692 THE BKAIN. 



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. 

 156, 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 tem- 

 poral half of the eye in which the nerve ends pass into one optic tract, 

 namely, the optic tract of the same side, while the fibres belonging to the 

 nasal half pass into another optic tract, namely the optic tract of the oppo- 

 site side. Thus the fibres of the temporal half of the right eye and of the 

 nasal half of the left eye pass into the right optic tract, and the fibres of 

 the nasal half of the right eye and of the temporal half of the left eye pass 

 into the left optic tract. Compare Fig. 156, in which the fibres forming the 

 right optic tract are shaded, while those forming the left optic tract are left 

 unshaded. Now the nasal half of one retina and the temporal half of the 

 other retina are " corresponding " parts. Hence, while each optic tract 

 contains fibres belonging to half of each eye, the two halves thus repre- 

 sented in each tract are corresponding halves. 



The amount and character of the decussation taking place in the optic 

 chiasma differs in different animal types, the difference having relation to 

 the amount of binocular vision, which in turn depends on the position of 

 the eyes in the head, that is, on the prominence of the face between the 

 eyes. In the fish, for instance, with laterally placed eyes, no binocular 

 vision at all is possible, and the decussation is complete ; the whole optic 

 nerve of each eye crosses over to the other optic tract. Between this and 

 the arrangement in man, just described, various stages obtain in various 

 animals. 



The chiasma also contains at its hinder part fibres which have no con- 

 nection with the optic nerves or the eyes, but are simply commissural tracts 

 passing from one side of the brain, namely, from the median corpus genicu- 

 latum ( 543) along one optic tract, through the chiasma to the other optic 

 tract, and so to the median corpus geniculatum of the other side of the 

 brain. These fibres are spoken of as the inferior or posterior (optic) com- 

 missure or arcuate commissure, or Gudden's commissure. It was once 

 thought that in a similar way fibres passed from one retina along one optic 

 nerve through the front part of the chiasma to the other optic nerve, and 

 to the other retina, forming an anterior (optic) commissure ; but this seems 

 to be an error. 



581. The optic vesicle is, as we have seen, budded off from the fore- 

 brain or forerunner of the third ventricle, and the optic chiasma is 



optic tract (shaded) supplying, through Op 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.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 011 the left side an image at a'. The right optic tract 

 is represented as ending in GL, the lateral corpus geniculatum ; in Pv, the pulvinar ; and in AQ, 

 the anterior corpus quadrigeminum, all three stippled; op rod, the optic radiation from these 

 bodies to R.Oc, the right occipital lobes 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, descend- 

 ing 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, shown as a broken 

 line; NC, the nucleus caudatus; LN, the nucleus lenticularis; and TH, optic thalamus shown in 

 outline; cia, the front limb; cig, the knee, and tip, the hind limb of the internal capsule. The 

 outlines of the fourth ventricle, 4tb Vn, and of the posterior corpora quadrigemina are shown by 

 dotted lines, that of the bulb is shown by a fine line; p, the pineal gland. 



