FORCED MOVEMENTS 151 



from the nasal side of the retinae undergo a decussa- 

 tion, i. e., they cross to the other side of the head and 

 brain. The left optic tract contains fibres from the 

 temporal side of the left eye, and from the nasal, 

 or internal, side of the right eye. If the left tract be 

 cut, the left sides of both retinae become blind, and 

 the patient recognises nothing more in the right half 

 of the field of vision. This is a case of hemianopia. 



A similar semidecussation also occurs in the motor 

 nerves of the eye. For the time being we may con- 

 sider the various muscles of each eye as a unit. In 

 the lateral movements of our eyes the rectus externus 

 of one eye and the rectus internus of the other co- 

 operate. If we assume an inherited connection be- 

 tween the retinal elements and the movements of the 

 eyes, the right externus and the left internus must be 

 innervated by the left half of the brain. The nerve- 

 fibres of the externi must thus be crossed, those of 

 the interni not crossed. The semidecussation in this 

 case naturally occurs in the brain, and not peripherally. 

 The pathological expression of this motor semidecus- 

 sation is the deviation conjugte, which is a motor 

 affection, corresponding to the sensory affection, 

 hemianopia. We can only expect to find these 

 semidecussations where symmetrical organs always 

 receive equal innervations, as in the case of our eyes. 

 Our arms and legs can move independently of each 

 other, but in lower Vertebrates the case is different. 

 The symmetrical fins of the fish receive equal innerv- 

 ations. I have shown that associated changes of 



