THE CEREBRAL NERVES. 427 



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 filaments after joining the sixth 

 nerve, are quite unknown; and there is no reason to believe 

 that the sixth nerve is, in function, more closely connected with 

 the sympathetic than any other cerebral nerve is. 



The question has often suggested itself why the six muscles 

 of the eyeball should be supplied by three motor nerves when 

 all of them are within reach of the branches of one nerve; and 

 the true explanation would have more interest than attaches 

 to the movements of the eye alone; since it is probable that 

 we have, in this instance, within a small space, an example of 

 some general rule according to which associate or antagonist 

 muscles are supplied with motor nerves. 



Now, in the several movements of the eyes, we sometimes 

 have to act with symmetrically placed muscles, as when both 

 eyes are turned upwards or downwards, inwards or outwards. 1 

 All the symmetrically placed muscles are supplied with sym- 

 metrical nerves, i. e., with corresponding branches of the same 

 nerves on the two sides; and the action of these symmetrical 

 muscles is easy and natural, as we have a natural tendency to 

 symmetrical movement in most parts. But because of this 

 tendency to symmetrical movements of muscles supplied by 

 symmetrical nerves, it would appear as if, when the two eyes 

 are to be moved otherwise than symmetrically, the muscles to 

 effect such a movement must be supplied with different nerves. 

 So, when the two eyes are to be turned towards one side, say 

 the right, by the action of the rectus extern us of the right eye 

 and the rectus internus of the left, it appears as if the tendency 

 to action through the similar branches of corresponding nerves 

 (which would move both eyes inwards or outwards) were cor- 

 rected by one of these muscles being supplied by the sixth, and 

 the other by the third nerve. So with the oblique muscles : 

 the simplest and easiest actions would be through branches of 

 the corresponding nerves, acting similarly as symmetrical 

 muscles ; but the necessary movements of the two eyes require 

 the contraction of the superior oblique of one side, to be asso- 

 ciated with the contraction of the inferior oblique, and the re- 

 laxation of the superior oblique, of the opposite side. For this, 

 the fourth nerve of one side is made to act with a branch of 



1 It is sometimes said that the external reeti cannot be put in action 

 simultaneously: yet they are so when the eyes, having been both di- 

 rected inwards, are restored to the position which they have in looking 

 straight forwards. 



