542 THE NERVOUS SYSTEM. 



ments 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.* All the symmetrically-placed muscles are sup- 

 plied with symmetrical 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 symme- 

 trical 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 externus 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 in- 

 wards or outwards) were corrected by one of these muscles 



* It is sometimes said, that the external recti cannot be put ' in 

 action simultaneously : yet they are so when the eyes, having been 

 both directed inwards, are restored to the position which they have in 

 looking straight forwards. 



