THE THIRD CEREBRAL NERVE. 541 



of the eyes, double-sight is often experienced when a single 

 object is within view of both the eyes : thirdly, the eye 

 cannot be moved either upwards, downwards, or inwards ; 

 fourthly, the pupil is dilated. 



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 contraction of the iris in 

 which its fibres are distributed. 



The contraction of the iris thus shows all the character 

 of a reflex act, and in ordinary cases requires the concurrent 

 action of the optic nerve, corpora quadrigemina, and third 

 nerve ; and, probably also, considering the peculiarities of 

 its perfect mode of action, 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 amaurosis of 

 one eye, its pupil may contract when the other eye is ex- 

 posed to a stronger light : and generally the contraction of 

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

 total quantity 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 mus- 

 cles 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 



