NERVES OF SPECIAL SENSE. OPTIC. 717 



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 Fig- 191. 



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 ap- 

 pear that a portion of the fibres decus- 

 sates, whilst another portion passes 

 directly from each Optic ganglia into 



the Corresponding eye. The fibres which Course of fibres in the chiasma, as exhibited by 

 proceed from the ganglia to the retinse, tearin & off the superficial bundles from a specimen 



and constitute the proper Optic Nerves, ^ a f rdened /' n f rit ' * Anterio ; fi f bres ' c fi " >al 



, ,. . . f f . , " . ' between the two retinae, p. Posterior fibres, corn- 



may be distinguished into an internal missural between the thalamL fl , y Diagram of 



and an external tract. Of these, the the preceding. 

 external on each side passes directly on- 

 wards 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 j 1 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 exclusively with the outer side 

 of the left retina, and with the inner side of the right. Now, as either 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 the visual impressions, which are so important in directing the movements 

 of the body, into proper harmony with the motor apparatus ; so that, the de- 

 cussation of the motor fibres in the pyramids being accompanied by a decussa- 

 tion of the optic nerves, the same effect is produced as if neither decussated 

 which last is the case with Invertebrated animals in general. 



743. The functions of the Auditory nerve, or Portio Mollis of the 7th, are 

 easily determined, by anatomical examination of its distribution, and by obser- 

 vation of pathological phenomena, to be analogous to those of the two preced- 

 ing. Atrophy or lesion of the trunk destroys the sense of Hearing; whilst 

 irritation of it produces auditory sensations, but does not occasion pain. From 

 experiments made upon the nerve before it leaves the cranial cavity, it appears 

 satisfactorily ascertained that this nerve is not endowed either with common 

 sensibility, or with the power of directly stimulating muscular movement. 

 Nor can any obvious reflex actions be executed by irritation of this nerve; 

 but it seems nevertheless by no means improbable, that the muscles which 

 regulate the tension of the tympanum are called into action by impressions 

 made upon it and reflected through the auditory ganglion, in the same manner 



1 This arrangement was first hypothetically suggested by Dr. Wollaston ("Philos. 

 Trans.," 1824), as facilitating the explanation of some of the phenomena of vision, and 

 more particularly single vision with two eyes. We shall hereafter see, however, that the 

 singleness of the impression resulting from the formation of two pictures upon our retinas 

 is not attributable to any such anatomical arrangement, their combination being a mental 

 process, and the fusion of two dissimilar pictures being requisite to enable us to exercise 

 one of the highest attributes of the visual sense, the perception of projection. (See CHAP. 

 xv. SECT. 5.) 



