542 NERVOUS SYSTEM. 



thick filament to the lenticular, or ophthalmic ganglion of the sympathetic. 

 It is this branch which is supposed, through the short ciliary nerves passing 

 from the lenticular ganglion, to furnish the motor influence to the iris. In 

 its course this nerve receives a few very 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, be applied a 

 little farther on in the course of the nerve, there are evidences of sensibility ; 

 and this is readily explained by its communications with the ophthalmic 

 branch of the trifacial. At its root, therefore, this nerve is exclusively motor, 

 and its office is connected entirely with the action of muscles. 



The phenomena which are observed after section of the motor oculi com- 

 munis 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 di- 

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

 directions, with 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 paralysis 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 voluntary effort. In the 

 human subject the falling of the lid gives to the face a peculiar and char- 

 acteristic expression. The complete loss of power shows that the levator 

 palpebrae superioris muscle depends upon the third nerve entirely for its mo- 

 tor filaments. In pathology, external strabismus is frequently observed with- 

 out falling of the lid, the filaments distributed to the levator muscle not be- 

 ing affected. 



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 condi- 

 tion requires no farther explanation. These points are illustrated by the 

 experiment of dividing the nerve in rabbits. If the head of the animal be 

 turned inward, exposing the eye to a bright light, the globe will turn outward, 

 by the action of the external rectus ; but if the head be 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, act- 

 ing from its origin at the inferior and internal part of the circumference 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 muscle is paralyzed, the superior oblique, 



