The anatomy of the nervous system, from the standpoint of development and function . ion to corticipetal fibers arising in the pulvinarand lateral geniculate body, the optic radiation contains corticifugal fibers 221 THI. NERVOUS SYSTEM arising in the cortex and terminating in the pulvinar. lateral geniculate body,and superior colliculus of the corpora quadrigemina. The significance of the partial decussation of the nerves is made clear byFigs. 162 and 163. The properties of the refracting media of the eyes are suchthat images of objects to the left of the axis of vision are produced on the na
The anatomy of the nervous system, from the standpoint of development and function . ion to corticipetal fibers arising in the pulvinarand lateral geniculate body, the optic radiation contains corticifugal fibers 221 THI. NERVOUS SYSTEM arising in the cortex and terminating in the pulvinar. lateral geniculate body,and superior colliculus of the corpora quadrigemina. The significance of the partial decussation of the nerves is made clear byFigs. 162 and 163. The properties of the refracting media of the eyes are suchthat images of objects to the left of the axis of vision are produced on the nasalside of the left eye and the temporal side of the right eye. And. due to the man-ner of decussation of the optic nerve-fibers, impulses from both these sourcesreach the visual area of the right cortex. In the same way the visual cortexof the left side receives impressions from objects to the right of the axis of is to say. the sensory representation of the outer world in the cerebralcortex is contralateral in the case of sight just as it is in the case of cutaneous. Fig. 163.—Diagram to show why a destruction of one optic tract causes blindness in both eyes forthe opposite lateral half of the field of vision. and auditory sensations. Furthermore, it will be evident that, while destruc-tion of one optic nerve causes total blindness in the corresponding eye. destruc-tion of one optic tract, its thalamic connections, their optic radiations, or thevisual cortex in which these radiations terminate, will produce blindness in botheyes for the opposite lateral half of the field of vision. This condition is knownas hemianopsia, and is produced by a lesion in the optic pathway anywhere be-hind the chiasma. CHAPTER XV THE EXTERNAL CONFIGURATION OF THE CEREBRALHEMISPHERES Development.—The cerebral hemispheres are formed by the examination ofthe alar lamina of the telencephalon, tin- rest of which remains a- tin- boundaryof the rostral part of the third ventricle, and i-
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