THE CRANIAL NERVES. 



453 



FIG. 121. 



on the same side. If the decussation were also complete in quadru- 

 peds, a longitudinal section of the chiasma at the median line would 

 divide at once all the optic nerve 

 fibres, and produce blindness of 

 both eyes. But Nieati* has 

 shown, by experiments on cats, 

 that after such a section vision 

 still exists in these animals in 

 an unmistakable degree ; show- 

 ing that the eyes receive through 

 the optic nerves some fibres 

 which have not crossed the 

 median line. From a compari- 

 son of the form and section sur- 

 faces of the optic tracts and 

 chiasma, he finds that the same 

 conclusion is applicable to man. 

 Disturbances of Vision from 

 Lesion of the Optic Nerves or 

 Tracts. There are certain va- 

 rieties of partial or complete 

 blindness in one or both eyes, 

 occurring in man, which are 

 only explainable on the suppo- 

 sition of incomplete decussa- 

 tion of the Optic nerves. They DIAGRAM OF THK OPTIC NERVES AND TRACTS, in Man. 

 f . 1. Left eyeball. 2. Right eyeball. 3, 3. Corpora 



depend On lesion Or COmpreS- geniculata interna. 4, 4. Corpora geniculata externa. 



5. Tubercula quadrigemina. 6, 6. Centres of vision in 



the cerebral hemispheres. 



SlOn Of the Optic fibres at differ- 

 , , . ~ 



ent parts of their course. Com- 

 plete blindness of one eye is produced by a lesion involving the whole 

 of one optic nerve, between the chiasma and the eyeball, as at A, Fig. 

 122 ; since such an injury interrupts all the nerve fibres, from whatever 

 source, going to the retina of the corresponding eye. 



In the affection known as hemiopia, the patient sees only one lateral 

 half of objects presented to his view. His field of vision, instead of 

 being circular, has the form of a semicircle ; being divided at its mid- 

 dle by a vertical diameter, on one side of which everything is invisible. 

 Such a condition may be produced by lesions affecting one of the optic 

 tracts behind the chiasma, as at B, Fig. 122. As the direct fibres, on 

 the outer border of each tract, pass to the external portion of the retina 

 on the same side, and the cross fibres pass to the internal portion of the 

 opposite eye, both eyes will be blinded in the corresponding half of the 

 retina, and for the opposite half of the field of vision. If the lesion 

 involve the left optic tract, as in Fig. 122, the right lateral half of the 



* Archives de Physiologie. Paris, 1878, 2me serie, tome v., p. 658. 



