MOTOR OCULI COMMUNIS 497 



the iris. In its course this nerve receives a few delicate filaments from the 

 cavernous plexus of the sympathetic and a branch from the ophthalmic 

 division of the trifacial. 



Properties and Uses of the Motor Oculi Communis. Stimulation of 

 the root of the third nerve in a living animal produces contraction of the 

 muscles to which it is distributed, but no pain. If the stimulus, however, 

 is applied a little farther on in the course of the nerve, there are evi- 

 dences of sensibility ; and this is readily explained by its communica- 

 tions with the ophthalmic branch of the trifacial. At its root, therefore, 

 this nerve is exclusively motor and its office is connected with the action 

 of muscles. 



The phenomena observed after section of the motor oculi communis 

 in living animals are the following : 



1. Falling of the upper eyelid, or blepharoptosis. 



2. External strabismus, immobility of the eye except in an outward 

 direction, inability to rotate the eye on its antero-posterior axis in certain 

 directions and slight protrusion of the eyeball. 



3. Dilatation of the pupil, with a certain degree of interference with 

 the movements of the iris. 



The falling of the upper eyelid is constantly observed after division 

 of the third nerve in living animals and always follows its complete paral- 

 ysis in the human subject. An animal in which the nerve has been 

 divided can not raise the lid but can press the lids together by a volun- 

 tary effort. In the human subject the falling of the lid gives to the face 

 a peculiar and characteristic expression. The complete loss of power 

 shows that the levator palpebrae superioris muscle depends on the third 

 nerve entirely for its motor filaments. 



The external strabismus and the immobility of the eyeball except in 

 an outward direction are due to paralysis of the internal, superior and 

 inferior recti muscles, the external rectus acting without its antagonist. 

 This condition requires no further explanation. These points are 

 illustrated by the experiment of dividing the nerve in rabbits. If the 

 head of the animal is turned inward, on exposing the eye to a bright 

 light, the globe will turn outward by the action of the external rectus ; 

 but if the head is turned outward, the globe remains motionless. 



It is somewhat difficult to note the effects of paralysis of the inferior 

 oblique muscle, which also is supplied by the third nerve. This muscle, 

 acting from its origin at the inferior and internal part of the circum- 

 ference of the base of the orbit to its attachment at the inferior and 

 external part of the posterior hemisphere of the eyeball, gives to the 

 globe a movement of rotation on an oblique horizontal axis, downward 

 and backward, directing the pupil upward and outward. When this 



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