524 THE CRANIAL NERVES. 



that the eye can no longer be opened to its full extent ; although it can 

 still be closed as usual by the action of the orbicularis oculi, which 

 does not depend upon the oculomotorius, but is animated by branches 

 derived from the seventh pair, or facial nerve. The superior eyelid 

 therefore droops, resting by. its own weight in such a position as to 

 cover the upper portion of the cornea, and the greater part or even the 

 whole of the pupil. In man this condition of the eyelid is known as 

 ptosis, and is one of the consequences following paralysis of the oculo- 

 motorius nerve. 



The influence of the oculomotorius upon the contractile movements of 

 the iris is important, though less distinct and uniform in its action, as 

 shown by experiment, than that exerted upon movements of the eyeball 

 itself. The connection of the oculomotorius with the muscular appa- 

 ratus of the iris is not a direct one, but takes place through the inter- 

 vention of the ophthalmic ganglion, to which this nerve sends a commu- 

 nicating motor branch, and which in turn gives off the ciliary nerves 

 destined for the iris. Some observers (Herbert Mayo, Longet) have 

 found well-marked paralysis of the iris following division of the oculomo- 

 torius nerve, and enumerate, as consequences of this injury, a permanent 

 dilatation and immobility of the pupil. In the experiments of Longet, 

 which were performed on dogs, rabbits, 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 upon the operated side. Bernard has also found 

 that division of the oculomotorius is followed, in the rabbit, by dilata- 

 tion of the pupil, and that in the operated e}^e the iris contracts only 

 very slowly and imperfectly under the influence of light. It is not, 

 however, completely paralyzed, since it may still move with considera- 

 ble promptitude under the influence of painful impressions conveyed by 

 the fifth pair. The action of the oculomotorius upon the pupil, there- 

 fore, is energetic and constant in the ordinary reflex movement of con- 

 traction under the stimulus of light ; but it takes place through the 

 ophthalmic ganglion, to which it communicates, in a certain degree, its 

 motive power. 



Fourth Pair. The Patheticus. 



This nerve presents a variety of peculiarities, which have always, 

 notwithstanding its minute size, attracted to it more or less special 

 attention. It is distributed exclusively to the superior oblique muscle 

 of the eyeball ; its name having been derived from the mistaken idea 

 that this muscle turned the e} r e upward and inward. The two oblique 

 muscles, 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 two eyeballs in the same position. 

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



