. The relations of the eye and its diseases to diseases of the body : forming a supplementary volume to every manual and textbook of practical medicine and opthalmology . the larger part of the nerve and undergodecussation. The fibres from the outer half of the retina, and whichanswer to the nasal side of the field, pass without decussation to thecerebral hemisphere of the same side. The optic tract thus containsall the fibres from the halves of the retinae on the same side, theright tract containing the fibres of the right retinal halves (the tern- DISEASES OF THE NERVOUS SYSTEM. 3 poral half
. The relations of the eye and its diseases to diseases of the body : forming a supplementary volume to every manual and textbook of practical medicine and opthalmology . the larger part of the nerve and undergodecussation. The fibres from the outer half of the retina, and whichanswer to the nasal side of the field, pass without decussation to thecerebral hemisphere of the same side. The optic tract thus containsall the fibres from the halves of the retinae on the same side, theright tract containing the fibres of the right retinal halves (the tern- DISEASES OF THE NERVOUS SYSTEM. 3 poral half of the right eye and the nasal half of the left eye). Bothtogether supply the left half of the field of vision, so that the righttract corresponds to the binocular left half of the field of vision andvice versa. Hence there is complete decussation so far as regardswhat is seen to the right and left. But this is only the general course, and the conditions in detailare much more complicated. In the first place it is to be noted thatthe line of separation between the two halves of the field of vision,which passes vertically through the point of fixation (not through. /V Fig. 2.—Course of the Fibres in the Right Retina, Seen from in Front. E, Optic nerve en-trance; F, fovea centralis; T, external (temporal); N, internal (nasal) side. From the stripAB, corresponding to the overlapping portion of the field of vision, spring crossed (light lines)and uncrossed (heavy lines) fibres. the entrance of the optic nerve) is usually not a sharp line. Asshown in,Fig. 1, fibres from both halves of the field of vision passfrom one side to the other in a strip which may be 10° in width(overlapping part of the field of vision). Hence, in loss of the regionof destination of one optic tract, the boundary of the visual fieldusually passes to the inside or outside of the point of fixation, andthe latter remains intact in both eyes. Fig. 2 gives the probable dis-tribution of the fibres of the optic
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