roots of the small wing of the sphenoid and the body of that bone, and it 

 is in relation below and laterally with the ophthalmic artery. 



The intra-cranial portion, which is from ten to twelve millimetres long, runs 

 backward and medialward, beneath the posterior end of the olfactory tract, and 

 above the ophthalmic artery, the medial border of the internal carotid artery 

 and the diaphragma sellse to the chiasma. From the chiasma to the central 

 connections of the nerve, the path is known as the optic tract. 



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

 considered with the optic chiasma and the optic tract (see p. 849). 



Fig. 732. — Nerves of the Nasal Cavity. 



Nasal branch 

 of ethmoidal Olfactory 

 Frontal sinus nerve nerve plexus 



Superior nasal concha 

 Sphenoidal sinus 



j > B "&% g Vidian nerve 



__^ Spheno-palatine 



>^?i ganglion 

 "7^ — Palatine nerves 



Orifice of Eustachian 

 tube 

 Nasal branches 



— ', Posterior palatine 



i 



J — Anterior palatine 



Middle palatine 



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 outward along it to the posterior part of the sclera. 



THE OCULO-MOTOR NERVES 



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

 pHes seven muscles connected with the eye, two of which, the sphincter of the iris 

 and cihary muscle, are within the ocular bulb. The remaining five are in the orbi- 

 tal cavity, and four of them — the superior, inferior, and medial recti and the 

 inferior oblique — are attached to the bulb, while the fifth, the levator palpebrae 

 superioris, is inserted into the upper eyelid. 



The fibres of the oculo-motor nerve spring from their nucleus of origin situated in the grey 

 substance of the floor of the cerebral aqusduct in the region of the superior quadrigeminate 

 body (fig. 662). The cells of this nucleus are divided into two main groups, a superior and an 

 inferior (fig. 663). 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 five secondary nuclei, according to the eye-muscles the cells of each group innervate. 

 Three of the five He lateral to the others and somewhat dorsally, and of the remaining two, 

 which are placed more medially, one encroaches upon the mid-line {nucleus medialis) and is con- 



