426 THE NERVOUS SYSTEM. 



The contraction of the iris thus shows all the character of a 

 reflex act, and in ordinary cases requires the concurrent ac- 

 tion 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 exposed to a stronger light : and gener- 

 ally the contraction of each of the pupils appears to be in di- 

 rect 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 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 voluntary 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 excluding 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 forwards, as in looking at a distant object, per- 

 mits the admission of the largest number of rays, of which 

 none are too divergent to be so refracted. 



The fourth nerve, or Nervus trochlearis or patheticus, is ex- 

 clusively motor, and supplies only the trochlearis or obliquus 

 superior muscle of the eyeball. 



The sixth nerve, Nervus abducens or ocularis extermis is also, 

 like the fourth, exclusively motor, and supplies only the rectus 

 extern us muscle. 1 The rectus externus is, therefore, convulsed, 

 and the eye is turned outwards, when the sixth nerve is irri- 

 tated ; and the muscle paralyzed when the nerve is disorganized, 

 compressed, or divided. In all such cases of paralysis, the eye 

 squints inwards, and cannot be moved outwards. 



1 In several animals it sends filaments to the iris (Radctyffe Hall) ; 

 and it has probably done so in man, in some instances in which the 

 iris has not been paralyzed, while all the other parts supplied by the 

 third nerve were (Grant). 



