CHAP, ii.] THE BRAIN. 1073 



FK;. 133. DIAGRAM TO ILLUSTRATE THE NERVOUS APPARATUS OF Vision IN MAN. 



(Sherrington.) 



/. the left eye, R. the right eye, o.x. the optic axis. F. the outline of the face 

 between the eyes, Op.T. the right optic tract (shaded) supplying, through Op. 

 I)e. the optic decussation, the temporal side or the retina of the right eye and 

 the nasal side of the retina of the left eye. L. F. L. and L. F. R. the left 

 visual fields of the left and right eye respectively; the two fields and the parts 

 of the two retinas whose excitation produces vision over the fields are shaded, 

 the object a in the field of the right side giving rise to an image at a', and a 

 on the left side an image at a'. 



The right optic tract is represented as ending in GL. the lateral corpus genicu- 

 latum, in Pv. the pulviuar, and in AQ. the anterior corpus quadrigeminum, all 

 throe stippled; op. rod. the optic radiation from these bodies to It. Oc. the 

 right occipital lobe, whose stippled cortex indicates the "visual area." d. the 

 'direct' tract to the cortex, c. c. corpus callosum, cut across at the splenium, 

 I. v. d. descending horn of the lateral ventricle. 



The left side has been utilized to indicate at F. shaded with lines, the cortical 

 motor area for the eyes; fm. c. indicates the path from it to III. IV. VI. the 

 nuclei of the third, fourth and sixth nerves, p. b. the posterior longitudinal 

 bundle, shewn as a broken line. NC. the nucleus caudatus, LN. the nucleus 

 lenticularis and TH. optic thalamus shewn in outline, Cia. the front limb, 

 Gig. the knee, and dp. the hind limb of the internal capsule. The outlines 

 of the fourth ventricle 4th Vn. and of the posterior corpora quadrigemina are 

 shewn by dotted lines, that of the bulb is shewn by a fine line. p. the pineal 

 gland. 



of each retina is not employed in binocular vision. Owing to the 

 position of the two eyes in relation to the nose, it comes about that 

 an object held very much on one side, to the left-hand side for 

 instance, while it is capable of producing an image on the extreme 

 nasal side of the left eye, and can be seen therefore by that eye, 

 cannot produce an image on the temporal side of the right eye ; 

 the nose blocks the way. It is therefore not seen by the right 

 eye, and the vision of it is monocular, by the left eye only. In 

 Fig. 133 it may be seen that the left visual field of the left eye 

 (L.F.L) extends more to the left, and is larger than the left visual 

 field of the right eye (L.F.R) and that the right retinal area, 

 corresponding to the left visual field, extends farther along the 

 nasal side of the left side (a), than it does along the temporal side 

 of the right eye (a), the difference being due to the presence of 

 the nose (F). And similar conditions obtain with regard to the 

 extreme right-hand side of the visual field. 



667. After these preliminary statements, we may now turn 

 to consider some anatomical facts concerning the ending of the 

 optic nerve in the brain. 



The optic nerve of each eye consists of nerve fibres coming from 

 all parts of the retina of that eye ; but the two optic nerves meet, 

 ventral to the floor of the third ventricle, cross each other at 

 the optic chiasma (Fig. 133, op. De), and are thence continued on 

 under the name not of optic nerves but of optic tracts (Op.T.). 

 The decussation of fibres which takes place in the chiasma has 

 peculiar characters. At their decussation (we are speaking now of 

 man) the fibres in the optic nerve belonging to the temporal half 



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