776 
Plaited optic nerve. 
The optic nerve in some animals exhibits a 
peculiar plaited appearance; this condition 
occurs in perfection in certain fishes, 
and in many birds a somewhat similar organi- 
zation may be detected, though not at all so 
perfectly as in the fish tribes. 
When the plaiting prevails in perfection, the 
nerve consists essentially of a thin membrane 
folded on itself exactly like a closed fan or a 
plaited frill ; but the arrangement referred to is 
not at first sight apparent, particularly if the 
nerve be inspected in that part of its course only 
which is external to the cranial cavity, for there 
the neurilema is so thick, dense, and tough, 
that a correct estimate of the disposition of the 
bervous structure cannot be formed until this 
investment has been removed. 
Deprived of its neurilema, the nerve seems to 
consist of a number of parallel lamine placed 
in juxta-position ; on closer examination these 
lamine turn out to be continuous with each 
other at their edges, and by a little care the 
nerve can be unfolded into a membrane of 
which the breadth is proportional to the num- 
ber and depth of the original lamine. 
When the plaited condition is perfect, it pre- 
vails along the entire length of the optic nerve, 
becoming manifest at its cerebral attachment, 
and continuing to the eye-ball; even the retina 
seems to participate in the same disposition, 
as folds or plaits are observable on the surface 
of that nervous expansion ; and in certain fish 
the optic lobes themselves present a similar 
organization, for the walls of the cavities which 
these tubercles contain are in some instances 
covered with lamine. 
The plaiting must be considered an essential 
attribute of the nervous substance and totally 
independent of the neurilematous investment, 
for this disposition of the nervous material 
occurs occasionally in the optic lobes and re- 
tina, structures which are devoid of neurilema. 
( Fig. 425.) 
Fig. 425. 
Plaited optic nerve of a 
Mullet. (From nature. ) 
a, optic nerve deprived 
of neurilema, exhibiting 
the plaited disposition ; 
b, sclerotic coat of the 
eye through which the 
nerve is passing; ¢, 
retina, in which the 
nerve terminates. 
Laminated optig nerve. 
In certain birds the optic nerve is laminated 
and bears a close resemblance to the plaited 
condition just described. 
In these birds a careful examination of the 
nerve is required for the discovery of its true 
texture, for the neurilema (endowed with un- 
common strength) adheres so firmly to the 
' nervous structure that without a careful dis- 
section its lamine elude observation. 
When viewed on one side the nerve seems 
perfectly smooth, but on the opposite side 
numerous lamine may be distinctly observed ; 
these are parallel to each other, and runplong 
OPTIC NERVES. 
the nerve in the longitudinal direction: they 
are of considerable depth and variable thick- 
ness; they are applied closely to each other, 
being separated by thin processes of neuri- 
lema only, and altogether (their straight course 
excepted) this arrangement bears a 1g 
similarity to the disposition of the lamine on 
the human cerebellum. A short section of the 
mate gare of its neurilema is not unlike a 3 
c k, the laminz representing the leaves, 
and the opposite smooth pales | the nerve ~ 
bearing a resemblance to the back and cover — 
of the book. The laminated optic nerve of the 
bird does not admit of being unfolded into a 
flat membrane, a proceeding which can be 
accomplished with care in the plaited optic 
nerve of the fish: and in birds \ 
structure occurs in that part of the nerve only 
which is in front of the chiasma; in its cerebr 
perm no such organization prevails. ( Fig. 
426. = 
4 
. 
aminatec 
ee. 
Fig. 426. 
, Eagle. ( From per 
a, chiasma; b, ‘ 
divested of neuri a 
exhibiting laming on o1 
surface ; ¢, : 
Desmoulins, who has paid the greatest ¢ 
tention to these varieties, 1 as the re 
of his extensive researches, that the plait 
arrangement of the optic nerve in fish, at 
its laminated condition in birds bears 
portion to the perfection of vision in the at 
mals under consideration; for those bi 
which are endowed with the most po 
piercing sight possess the laminated struet 
at a maximum of developement. For exat 
birds of prey, such as the eagle, the fal 
and the kite, evince an acuteness of this fac 
truly surprising; from heights in the ata 
phere, at which they are themselves alm 
invisible, they discover their prey on the gro 
and pounce on it with the most unerring 
tainty, whilst at the trifling distance of a 
yards other animals recognise such objects: 
difficulty: now birds of this class affore 
most perfect specimens of the laminated 
nerve. It is further stated on the same at 
rity that birds which at short distances po 
remarkably quick and accurate pe 
vision (more especially when in suc 1 
this faculty is exercised in media of di 
laminated optic nerves, and that me t 
A 
j 
degrees of density) are also provi 
which the plaited optic nerves occur are 
dacious habits, and consequently require 
erful organs of vision just as the birds: 
propensities. a 
e writer has examined the optie ne 
the stork, kingfisher, eagle, &e., and i 
as well as in all the fishes which have 
within his reach, his dissections have 
verified the anatomical descriptions givé 
Desmoulins. oo 
It is difficult to explain the superior 
bility supposed to be thus conferred” 
