52 THE NERVOUS SYSTEM 



centres also establish communications with the nuclei of 

 other cerebral nerves. 



Two neurones, therefore, are concerned in the transference 

 of stimuli from the retina to the cortex. Lesions which 

 affect either the higher neurone or the lower neurone behind 

 the optic chiasma produce exactly similar results. The 

 temporal half of the homo-lateral retina (nasal side of the 

 field of vision) and the nasal half of the hetero-lateral retina 

 (temporal side of the field of vision) are both blind. Whether 

 the lesion affects the upper or the lower neurone can be de- 

 termined by the test for \VernickJs sign. 



Under normal conditions when a strong ray of light 

 stimulates the retina, both pupils become contracted. This 

 is known as the light reflex, and it is believed that a special 

 set of fibres is concerned in its production. These pupillary 

 fibres leave the retina in the optic nerve, and at the optic 

 chiasma they undergo a decussation which is precisely 

 similar to the partial decussation of the visual fibres. They 

 then pass backwards in the optic tract, but do not terminate 

 in the lower visual centres. Instead, they terminate in the 

 iris nucleus, which is a special collection of cells in the 

 nucleus of the oculo-motor nerve. 



In Wernicke's test, a ray of light is reflected on to the 

 blind half of the retina and, as the test is extremely delicate, 

 great care must be taken to ensure that the light does not 

 impinge on the unaffected part of the retina. When the 

 upper neurone is the site of the lesion, the light stimulus is not 

 interrupted in its passage to the iris nucleus and the light reflex 

 is present, so that in this condition Wernicke's sign is positive. 

 In affections of the lower neurone, the pupillary fibres may 

 escape when the lesion is situated in the lower visual centres. 

 On the other hand, when the lesion involves the optic tract, the 

 pupillary fibres are affected to the same extent as the visual 

 fibres, and just as the visual stimulus is cut cff from the lower 

 centres, causing homonymous hemianopia, so Wernicke's test 

 applied to the blind halves of the retinae gives a negative result. 



