58 THE NERVOUS SYSTEM 



principal movement is complicated by a deviation medially. 

 This is counter-balanced by the action of the superior 

 oblique, which, acting through a fibrous pulley (Fig. 33), rotates 

 the eye so as to make the pupil look downwards and laterally. 

 Pure downward rotation is obtained when the two muscles 

 act together. The inferior rectus is supplied by the third and 

 the superior oblique by the fourth cerebral nerve. 



The lateral and the medial recti are applied, respectively, to 

 the lateral and the medial aspects of the eyeball. The former is 

 a pure abductor of the eye, i.e., it rotates the eyeball so as to 

 make the pupil look laterally. It is supplied by the abducent 

 nerve. The medial rectus rotates the eyeball in the opposite 

 direction. Its nerve-supply is derived from the oculo-motor. 



The intrinsic muscles of the eye are supplied directly from 

 the ciliary ganglion (p. 66). The sphincter of the pupil and 

 the ciliary muscle (the muscle of accommodation) are inner- 

 vated primarily from the oculo-motor nerve, while the dilator 

 of the pupil gains its supply from the sympathetic system 

 (p. 187). 



PARALYSIS OF THE ORBITAL MUSCLES 



The third, fourth and sixth cerebral nerves are frequently 

 all affected by the same lesion, since they follow the same 

 intra-cranial course after they emerge from the brain-stem, and 

 their upper neurones are closely related in the internal capsule, 

 the corona radiata and the cortex. 



SUPRA-NUCLEAR LESIONS, unless they are bilateral, rarely 

 produce complete paralysis of any of the muscles of the orbit. 

 Under normal conditions, the movements of the two eyes are 

 always associated. Thus, except in the comparatively rare 

 movement of convergence, the lateral rectus always works in 

 association with the medial rectus of the opposite side. In 

 order that perfect harmony may be obtained, both these 

 muscles are bilaterally represented in the cortex. The arrange- 

 ment is illustrated in Fig. 20, where it is seen that each 

 nucleus receives fibres from the cortex on both sides. Further, 



