CHAPTER VII. 



THE INHIBITORY NERVES TO THE INVOLUNTARY MUSCLES OF 



THE EYE. 



THE last muscular group to be considered, with respect to this 

 question of inhibitory fibres, is the prosomatic group of involun- 

 tary muscles, the motor cells of which are found in the ciliary 

 ganglion. The movements of the pupil are regulated by 

 two muscles of antagonistic action, the sphincter muscle and 

 the dilator muscle. One would expect therefore to find, in ac- 

 cordance with the law of reciprocal innervation already given in 

 so many instances, that the motor nerves to the sphincter muscle 

 are accompanied by inhibitory nerves to the dilator muscle and 

 vice versa. In other words, stimulation of the roots of the third 

 nerve ought to cause relaxation of the dilator muscle, and stimu- 

 lation of the cervical sympathetic ought to produce relaxation of 

 the sphincter muscle. 



There is a certain amount of evidence that such is the case. 

 Weymouth Reid investigated the electrical changes in the two 

 muscles, which occur on stimulation of the cervical sympathetic 

 and roots of the third nerve respectively, by the same method as 

 I had used in the case of the heart. The iris being exposed, he 

 made a thermal injury across the sphincter muscle in one case, 

 and across the radial muscle in the other, and obtained in each 

 case a marked demarcation current when the electrodes were 

 placed upon the injured and uninjured parts of the muscle. The 

 first position of the electrodes he calls the concentric position, 

 the second the radial. With the electrodes in the radial position 

 stimulation of the cervical sympathetic gave a negative variation 

 of the current indicative of a contraction of the dilator muscle, 

 and in the concentric position a positive variation of the current 

 indicative of a relaxation of the sphincter muscle. On the other 

 hand stimulation of the roots of the third nerve gave the reverse 

 effects, a negative variation in the concentric position, and a 

 positive variation in the radial position. These experiments in- 

 dicate that a true reciprocal innervation is present in this system 

 also. 



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