542 THE NERVOUS SYSTEM. 



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 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, permits 

 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 

 exclusively motor, and supplies only the trochlearis or 

 obliquus superior muscle of the eyeball. 



The sixth nerve, Nervus abducens or ocularis externus^ is 

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

 the rectus externus muscle.* The rectus externus is, 

 therefore, convulsed, and the eye is turned outwards, when 

 the sixth nerve is irritated; 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. 



In its course through the cavernous sinus, the sixth 

 nerve . forms larger communications with the sympathetic 

 nerve than any other nerve within the cavity of the skull 

 does. But the import of these communications with the 

 sympathetic, and the subsequent distribution of its fila- 



* In several animals it sends filaments to the iris (KadclyfFe 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). 



