CRANIAL NERVES. 103 



2d Pair. Optic. 



Apparent Origin. From the anterior portion of the optic commissure. 



Deep Origin. The origins and connections of the optic tract are very 

 complex. The immediate origins are bands of fibres from the thalamus 

 opticus and anterior corpora quadrigemina. The corpora geniculata are 

 interposed ganglia. The ultimate roots are traced 



1. By a broad band of fibres " the optic radiation of Gratiolet " to the 



psycho-optic centres in the occipital lobes. 



2. To the gyrus hippocampi and sphenoidal lobes. 



3. Through the corpus callosum to the motor areas of the opposite cere- 



bral hemispheres. 



4. To the frontal region by " Meynert's Commissure." 



5. To the spinal cord. 



6. To the corpora geniculata, pulvinar, and anterior corpora geniculata by 



ganglionic roots. 



Distribution. The two roots unite to form a flattened band, the optic 

 tract, which winds around the crus cerebri to decussate with the nerve of 

 the opposite side, forming the optic chiasm. The decussation of fibres is 

 not complete ; some of the fibres of the left optic tract going to the outer 

 half of the eye of the same side, and to the inner half of the eye of the 

 opposite side ; the same holds true for the right optic tract. 



The optic nerves proper arise from the commissure, pass forward through 

 the optic foramina, and are finally distributed in the retina, 



Properties. They are insensible to ordinary impressions, and convey 

 only the special impressions of light. Division of one of the nerves is 

 attended by complete blindness in the eye of the corresponding side. 



Hemiopia and Hemianopsia. Owing to the decussation of the fibres 

 in the optic chiasm division of the optic tract produces loss of sight in 

 the outer half of the eye of the same side, and in the inner half of the 

 eye of the opposite side, the blind part being separated from the normal part 

 by a vertical line. The term hemiopia is applied to the loss of function or 

 paralysis of the one-half of the retina; hemianopsia is applied to the blind- 

 ness in the field of vision. If, for example, the right optic tract be divided, 

 there will be hemiopia in the outer half of the right eye and inner half of 

 left eye, thus causing left lateral hemianopsia, and as the two halves are 

 affected which correspond in normal vision, it is spoken of as homonymous 

 hemianopsia. Lesion of the anterior part of the optic chiasm causes blind- 

 ness in the inner half of the two eyes. 



