THE CRANIAL NERVES. 



the eye, it is the upper eyelid alone which moves, being rafct 

 uncover the cornea and pupil, by the levator palpebraB superioris. 

 this muscle is animated by a branch of the oculomotorius, it is paralyzed 

 by section of this nerve at the same time with the muscles moving the 

 eyeball. The consequence is that the eye can no longer be fully opened ; 

 though it can be closed as usual by the action of the orbicularis oculi, 

 which is- supplied from the seventh pair. The eyelid therefore droops, 

 resting in such a position as to cover the upper portion of the cornea, 

 and the greater part or even the whole of the pupil. This condition is 

 known as ptosis, and is one of the consequences following paralysis of 

 the oculomotorius nerve. 



The influence of the oculomotorius on the contractile movements of 

 the iris is important, though less distinct and uniform than that 

 exerted on the movements of the eyeball. The connection of the 

 oculomotorius with the muscular apparatus of the iris is indirect, tak- 

 ing place through the intervention of the ophthalmic ganglion, to which 

 it sends a motor branch, and which in turn gives off the ciliary nerves 

 for the iris. Some observers (Mayo, Longet) have found well-marked 

 paralysis of the iris following division of the oculomotorius nerve, and 

 enumerate, as consequences of this injury, permanent dilatation and 

 immobility of the pupil. In the experiments of Longet, on dogs, rab- 

 bits, and pigeons, irritation of the cephalic extremity of the optic nerve 

 caused contraction of the pupil in both eyes ; but after division of the 

 oculomotorius nerve the effect was no longer produced on the operated 

 side. Bernard has also found that division of the oculomotorius is fol- 

 lowed, in the rabbit, by dilatation of the pupil, and that in the operated 

 eye the iris contracts only very slowly and imperfectly under the influ- 

 ence of light. It is not, however, completely paralyzed, since it may 

 still move with considerable promptitude under the influence of painful 

 impressions conveyed by the fifth pair. The action of the oculomo- 

 torius on the pupil, therefore, is energetic and constant in the ordinary 

 reflex movement of contraction under the stimulus of light ; but it 

 takes place through the ophthalmic ganglion, to which it communi- 

 cates, in a certain degree, its motive power. 



Fourth Pair. The Patheticns. 



This nerve presents certain peculiarities, which, notwithstanding its 

 minute size, have attracted to it special attention. It is distributed ' 

 exclusively to the superior oblique muscle of the eyeball ; its name hav- 

 ing been derived from the erroneous idea that this muscle turned the 

 eye upward and inward. Both the superior and inferior oblique mus- 

 cles, however, have been fully shown to cause in the eyeball a nearly 

 simple movement of rotation about its longitudinal axis. They are 

 antagonistic to each other ; and by their contraction and relaxation, 

 during movements of inclination of the head from side to side, they 

 maintain the horizontal planes of the eyeballs in the same position. 

 If this parallelism were not preserved, objects would appear to stand in 



