654 FUNCTIONS OF THE CEREBRO-SPINAL NERVOUS SYSTEM. 



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, would thus be 

 such that, as Mr. Mayo concluded, the fibres from either optic ganglion is 

 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; whilst the left optic ganglion 

 is 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 

 the distribution of the nerves be correct, that in Man, as in the lower ani- 

 mals, each ganglion receives the impressions of objects situated on the oppo- 

 site side of the body. The recent researches of Biesiadecki, 1 Mandelstamm, 2 

 and Michel, 3 have thrown much doubt upon this view, and seem to prove 

 that the decussation of the optic nerves is complete in Man, though the fibres 

 pursue a wavy course, which might readily lead to the assumption of the 

 existence of the commissures and tracts above mentioned. In full accord- 

 ance with this is the observation of M. Brown-Sequard 4 that section of one 

 optic nerve causes complete amaurosis of the opposite eye, whilst an antero- 

 posterior section of the chiasma produces complete blindness of both eves. 

 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 decussatiou of the motor 

 fibres in the pyramids being accompanied by a decussation of the optic 

 nerves, the same effect is produced as if neither decussated, which last is the 

 case with Invertebrated animals in general. The view maintained by Hann- 

 over receives some support from the interesting observation of Mr. Towue, 

 that a blending of two colors occurs if their images are simultaneously pre- 

 sented to the inner side of one retina and the outer side of the other retina, 

 but not otherwise. 5 



519. The functions of the Auditory nerve, or Portio Moll is of the 7th, are 

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

 observation of pathological phenomena, to be analogous to those of the two 

 preceding. 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 as the diameter of the pupil is regulated 

 through the optic nerve. In the involuntary start, however, which is occa- 

 sioned by a loud and sudden sound, we have an example of a consensual 

 movement excited through the Auditory nerve, which is evidently analogous 

 to the closure of the eyes to a strong light. In certain morbidly impressible 

 states of the nervous system, as will be presently shown ( 533), the effect 

 of sounds on the motor apparatus is far more remarkable. It has been 



1 Wien Akitd. Sitz.-ber , Hd. xlii, 1801, p. 86. 



2 Centralblntt f. d. Med. Wiss., 1873, No. 22. 



1 Archiv f. Oplithalmol., Bd. xix, Heft 2, 1873. 



4 Brown-Si'-tjiiiird, Archives de Physiologic, 1872, p. 261. 



5 For additional facts bearing on this subject, see the observations of Mr. Towne, 

 On the Stereoscope and Stereoscopic Results, in Guy's Hospital Reports for 1862, p. 

 <!9, and 1863, p. 103. 



