NERVES OF SPECIAL SENSE OPTIC. 653 



of light on the retina of the sound eye, which produces a motor change in 

 the third pair on both sides. Further, as already shown ( 508), the impres- 

 sion only of light upon the retina may give rise to contraction of the pupil, 

 by reflex action, when the Optic nerve is itself sound ; whilst no sensations 

 are received through the eye, in consequence of disease in the sensorial por- 

 tion of the nervous centres. Although the contraction of the pupil is effected 

 by the influence of motor fibres which proceed to the sphincter of the Iris 

 from the third pair of nerves, through the Ophthalmic ganglion, its dilata- 

 tion, as \ve shall hereafter see, depends upon the influence it derives from the 

 Sympathetic system, of which that ganglion forms a part. Besides the con- 

 traction of the pupil, another action of a " reflex " character is produced 

 through the Optic nerve; namely, the contraction of the Orbicularis muscle 

 under the influence of strong light, or when a foreign body is suddenly 

 brought near the eye. But this cannot be excited without a consciousness 

 of the visual impression ; in fact, it is a movement of a " consensual " kind, 

 produced by the painful sensation of light, which gives rise to the condition 

 well characterized by the term photophobia. The involuntary character of 

 it must be evident to every one who has been engaged in the treatment of 

 diseases of the eyes ; and the effect of it is aided by a similarly involuntary 

 movement of the eyeball itself, which is rotated upwards and inwards, to a 

 greater extent than the Will appears able to effect. Another reflex move- 

 ment excited through the visual sense is that of Sneezing, which is induced 

 in many individuals by the sudden exposure of the eyes to a strong light: 

 of the purely automatic character of this movement there can be no ques- 

 tion, since it cannot be imitated voluntarily ; and that it is not excito-motor, 

 is proved by the fact that it is not excited unless the light be seen. 1 



518. 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 

 undergo more or less completely, whilst passing between their ganglia and 

 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, etc.) ; and also, in great part at least, 

 with Birds. 2 In the Human subject, however, and in animals which, like 

 him, have the axes of both eyes directed to the same object, the view enter- 

 tained by many excellent anatomists and microscopists up to a recent period 

 was that suggested by Hannover, 3 namely, that the posterior border of the 

 Optic Chiasma is formed exclusively of eommissural fibres, which pass from 

 one optic ganglion to the other, without entering the real optic nerve ; 

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

 in like. manner, to act as a commissure between the two retime; 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, 

 Hannover considers to be the true Optic Nerve ; and in this he thought that 

 a portion of the fibres decussates, whilst another portion passes directly from 

 each Optic ganglion into the corresponding eye. The fibres proceeding from 

 the ganglia to the retinae, constituting the proper Optic Nerves, he distin- 

 guished into an internal and an external tract. Of these the external, on each 

 side, he maintained, passes directly onwards to the eye of that side ; whilst 



1 A patient was for some time in the London Hospital, in whom there was such an 

 undue impressibility of the retina, that she could not remain in even a moderate light 

 without a continual repetition of the act of Sneezing. 



2 See Solly on The Human Brain, 2d edit., p. 288. 



3 Das Auge, 1852. 



