Cartilaginous fish.—In cartilaginous fish a well-marked chiasma occurs. In this class the junction of the two optic nerves is no longer effected by means of mere cellular adhesions, as in the osseous fish, but a perfect union of the proper substance of the nerves constitutes a true cbiasma. The optic nerves arise each from the corresponding optic lobe chiefly; they quickly converge, and soon become confounded with each other in the chiasma; and so inti mate is their connection, that anatomists possess little information as to the exact arrangement of the nervous filaments in this structure (fig. 410).
Birds.—In birds the chiasma is large, being proportional to the size of the optic nerves ; by . a little management its organization can be accurately demonstrated. Maceration for a few days in spirits hardens this structure suffici ently to enable the operator to strip off the neurilemnia, and then, even without the aid of a lens, the chiasma may be seen to consist chiefly of laminm. Forcible extension of the optic nerves in such a manner as to tear through the superficial stratum of the chiasma on its lateral aspect, greatly facilitates the examination.
The laminx originate in the tr.actus opticus, and appear to spring from the inner part alone pf that mass ; they gain the chiasma, and here lhose derived from opposite sides of the brain form a reciprocal interlacement. A perfect and lPgular decussation of the inner filaments of the Iwo tractus optici thus takes place in the chiasma, in such manner that a large proportion of the tractus of one side is evidently traceable to the opposite optic,,nerye,,and vice versa-. but the outer part of each tractus opticus con tinues on to form the outer part of the optic nerve of its own side, and has no concern in the formation of the decussating laminm.
The number of laminre in the chiasma of different birds is subject to some variety, but in the entire class, without exception, the lami nated structure prevails (fig. 417).
Anzphibia and reptiles.—In amphibia and reptiles a laminated chiasma, sornewhat similar to that just described in birds, occurs ; but the decussating laminw are very variable in number, and in general much fewer than in Thus, in Amphisbcena, according to Muller, there are only five lamince in all, two from one side, and three from the other ; and in lacerta ocellata, according to the same authority, as many as eight have been counted, four on either side N. 418), In some reptiles the posterior part of the chiasma is strictly commissural, the inner part of each tractus opticus being appro priated to the formation of a band-like corn missure : in Amphisbcena a triangular space separates this band from the remainder of the chiasma (fig. 418).
MaTionalia and man.—In mammalia and mart the chiasrna is no longer laminated, and great difficulty occurs in attempts to display its real structu re.
The older anatomists were evidently unable to trace the filaments of the human optic nerves satisfactorily through the chiasma, and in con sequence they relied either on pathological facts, or the results of experiments, or the data furnished by comparative anatomy, to determine the mutual relations of the second pair in this part of their course. The conclusions arrived
at by such rnodes of investigation were un satisfactory, and remarkable for much dis crepancy.
i. 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 tiactus 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 corrcspond ing optic nerve atrophied as far back as the chiasma, and the tractus opticus of the sante 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 other, 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 StalIC 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 jig. 407) was also consideied 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, llertwig, Flourens, and others led to the conclusion, that if in the Mainmalia 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 Unpaired 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 retin : if one of the optic tubercles be removed, the sight of the opposite eye fails, and if both be removed, per fect blindness ensues.