532 HANDBOOK OF PHYSIOLOGY. 



raised by the levator palpebrse, but droops (ptosis) and remains gently 

 closed over the eye. under the unbalanced influence of the orbicularis 

 palpebrarum, which is supplied by the facial nerve: (2) the eye is 

 turned outwards (external strabismus) by the unbalanced action of the 

 rectus externus, to which the sixth nerve is appropriated: and hence, 

 from the irregularity of the axes of the eyes, double sight, diplopia, is 

 often experienced when a single object is within view of both the eyes: 



(3) the eye cannot be moved either upwards, dowmvards, or inwards: 



(4) the pupil becomes dilated (mydriasis), and insensible to light: (5) the 

 eye cannot accommodate for short distances. 



Contraction and Dilatation of the Pupil. The relation of the third 

 nerve to the muscles of the iris is of peculiar interest. Under ordinary 

 circumstances the contraction of the iris is a reflex action, which is pro- 

 duced by the stimulus of light on the retina which is conveyed by the 

 optic nerve to the brain (probably to the corpora quadrigemina or me- 

 dulla), 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 centre is destroyed or much compressed. But 

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

 cited 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, its centre, and the third nerve; and, probably also, considering 

 the peculiarities of its perfect mode of action, of the ophthalmic gan- 

 glion. But, besides, both irides will contract under the reflected stimu- 

 lus of light falling upon one retina or under irritation of one optic nerve 

 only. Thus in amaurosis of one eye, its pupil may contract when the 

 other eye is exposed to a stronger light; and generally the contraction of 

 each of the pupils appears to be in direct proportion to the total quan- 

 tity of light which stimulates either one or both retinae, according 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 inwards, or upwards 

 and inwards, by the action of the third nerve distributed in the rectus 

 internus and rectus superior, the iris contracts, as if under direct volun- 

 tary influence. 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 

 inwards, 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 



