Q12 NERVOUS SYSTEM. 



third nerve upon the iris are the following : Herbert Mayo made experiments on thirty 

 pigeons, living or just killed, upon the action of the optic, the third, and the fifth nerves 

 on the iris. He states that, when the third nerves are divided in the cranial cavity in a liv- 

 ing pigeon, the pupils become fully dilated and do not contract on the admission of intense 

 light ; and, when the same nerves are pinched in the living or dead bird, the pupils are 

 contracted for an instant on each stimulation of the nerves. The same results follow 

 division or irritation of the optic nerves under similar conditions ; but, when the third 

 nerves have been divided, no change in the pupil ensues upon irritating the entire or 

 divided optic nerves. 



The above experiments are accepted by nearly all physiological writers ; and the 

 assumption is that the third nerves animate the muscular fibres that contract the pupil, 

 the contraction produced by irritation of the optic nerves being reflex in its character. 

 Later observers, however, have carried their experiments somewhat farther. Longet 

 divided the motor oculi and the optic nerve upon the right side. He found that irrita- 

 tion 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 its operations are rather 

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

 been found, also, by Bernard, in experiments upon rabbits, that the pupil is not immedi- 

 ately dilated after division of the third nerve. The method employed by Bernard, intro- 

 ducing a hook into the middle temporal fossa through the orbit and tearing the- nerve, 

 can hardly be accomplished without touching the ophthalmic branch of the fifth, which 

 produces intense pain and is always followed by a more or less persistent contraction of 

 the pupil. Several hours after the operation, however, the pupil is generally found 

 dilated, and it may slowly contract when the eye is exposed to the light. In one experi- 

 ment, this occurred after the eye had been exposed for an hour. But farther experi- 

 ments by Bernard show 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 upon other nerves. It is well 

 known, for example, that division or irritation 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 facts show that the third nerve is not the only one 

 capable of acting upon the iris, and that it is not the sole avenue for the transmission 

 of reflex influences. 



Bernard also found that galvanization of the motor oculi itself did not produce con- 

 traction of the pupil, but this result followed when he galvanized the ciliary nerves 

 coming from the ophthalmic ganglion. Chauveau states that, in experiments upon 

 horses, he has not 'observed contraction of the pupil following galvanization of the motor 

 oculi, although he has sometimes seen it in rabbits. At all events, contraction is by no 

 means constant ; and, when it occurs, it probably depends upon stimulation of the ciliary 

 nerves themselves or irritation of the ophthalmic branch of the fifth, and not upon stimu- 

 lation of the trunks of the third pair. 



The movements of the iris will be treated of again, in connection with the physiology 

 of vision ; but we may here allude to an interesting fact observed by Miiller, which relates 

 to the action of the motores oculorum. When the eye is turned inward by a voluntary 

 effort, the pupil is always contracted ; and when the axes of the two eyes are made to 

 converge strongly, as in looking at near objects, the contraction is very great. 



