THE CENTRAL NERVOUS SYSTEM 



819 



decussate. The number of uncrossed fibres is smaller than that of 

 crossed. The chiasma also contains fibres in its posterior portion, 

 which extend from one optic tract to the other, but are not con- 

 nected with the retinas or the optic nerves. They are commissural 

 fibres which connect the two mesial geniculate bodies across the 

 middle line, and are called Gudden's commissure. A sufficiently 

 extensive lesion involving the occipital cortex on one side, or the 

 posterior portion of the optic thalamus, or the optic tract, causes 

 hemianopia* or defect of the visual field on the side opposite to the 

 lesion, with blindness of the corresponding halves of the two retinae. 

 Thus, a lesion equivalent to complete section of the right optic tract 

 would cause blindness of the 



nasal half of the left, and of _ . te. . ^ 



the temporal half of the 

 right eye, and the left half 

 of the field of vision would 

 be blotted out the patient 

 would be unable, with his 

 eyes directed forwards, to 

 see an object at his left. 

 Such a complete hemianopia 

 is much rarer in disease of 

 the cortex than in disease 

 of the optic tract. A lesion 

 e.g., a tumour of the pitui- 

 tary body involving the 

 whole of the optic nerve in 

 front of the chiasma, would 

 cause complete blindness 

 in the corresponding eye. 

 Sometimes in disease of the 

 optic nerve vision is not 

 totally destroyed in the eye 

 to which it belongs, but the 

 field is narrowed by a cir- 

 cumference of blindness. In 

 this case the pathological 

 change involves the cir- 

 cumferential fibres of the 

 nerve. When the chiasma 

 is affected by disease, a very 

 frequent symptom is bitem- 

 poral hemianopia, blindness 

 of the nasal halves of the 

 retinae, with loss of the outer or temporal half of each field of 

 vision. The optic nerve and tract contain a few efferent fibres for 

 the retina, whose cell-bodies have not yet been certainly located. 



The third nerve, or oculo-motor, arises from an elongated nucleus, 

 or a series of nuclei, containing large nerve-cells in the floor of the 

 Sylvian aqueduct below the anterior corpora quadrigemina. The 

 root-bundles coming off from the most anterior of the nuclei carry 

 fibres that innervate the ciliary muscle, and thus have to do with 



* The terms ' hemiopia,' ' hemianopia/ ' hemianopsia/ are used with 

 reference sometimes to the blind side of the retinae, but ordinarily to the 

 half of the visual field which is deficient. We shall always use the word 

 ' hemianopia ' in the latter sense. 



522 



[II Nerve 



FIG. 342. SCHEME OF THE VISUAL PATH 

 (HALLIBURTON, AFTER SCHAFER). 



