156 



Irritation of the fifth nerve excites slight motion in the iris. Divi- 

 sion of the fifth produces temporary contraction of the pupil. In 

 the space of half an hour this effect will have ceased, and the pupil 

 will have resumed its former diameter. 



If a slight galvanic current be passed along the sympathetic, con- 

 traction of the pupil will be produced ; but let the sympathetic be 

 divided, at the superior cervical ganglion for instance, and instantly 

 the pupil shall forcibly contract, and again widely dilate. 



If a weak galvanic current be used, and the poles brought into 

 contact with the sclerotica at its junction with the cornea, contrac- 

 tion of the pupil to two- thirds of its actual diameter takes place, 

 and this effect continues so long as the current continues to be 

 formed; but on breaking connection, the pupil immediately resumes 

 its former diameter. So soon as life is extinct, galvanism ceases to 

 affect the iris, whether applied to the membrane itself, through the 

 external coats, or though the poles be in contact with the retina ; 

 but if applied to the sympathetic, movement may be excited in the 

 iris. 



The optic nerve being divided, the pupil is dilated : irritation of 

 the third nerve then produces merely a slight and momentary effect 

 upon the iris ; but if the sympathetic be divided, the pupil will con- 

 tract violently, and again dilate beyond its previous state of dilata- 

 tion. 



I'he sympathetic being divided, irritation of the cranial nerves 

 does not affect the iris; but though the cerebrum and corpora 

 quadrigemina be removed, division of the sympathetic will still ex- 

 cite the iris to motion. And, consequently, the author infers that 

 the basic or primary motion of the iris is derived from the vis motoria 

 of the excito-motor ganglionic system : he shows also, that where 

 the ophthalmic ganglion is wanting, as in fishes and reptiles, the iris 

 is motionless. Allusion is, lastly, made to some medicinal agents, 

 to show their influence upon the nervous centres, and their conse- 

 quent effect upon the iris : they are classed as follows : — 

 I. True depressors and pupil dilators. 

 II. True excitants and pupil dilators. 



III. Stimulants which become depressors, which dilate the pupil. 



IV. Exciters of voluntary nerves and pupil dilators. 



V. Sedatives which terminate as depressors, which first contract 



and then dilate the pupil. 

 VI. Excitants which become sedatives, which first dilate and then 



contract the pupil. 

 And from what has gone before, it is concluded, that contraction 

 of the pupil is the active state of the iris, and that dilatation is its 

 enervated condition ; that a healthy retina and cerebral nervous 

 arc are necessary to the motions of the iris, and the ophthalmic 

 ganglion to motion ; and that the primary motion of the iris is due 

 to organic nervous influence, but its forced or animal motion to the 

 reflected stimulus of light upon the retina. 



