498 NERVOUS SYSTEM 



muscle is paralyzed, the superior oblique, having no antagonist, rotates 

 the globe upward and inward, directing the pupil downward and out- 

 ward. The action of the oblique muscles is observed when the head is 

 moved alternately toward one shoulder and the other. In the human 

 subject, when the inferior oblique muscle on one side is paralyzed, the 

 eye can not 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 there is 

 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 can not close the lids. When the cornea is touched, 

 all 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. 



The third nerve sends a filament to the ophthalmic ganglion of the 

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

 origin and pass to the iris. While it is 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 on the ganglion, 

 not, perhaps, differing from the general effects on the sympathetic gan- 

 glia that follow destruction of their branches of communication with the 

 motor nerves. 



The third nerve animates the muscular fibres that contract the pupil, 

 the contraction produced by stimulation of the optic nerves being reflex 

 in its character. The reflex action by which the iris is contracted is not 

 instantaneous, like most of the analogous phenomena observed in the 

 cerebro-spinal system, and its operations are rather characteristic of the 

 action of the sympathetic system and non-striated muscular tissue. It 

 has been found, also, that the pupil is not immediately dilated after 

 division of the third nerve. Several hours after the operation, however, 

 the pupil usually is found dilated, and it may slowly contract when the 

 eye is exposed to the light. 



Experiments have shown, also, that although the pupil contracts 

 feebly and slowly under the stimulus of light after division of the motor 

 oculi, it will dilate under the influence of belladonna and can be made 

 to contract by operating on other nerves. It is well known, for example, 

 that division or stimulation of the fifth nerve produces contraction of 

 the pupil. This takes place after as well as before division of the third 

 nerve. Section of the sympathetic in the cervical region also contracts 

 the pupil, and this occurs after paralysis of the motor oculi. These 



