MOTOR OCULI COMMUNIS (THIRD NERVE). 611 



1. Falling of the upper eyelid, or blepharoptosis. 



2. External strabismus, immobility of the eye (except in an outward direction), ina- 

 bility to rotate the eye on its antero-posterior axis in certain directions, with slight pro- 

 trusion of the eye-ball. 



3. Dilatation of the pupil, with a certain amount 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 ani- 

 mal in which the nerve has been divided cannot raise the lid, but can approximate 

 the lids more closely, by a voluntary effort. In the human subject, the falling of the lid 

 gives to the face a very peculiar and characteristic expression. The complete loss of 

 power shows that the levator palpebrss superioris muscle depends upon the third nerve 

 entirely for its motor filaments. In pathology, external strabismus is very frequently 

 observed without falling of the lid, the filament distributed to the levator muscle not 

 being affected. 



The external strabismus and the immobility of the eyeball except in an outward direc- 

 tion are due to paralysis of the internal, superior, and inferior recti muscles, the external 

 rectus acting without its antagonist. This condition requires no farther explanation. 

 These points are well 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 out- 

 ward, the globe remains motionless. 



It is somewhat difficult to note the effects of paralysis of the inferior oblique muscle, 

 which is also supplied by the third nerve. This muscle, acting 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, having no antagonist, rotates the globe upward and inward, directing the pupil 

 downward and outward. The action of the oblique muscles is observed when we move 

 the head alternately toward one shoulder and the other. In the human subject, when 

 the inferior oblique muscle on one side is paralyzed, the eye cannot move in a direction 

 opposite to the movements of the head, as it does upon the sound side, so as to keep the 

 pupil fixed, and the patient has double vision. 



When all the muscles of the eyeball, except the external rectus and superior oblique, 

 are paralyzed, as they are by section of the third nerve, the globe is slightly protruded, 

 simply by the relaxation of most of its muscles. An opposite action is easily observed 

 in a cat with the facial nerve divided, so that it cannot close the lids. When the cornea 

 is touched, all of the muscles, particularly the four recti, act to draw the globe into the 

 orbit, which allows the lid to fall slightly, and projects the little membrane which serves 

 as a third eyelid in these animals. 



Observations with regard to the influence of the third nerve upon the movements of 

 the iris have not been so satisfactory in their results as those relating to the muscles of the 

 eyeball. It will be remembered that this nerve sends a filament to the ophthalmic gan- 

 glion of the sympathetic, and that, from this ganglion, the short ciliary nerves take their 

 origin and pass to the iris. The ganglia of the sympathetic system receive branches both 

 from motor and sensory nerves belonging to the cerebro-spinal system, and the ophthal- 

 mic ganglion is no exception to this rule. While it is undoubtedly true that division of 

 the third nerve affects the movements of the iris, it becomes a question whether this be 

 a direct influence, or an influence exerted primarily upon the ganglion, not, perhaps, 

 differing from the general effects upon the sympathetic ganglia that follow destruction of 

 their branches of communication with the motor nerves. 



The most important experimental observations with regard to the influence of the 



