OPTIC NERVES. 
nerves thus united; and for these reasons the 
physiology of the chiasma is inyested with 
uncommon interest. 
The existence of a chiasma is not general 
throughout the animal series, and even when 
present it exhibits much diversity of appearance 
and structure in different classes. <A brief ex- 
position of some of its more striking varieties, 
in animals, will probably constitute the best 
introduction to the study of the chiasma in man, 
Invertebrata.—In the invertebrate classes 
nothing like a chiasma has been demonstrated, 
nor has any mutual crossing of the special optic 
nerves been proved to exist. The nerves which 
are furnished to the compound eyes of insects 
and crustaceans pass in a direct course to their 
destination ; the same remark applies to the 
merves from which the lens-eyes of insecta, 
yj arachnida, crustacea, and mollusca, derive their 
p sensibility; and it may be presumed that the 
nerves which supply the simple eye-dots of 
__annelida and other inferior animals are similarly 
__circumstanced. 
Osseous fish—In osseous fish the optic 
nerves generally cross each other at an acute 
‘angle, in such manner that the nerve which 
comes from the right side of the brain goes 
‘distinctly to the /eft eye, and vice versa: at the 
“point of decussation the nerves lie one over the 
other ; they are usually flattened at this spot, and 
closely joined together, but this junction is 
‘effected by means of cellular or fibrous ad- 
hesions only, as no intermingling of the nervous 
laments takes place, and the nerves themselves 
ean be isolated without injury to their proper 
Structure (fig. 407). 
Cartilaginous fish—In cartilaginous fish a 
well-marked chiasma occurs. In this class the 
unction of the two optic nerves is no longer 
effected by means of mere cellular adhesions, 
as in the osseous fish, buta perfect union of the 
proper substance of the nerves constitutes a 
“tue chiasma. 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- 
aate is their connection, that anatomists possess 
fittle information as to the exact arrangement 
of the nervous filaments in this structure 
ig. 410). 
Birds.—In birds the chiasma is large, being 
Proportional to the size of the optic nerves; by 
@ 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 
meurilemma, and then, even without the aid ofa 
ens, the chiasma may be seen to consist chiefly 
of lamine. Forcible extension of the optic 
merves, in such a manner as to tear through the 
nperficial stratum of the chiasma on its lateral 
spect, greatly facilitates the examination. 
The laminz originate in the tractus opticus, 
and appear to spring from the inner part alone 
of that mass ; they gain the chiasma, and here 
those derived from opposite sides of the brain 
yorm areciprocal interlacement. A perfect and 
egular decussation of the inner filaments of the 
Wo tractus optici thus takes place in the 
VOL. 11. 
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763 
chiasma, in such manner that a large proportion 
of the tractus of one side is evidently traceable 
. 
to the opposite optic_nerve, and vice versa: 
but the outer part of waa toto 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 lamine. 
The number of lamine 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). 
Fig. 417. 
My: 
Chiasma of the common fowl. (After Miiller.) 
a, a, optic nerves; 6, chiasma, dissected so as 
to shew its decussating lamine; c, ¢, tractus 
optici. 
Amphibia and reptiles—In amphibia and 
reptiles a laminated chiasma, somewhat similar 
to that just described in birds, occurs; but 
the decussating lamine are very variable in 
number, and in general much fewer than in 
birds. 
Thus, in Amphisbeena, according to Miiller, 
there are only five laminz in all, two trom 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 (fig. 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 com- 
missure: in Amphisbeena a triangular space 
separates this band from the remainder of the 
chiasma (fig. 418). 
. Fig. 418. 
A. B. 
a 
pe 
© | 
Chiasma in Amphisbena. (After Miiller.) 
A, section of chiasma to exhibit the decussating 
laminz, of which there are three from one side and 
two from the other. ‘ 
B, chiasma seen from below. 
a, a, optic nerves; 5, b, tractus optici; ¢, com- 
missural band; d, triangular space; f, true chi- 
asma, 
Mammalia and man.—In mammalia and man 
the chiasma is no longer laminated, and great 
difficulty occurs in attempts to display its real 
structure. 
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 
3D 
