784 VISUAL SENSATIONS. [Book iti. 



Fig. 138. Diagram to illustrate the Nervous Apparatus ok Vision in 



Man. (Sherrington.) 



L. the left eye, B. the right eye, o.x. the optic axis. F. the outline of the 

 face between the eyes, OpT. the right optic tract (shaded) supplying, through 

 Oj>. De. the optic decussation, the temporal side of the retina of the right eye 

 and the nasal side of the retina of the left eye. L. F. L. and L. F. B. 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 geni- 

 culatum, in Pv. the pulvinar, and in AQ. the anterior corpus quadrigeminum, all 

 three stippled ; op. rad. the optic radiation from these bodies to B. Oe. 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 corti- 

 cal 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. NO. the nucleus caudatus, LN. the nucleus 

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

 the knee, and Cip. 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. 



two eyes it follows that the temporal side of the right and the 

 nasarside of the left eye are such corresponding parts, while 

 the nasal side of the right eye corresponds to the temporal side 

 of the left eye. But the whole of each retina is not employed in 

 binocular vision. Owing to the position of the two eyes in rela- 

 tion 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. 138 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. 



§ 495 . 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. 138, op. De), and are thence continued 

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



