770 



OPTIC NERVES. 



part of their course. The conclusions arrived 

 at by such modes of investigation were un- 

 satisfactory, and remarkable for much dis- 

 crepancy. 



1. Some maintained that the nerves are 

 merely placed in exceedingly close juxta- 

 position in the chiasma, without any inter- 

 crossing of their respective filaments, and that 

 each tractus opticus in reality passes on to form 

 the optic nerve of its own side. 



These views were supported by Vesalius,* 

 who detailed the particulars of a case in which 

 after death the two optic nerves were found 

 perfectly distinct from each other throughout 

 their whole course, and consequently no chiasma 

 existed, although, during life, vision had been 

 unimpaired ; and the same hypothesis was 

 strengthened by Santorini and others, who, in 

 certain instances where one eye had been 

 destroyed many years before death, observed on 

 the post mortem examination the correspond- 

 ing optic nerve atrophied as far back as the 

 chiasma, and the tractus opticus of the same 

 side wasted, while the nerve and tractus of the 

 opposite side were perfectly healthy. 



2. Others were persuaded that a perfect de- 

 cussation exists in the chiasma, and that all the 

 filaments of the tractus opticus of one side pass 

 fairly across to form the optic nerve of the oilier, 

 and vice versa. In favour of this opinion it 

 was urged that in the majority of cases of long- 

 continued blindness of a single eye the opposite 

 tractus opticus, and not the tractus on the same 

 side with the affected eye, becomes atrophied. 

 Scemmerring observed several such cases in the 

 human subject, and traced the same appearances 

 in the horse, dog, squirrel, rabbit, hog, cat, and 

 chamois : and Cuvier preserved in spirits the 

 brain of a horse in which the wasting of one 

 optic nerve continued backwards into the oppo- 

 site tractus. The evident manner in which the 

 optic nerves in osseous fish cross each other (see 

 fig. 407) was also considered favourable to this 

 view, and the results of experiments on living 

 animals were confidently appealed to in farther 

 confirmation of the same. Thus the experiments 

 of Rolando, Pourfour Petit, Saucerotte, Hertwig, 

 Flourens, and others led to the conclusion, that 

 if in the Mammalia one hemisphere of the ce- 

 rebrum be injured deeply or removed, vision 

 becomes impaired or destroyed in the opposite 

 eye ; if the two cerebral hemispheres be suc- 

 cessively subjected to the same treatment, vi- 

 sion becomes successively impaired or destroyed 

 in both eyes; if one of the nates be removed, the 

 animal sees no longer on the opposite side; and 

 if both be removed, blindness affects both eyes 

 in succession and occurs constantly in the eye 

 opposite to the injured tubercle; if in birds one 

 of the cerebral hemispheres be removed, vision 

 becomes extinguished in the opposite eye; if the 

 two hemispheres be removed in succession, a 

 cross paralysis affects the two retinae : if one of 

 the optic tubercles be removed, the sight of the 

 opjiositc eye fails, and if both be removed, per- 



jt'ct blindness ensues. 



3. Another class of physiologists believed 



* Vesalius dc Corp. Hum. Fab., 1. iv. c. iv. 



that a partial decussation occurs in the human 

 chiasma, and that some of the filaments of each 

 tractus opticus continue on into the nerve of their 

 own side, while others cross obliquely into the 

 optic nerve of the opposite side. Certain facts in 

 pathology seemed difficult of explanation on 

 any other supposition ; thus cases of long-con- 

 tinued blindness of a single eye, the result of 

 accident, have been met with, in which after 

 death the corresponding optic nerve was found 

 atrophied as far back as the chiasma (the optic 

 nerve belonging to the healthy eye being of 

 fully its ordinary dimensions, or even larger 

 than natural) while both tractus optici were 

 wasted. 



That a partial decussation does occur in the 

 human chiasma can now no longer admit of 

 doubt, for of late years the existence of this 

 organization has been proved by actual dis- 

 section, and it can be rendered apparent to the 

 naked eye by the precaution of hardening the 

 nervous substance before the dissection is com- 

 menced. According to the writer's experience 

 immersion in spirit hardens the preparation in 

 the most satisfactory manner. In performing 

 the dissection the neurilemma must be first care- 

 fully removed from the chiasma, and also from 

 the adjoining portions of the optic nerves and 

 tractus optici ; after this preliminary, each optic 

 nerve should be divided transversely a little in 

 front of the chiasma ; a transverse incision car- 

 ried horizontally into the cut surface of the 

 cerebral extremity of each nerve will then 

 enable the operator to split it in a direction 

 backwards towards the chiasma, and by pro- 

 ceeding cautiously a horizontal division of the 

 chiasma, and of a part of the optic nerves and 

 the tractus optici, will be effected. ( Fig. 419 ) 



The annexed fi- 



Fig. 419. gure represents a 



preparation made in 

 the manner descri- 

 bed, and it is essen- 

 tially similar to a 

 diagram of the hu- 

 man chiasma pub- 

 lished some years 

 si nee by Mayo; the 



primitive filaments 

 ^ neryes are 



c . .,, 



of course invisible, 

 a a, optic nerves ; b b, trac- but tlle direction of 



tus P tlcl< the larger fasciculi 



admits of not the slightest question ; the outer 

 fasciculi of each tractus opticus continue 

 onwards without interruption to form the outer 

 part of the optic nerve of the same side : the 

 middle fasciculi cross the chiasma obliquely, 

 and after decussating the corresponding fasciculi 

 of the other tractus, contribute to the formation 

 of the optic nerve of the opposite side ; and the 

 internal fasciculi cross the posterior part of the 

 chiasma transversely and uniting directly with 

 the corresponding fasciculi of the other tractus 

 seem to be strictly commissural ; across the 

 front of the chiasma some fasciculi take an 

 arched course, and being prolonged forwards 

 along the inner edges of the optic nerves they 

 are likewise apparently commissural. 



Human Chiasma. (Froma dis- 

 section made by the writer.) 



