HERMANN VON HELMHOLTZ 



itself. Normally, the superior rectus co-operates with 

 the inferior oblique, and the inferior rectus with the 

 superior oblique. 



The two eyeballs are also associated in their move- 

 ments in the most exquisite manner, an association 

 accomplished by a nervous mechanism which need not 

 here be discussed. Suffice it to say, that when we 

 look, say to the right side, the right external and the 

 left internal recti work together, and when we look 

 towards the left side, then there is harmonious co-opera- 

 tion between the left external and the right internal 

 recti. Again, the two superior recti act together if we 

 look upwards, and the two inferior if we look down- 

 wards ; if we look downwards pensively to the right 

 side, the inferior recti of each eye, along with the 

 superior oblique on the same side, and the internal 

 rectus on the left side all come into play ; while, if we 

 look to the left side, the corresponding muscles for 

 that movement are brought into action ; and, finally, 

 if we look upwards and to the right, then the superior 

 recti contract, and are aided by the inferior oblique on 

 the right side, and the internal rectus on the left ; 

 while, if we assume the same attitude to the left, the 

 superior recti again act, and are assisted by the inferior 

 oblique on the same side, and the internal rectus on the 

 right. As these muscles are innervated, in many cases, 

 from opposite sides of the brain, it is clear that the 

 cerebral actions must be of a very complicated char- 

 acter. All these movements are under the control of 

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