772 



OPTIC NERVES. 



impressions are referred to the same side of the 

 brain, and not to opposite sides of the brain as 

 occurs in other cases." 



Wollaston adopted this hypothesis in conse- 

 quence of the ready explanation it affords of 

 certain cases of visits ilimidiatus: he on several 

 occasions in his own person experienced a tem- 

 porary defect of vision, in consequence of which 

 one and the same lateral half of every object 

 became invisible, whilst he still continued to 

 see the other half distinctly ; and the same 

 amount of blindness subsisted whether he em- 

 ployed one or both eyes in looking at the 

 object. 



Examples of the same form of amaurosis are 

 of no very unfrequent occurrence, and according 

 to the theoretical notions just propounded this 

 partial loss of vision originates in some func- 

 tional affection of one tractus opticus whilst the 

 other remains healthy ; those parts of the two 

 retinae which derive their origin from \hejaulty 

 tractus being supposed to labour under a tem- 

 porary amaurosis, while all other parts retain 

 their ordinary sensibility. 



In further support of this hypothesis, Mul- 

 ler's researches have convinced him that in 

 man single vision by the two eyes occurs only 

 when certain parts of the two retina? are affected 

 simultaneously, and that under other circum- 

 stances double vision ensues. This conclusion 

 has been arrived at, chiefly from the results of 

 experiments upon the eye-ball. Thus, when 

 the eye-lids are closed in a dark room, if 

 pressure be applied deeply to the eye so as to 

 affect the retina,.luminous spectra are prod need. 

 When certain parts of the two eye-balls are 

 subjected to pressure at the same time, a single 

 spectrum occurs, and when other parts of the 

 two eye-balls are pressed upon simultaneously 

 two spectra appear. Those parts of the two 

 retinae which in the above experiment furnish a 

 single spectrum are styled " identical," in con- 

 sequence of their identity of sensation ; and 

 those parts which produce two spectra are de- 

 nominated " non-identical,"forobvious reasons. 

 Some of the conditions under which single and 

 double vision respectively take place, seem to 

 be confirmatory of this doctrine of" identical " 

 and " non-identical" parts in the two retinae; 

 thus double vision is a common consequence 

 of any cause having a tendency to disturb the 

 relative directions of the optic axes : for exam- 

 ple, diplopia frequently occurs in cases of 

 strabismus, and double vision may (according 

 to Mailer) be produced in perfectly healthy 

 eyes by a simple experiment : if a spectator fix 

 his eyes upon an object and then press on one 

 of them in such manner as to alter the direction 

 of its axis, the object which at first seemed 

 single will assume a double appearance. These 

 phenomena admit of explanation on the suppo- 

 sition that in consequence of the distortion of 

 the axes of the eyes, the visual impressions take 

 effect on " non-identical " parts of the two 

 retinae, which t her (fore propagate two impres- 

 sions instead of one to the sensorium. 



These views apparently strengthen Newton's 

 hypothesis, for it may be presumed that the 

 " identical " parts of the two retinae are those 



which derive their origin from the, same tractus 

 opticus, and the " non-identical " on the con- 

 trary those which come from different tractus 

 optici. 



The comparative anatomy of the nerves in 

 question furnishes some facts favourable to 

 Newton's hypothesis ; thus many animals in 

 which the eyes are directed laterally, in such 

 manner that each embraces a totally different 

 field of vision, have no chiasma, and their optic 

 nerves cross each other, so that the right retina 

 is in connection solely with the left side of the 

 brain, and vice versa. This arrangement pre- 

 vails in the majority of osseous fish, and, for so 

 far, affords negative proof of the hypothesis 

 under consideration. The following theoretical 

 explanation may be offered. " In these animals, 

 owing to the position of their eyes, the same 

 object can never be depicted on the two re- 

 tinae simultaneously ; consequently, in them no 

 provision to ensure single vision of the same 

 object by both eyes is required, and therefore 

 no parts of the two retinae have a common con- 

 nection with one and the same side of the brain, 

 the two optic nerves being derived respectively 

 from opposite sides of the organ." 



Again, Mr. Solly has shown that in many 

 fish, such as the skate, (in which the eyes are 

 so set that the respective fields of vision may 

 comprise in a great measure the same objects,) 

 a chiasma exists : and the anatomy of the 

 chiasma in birds is likewise on the whole fa- 

 vourable to the hypothesis; for in these animals 

 the optic axes are in general very divergent, 

 and consequently the respective fields of vision 

 can have but little identity ; a fact which agrees 

 theoretically with the almost perfect decussation 

 observable in their laminated chiasma. 



Although this explanation of single vision 

 has been sanctioned by the authority of Newton 

 and Wollaston, and supported by strong ana- 

 tomical facts and analogies, it will scarcely 

 stand the test of critical examination ; its va- 

 lidity has therefore been much questioned and 

 apparently with ju<tice,forthefollowing reasons : 



1. " Identity of sensation" is not exclusively 

 a special attribute of the second pair ; although 

 it exists in them in great perfection, other nerves 

 must also be admitted to possess the same pro- 

 perty ; thus notwithstanding that both ears are 

 commonly employed simultaneously for the ordi- 

 nary purposes of hearing, the sensation ofasingle 

 impression of sound is in general propagated to 

 the sensorium ; and although both nares are 

 used in the appreciation of odours, the sensation 

 of single impressions of scents is most usually 

 produced : now since neither the olfactory 

 nor the auditory nerves are provided with a 

 chiasma, and nevertheless these nerves unde- 

 niably evince a unity of sensation, there is good 

 reason for scepticism when the very same pro- 

 perty in the optic nerves is attributed to the pre- 

 sence of a chiasma. 



2. Many facts in pathology are obviously at 

 variance with Newton's theory ; if it were the 

 true explanation of single vision, morbid affec- 

 tions of one side of the brain (whenever pro- 

 ductive of amaurosis) ought to implicate more 

 or less of one half of each retina, whereas expe- 



