342 



FUNCTIONS OF THE NERVOUS SYSTEM. 



445. There is a further peculiarity, of a very marked kind, attending the 

 course of the Optic nerves ; this is the crossing or decussation which they 



[Fig. 159. 



[Fig. 160. 



Plan of the oplic nerves on a 

 small scale, showing their diver- 

 gence from the chiasma. c, and 

 their junction with the globe, on 

 the inner side of the axis of the 

 humors.] 



Course of fibres in the chiasma, as exhibited by 

 tearing; oft" the superficial bundles from a specimen 

 hardened in spirit, a. Anterior fibres, commissural 

 between the two retinrc. p Posterior fibres, com- 

 missural between the thalami. a', p'. Diagram of 

 the preceding.] 



undergo, more or less completely, whilst proceeding from their ganglia to the 

 eyes. In some of the lower animals, in which the two eyes (from their lateral 

 position) have entirely different spheres of vision, the decussation is complete; 

 the whole of the fibres from the right optic ganglion passing into the left eye, 

 and vice versa. This is the case, for example, with most of the Osseous 

 Fishes (as the cod, halibut, &c.) ; and also, in great part at least, with Birds. 

 In the Human subject, however, and in animals which, like him, have the two 

 eyes looking in the same direction, the decussation seems less complete ; but 

 there is a very remarkable arrangement of the fibres, which seems destined to 

 bring the two eyes into peculiarly consentaneous action. The posterior border 

 of the Optic Chiasma is formed exclusively of commissural fibres, which pass 

 from one optic ganglion to the other, without entering the real optic nerve. 

 Again, the anterior border of the Chiasma is composed of fibres, which seem, 

 in like manner, to act as a commissure between the two retinae ; passing from 

 one to the other, without any connection with the optic ganglia. The tract 

 which lies between the two borders, and occupies the middle of the Chiasma, 

 is the true Optic Nerve ; and in this it would appear that a portion of the fibres 

 decussates, whilst another portion passes directly from each Optic ganglion 

 into the corresponding eye. The fibres which proceed from the ganglia to 

 the retina?, and constitute the proper Optic Nerves, may be distinguished into 

 an internal and an external tract. Of these, the external on each side, passes 

 directly onwards to the eye of that side; whilst the internal crosses over to 

 the eye of the opposite side. The distribution of these two sets of fibres in 

 the retina of each eye respectively, is such that, according to Mr. Mayo, the 

 fibres from either optic ganglion will be distributed to its own side of both 

 eyes ; the right optic ganglion being thus exclusively connected with the outer 

 part of the retina of the right eye, and with the inner part of the retina of the 

 left eye ; and the left optic ganglion being, in like manner, connected exclu- 

 sively with the outer side of the left retina, and with the inner side of the 

 right. Now as cither side of the eye receives the images of objects, which 

 are on the other side of its axis, it follows, if this account of their distribution 

 be correct, that in Man, as in the lower animals, each ganglion receives the 

 sensations of objects situated on the opposite sides of the body. The purpose 

 of this decussation may be, to bring UK: visual impressions, which are so im- 

 portant in directing the movements of the body, into proper harmony with the 



