138 HAND-BOOK OF PHYSIOLOGY. 



Contraction and Dilatation of the Pupil. The relation of the 

 third nerve to the iris is of peculiar interest. In ordinary circumstances 

 the contraction of the iris is a reflex action, which may be explained as 

 produced by the stimulus of light on the retina being conveyed by the 

 optic nerve to the brain (probably to the corpora quadrigemina), and 

 thence reflected through the third nerve to the iris. Hence the iris ceases 

 to act when either the optic or the third nerve is divided or destroyed, 

 or when the corpora quadrigemina are destroyed or much compressed. 

 But when the optic nerve is divided, the contraction of the iris may be 

 excited by irritating that portion of the nerve which is connected with 

 the brain: and when the third nerve is divided, the irritation of its distal 

 portion will still excite the contraction of the iris. 



The contraction of the iris thus shows all the characters of a reflex 

 act, and in ordinary cases requires the concurrent action of the optic 

 nerve, corpora quadrigemina, and third nerve; and, probably also, con- 

 sidering the peculiarities of its perfect mode of action, of the ophthalmic 

 ganglion. But, besides, both irides will contract their pupils under the 

 reflected stimulus of light falling only on one retina or under irritation 

 of one optic nerve. Thus, in blindness of one eye, its pupil may contract 

 when the other eye is exposed to a stronger light: and generally the con- 

 traction of each of the pupils appears to be in direct proportion to the 

 total quantity of light which stimulates either one or both retinge, accord- 

 ing as one or both eyes are open. 



The iris acts also in association with certain other muscles supplied by 

 the third nerve: thus, when the eye is directed inward, or upward and in- 

 ward, by the action of the third nerve distributed in the rectus internus 

 and rectus superior, the iris -contracts, as if under direct voluntary in- 

 fluence. The will cannot, however, act on the iris alone through the 

 third nerve; but this aptness to contract in association with the other 

 muscles supplied by the third, may be sufficient to make it act even in 

 total blindness and insensibility of the retina, whenever these muscles 

 are contracted. The contraction of the pupils, when the eyes are moved 

 inward, as in looking at a near object, has probably the purpose of ex- 

 cluding those outermost rays of light which would be too far divergent to 

 be refracted to a clear image on the retina; and the dilatation in looking 

 straight forward as in looking at a distant object, permits the admission 

 of the largest number of rays, of which none are too divergent ^to be so 

 refracted. (For further remarks on the contraction and dilatation of the 

 pupil, see pp. 205, 206, Vol. II.) 



FOURTH NERVE. 



Functions. The fourth nerve, or Nervus trocJilearis or patlieticus, 

 is exclusively motor, and supplies only the trochlearis or obliquus superior 

 muscle of the eyeball. 



