THE CENTRAL NERVOUS SYSTEM 719 



these nerve-cells, which proceed in the optic radiation (p. 692) to 

 the occipital cortex (Figs. 258, 271, and 288). In young animals all 

 these structures undergo atrophy after extirpation of the eyeball. At 

 the chiasma the fibres of the optic nerve decussate, partially in man 

 and some mammals, as the rabbit, dog, cat, and monkey, completely 

 in animals whose visual field is entirely independent for the two eyes, 

 as in fishes and birds. In man the fibres for the nasal halves of 

 both retinae cross the middle line at the chiasma, those for the 

 temporal halves do not. The chiasma also contains fibres in its 

 posterior portion, which extend from one optic tract to the other, 

 but are not connected with the retinae 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 

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



FIG. 257. SCHEME OF THE OLFACTORY NERVOUS APPARATUS (HALLI- 

 BURTON AFTER CAJAL). 



A, olfactory cells ; B, glomeruli ; C, mitral cells ; D, olfactory granule cell ; E, 

 lateral root of olfactory tract ; F, cortex of brain in the region of the uncinate gyrus ; 

 a, small cell of mitral layer ; b, brush of dendrite of a mitral cell ending in a glome- 

 rulus ; , thorns or spines on the processes of an olfactory granule ; e, collateral 

 coming off from the axon of a mitral cell ; /, collaterals ending in the molecular layer 

 of the uncinate gyrus ; g, pyramidal cells of the cortex ; A, supporting epithelial cells 

 of the olfactory mucous membrane. 



portion of the optic thalamus, or the optic tract, causes hemi- 

 anopia* (blindness of the corresponding halves of the two retinae) 

 on the side of the lesion. Thus, a lesion equivalent to complete 

 section of the right optic tract would cause blindness of the nasal 

 half of the left, and of 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. A lesion, e.g., a tumour of the pituitary body, involving the 



* The terms ' hemiopia,' ' hemianopia,' ' hemianopsia,' are sometimes 

 used with reference to the blind side of the retina?, sometimes to the dark 

 half of the visual field. We shall always use the word ' hemianopia ' 

 with reference to the retina. 



