MOTOR OCULI COMMUNIS. 543 



having no antagonist, rotates the globe upward and inward, directing the pupil 

 downward and outward. 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 the patient has 

 double vision. 



When all the muscles of the eyeball, except the external rectus and supe- 

 rior 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 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. 



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

 pathetic, and from this ganglion, the short ciliary nerves take their origin, 

 and pass to the iris. 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. 



Herbert Mayo (1823) made experiments on thirty pigeons, living or just 

 killed, upon the action of the optic, the third and the fifth nerves, on the 

 iris. When the third nerves were divided in the cranial cavity in a living 

 pigeon, the pupils became fully dilated and did not contract on the admission 

 of intense light; and when the same nerves were pinched in the living or 

 dead bird, the pupils were contracted for an instant on each stimulation of 

 the nerves. The same results followed division or stimulation of the optic 

 nerves, under similar conditions ; but when the third nerves had been 

 divided, no change in the pupil ensued upon stimulating the entire or 

 divided optic nerves. 



The third nerves animate the muscular fibres that contract the pupil, the 

 contraction produced by stimulation of the optic nerves being reflex in its 

 character. Longet divided the motor oculi and the optic nerve upon the right 

 side. He found that stimulation of the central end of the divided optic 

 nerve produced no movement of the pupil of the side upon which the motor 

 oculi had been divided, but caused contraction of the iris upon the opposite 

 side. This, taken in connection with the fact that in amaurosis affecting one 

 eye, the iris upon the affected side will not contract under the stimulus of 

 light applied to the same eye, but will act when the uninjured eye is exposed 

 to the light, farther illustrates the reflex action which takes . place through 

 these nerves. 



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 



