. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. OPTIC NERVES. Plaited optic nerve. The optic nerve in some animals exhibits a peculiar plaited appearance; this condition occurs in greatest 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
. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. OPTIC NERVES. Plaited optic nerve. The optic nerve in some animals exhibits a peculiar plaited appearance; this condition occurs in greatest 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 nervous structure cannot be formed until this investment has been removed. Deprived of its neurilema, the nerve seems to consist of a number of parallel laminae placed in juxta-position; on closer examination these laminae 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 laminae. 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 laminae. 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 o
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