3. Another class of physiologists believed 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 &tutus optici were wasted.
That a partial decussatiou 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 froin 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 F backwards towards the chiasina, 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 &, gure represents ' preparation mad the manner des bed, and it is es tially similar t diagram of the man chiasma p fished some y since by Mayo; primitive fila of the nerves of course invis but the direct io the larger fasci admits of not the slightest question ; the o fasciculi of each tractus opticus cont onwards without interruption to form the o part of the optic nerve of the same side : rniddle fasciculi cross the chiasma obliq and after decussating the corresponding fasci of the other tractus, contribute to the forma of the optic nerve of the opposite side; and internal fasciculi cross the posterior part o chiasma transversely and uniting directly the corresponding fasciculi of the other tra seem to be strictly commissuml ; across front of the chiasma some fasciculi tak arched course, and being prolonged along the inner edges of the optic ne are likewise apparently commissural.
Some difficulty no doubt arises when the physiologist atternpts to reconcile with this anatomical arrangement certain facts already detailed : it may be remarked, however, that arguments derived from the pathology of the optic nerve can be but of little value, since they have been relied on in turn by the framers of each hypothesis as affording proof of their own peculiar views ; and in the Museum of the Richmond Surgical Hospital, Dublin, the writer has seen specimens of atrophied optic nerves in rnan which furnish the most contradictory evi dence upon the subject under discussion. The preparations alluded to were cases in which one eye had been destroyed either by local disease or accident, many years previous to death, and where in consequence the corresponding optic nerve became wasted from disuse, while the other optic nerve continued healthy. In the majority of these specimens the wasting has been propagated backwards to the opposite tractus opticus and has implicated that structure, while the corresponding tractus has been spared (seefig.420, A); in some examples both tractus optici have suffered a diminution of size and in general to an unequal amount ; and in one very remarkable instance the tractus opticus of the same side with the shrunken nerve has dwindled into a narrOw„band, while the other retains fully its normal dimenlions (fig. 420, B).
It may be fair to add, that the case quoted from Vesalius is considered by many, 'and amongst others by Gall and Spurzheim, as of doubtful authenticity, and the results of expe riments on living animals should be received with caution, for to argue from experiments on birds to the human subject is plainly fallacious, since the struCture of the chiasma is not iden tical in the two cases, and the great obstacles encountered in the performance of such expe riments on the mammalia renders them of trifling value.
Use of the chianna.—In the direct junction between two corresponding nerves of opposite sides displayed in the chiasma, the second pair form an exception to a general law ; for in no other known instance does a similar union occur ; it therefore becomes a subject of great interest to determine how far this anomaly admits of explanation by any unusual properties in the nerves so circumstanced.