1. VISUAL PIGMENTS
3
The lens is usually spherical and protrudes through the pupil. In
lampreys it is held against the cornea by the vitreous humor; no suspensory ligaments or muscles are attached to the lens. In most teleosts the
pupil is immobile, but in many elasmobranchs the iris is capable of extensive contraction. In the absence of corneal refraction, protrusion of
the lens through the pupil assures a wide field of view. The eye accommodates by small movements of the lens. In teleosts, it is pulled backward
by a retractor muscle; in elasmobranchs, it is pulled forward by a protractor muscle. The lens or cornea of many fishes contains pigments that
filter out violet or ultraviolet radiation, probably improving visual acuity
(Kennedy and Milkman, 1956; Denton, 1957; Motais, 1957). Little is
known about the aqueous or vitreous humors; among different species,
the vitreous ranges in consistency from a liquid to a firm gel.
Continuous with the peripheral border of the iris is the complex tissue
named “choroid” ( alternatively spelled chorioid) . The choroid combines
the functions of nourishing the retina and of absorbing stray light or reflecting it by a tapetum lucidum back through the retina (Section I, C).
In common with other nervous tissue, the retina has a high oxygen consumption (Lindeman, 1943). The innermost part of the choroid, lying
just behind the retina, is modified into a choriocapillary structure. In
most teleosts, but not in elasmobranchs, the choroid projects through the
optic cleft into the posterior chamber of the eye, as the richly vascular,
pigmented falciform process (Fig. 1; see Hanyu, 1959). The implication
of a nutritive function for this process is strengthened by the occurrence
of vitreal blood vessels closely applied to the retinal surface (Fig. 1) only
in those species lacking the falciform process (e.g., eels, puffers, and
anglerfishes) . Most teleosts ( and Amia) have a peculiar, specialized
“choroid gland,” which is actually a rete mirabile, located behind the
retina. Its structural similarity to the gas gland of teleost swim bladders
led Wittenberg and Wittenberg (1962) to measure the oxygen pressure
in the eyes of living marine fishes. In the vitreous, immediately in front
of the retina, the partial pressure of oxygen was highest (average values
250420 mm Hg) in teleosts with a prominent rete. Teleosts with smaller
retia had lower oxygen pressures (20-210 mm); elasmobranchs and
teleosts which lack a choroid gland had still lower pressures ( 10-20 mm) .
Clearly, active secretion of oxygen (the second case known in animals)
is associated with the choroid gland. Those fishes that have lost the
pseudobranch invariably lack a choroid gland also.
Only a general description of the retina is given here; additional information may be found in the section on visual electrophysiology. Fish
retinae are organized according to the ordinary vertebrate plan. Innermost
are the various neuronal and glial elements, which are relatively transparent. Light passes through these to the photoreceptor (“visual”) cells.
3
The lens is usually spherical and protrudes through the pupil. In
lampreys it is held against the cornea by the vitreous humor; no suspensory ligaments or muscles are attached to the lens. In most teleosts the
pupil is immobile, but in many elasmobranchs the iris is capable of extensive contraction. In the absence of corneal refraction, protrusion of
the lens through the pupil assures a wide field of view. The eye accommodates by small movements of the lens. In teleosts, it is pulled backward
by a retractor muscle; in elasmobranchs, it is pulled forward by a protractor muscle. The lens or cornea of many fishes contains pigments that
filter out violet or ultraviolet radiation, probably improving visual acuity
(Kennedy and Milkman, 1956; Denton, 1957; Motais, 1957). Little is
known about the aqueous or vitreous humors; among different species,
the vitreous ranges in consistency from a liquid to a firm gel.
Continuous with the peripheral border of the iris is the complex tissue
named “choroid” ( alternatively spelled chorioid) . The choroid combines
the functions of nourishing the retina and of absorbing stray light or reflecting it by a tapetum lucidum back through the retina (Section I, C).
In common with other nervous tissue, the retina has a high oxygen consumption (Lindeman, 1943). The innermost part of the choroid, lying
just behind the retina, is modified into a choriocapillary structure. In
most teleosts, but not in elasmobranchs, the choroid projects through the
optic cleft into the posterior chamber of the eye, as the richly vascular,
pigmented falciform process (Fig. 1; see Hanyu, 1959). The implication
of a nutritive function for this process is strengthened by the occurrence
of vitreal blood vessels closely applied to the retinal surface (Fig. 1) only
in those species lacking the falciform process (e.g., eels, puffers, and
anglerfishes) . Most teleosts ( and Amia) have a peculiar, specialized
“choroid gland,” which is actually a rete mirabile, located behind the
retina. Its structural similarity to the gas gland of teleost swim bladders
led Wittenberg and Wittenberg (1962) to measure the oxygen pressure
in the eyes of living marine fishes. In the vitreous, immediately in front
of the retina, the partial pressure of oxygen was highest (average values
250420 mm Hg) in teleosts with a prominent rete. Teleosts with smaller
retia had lower oxygen pressures (20-210 mm); elasmobranchs and
teleosts which lack a choroid gland had still lower pressures ( 10-20 mm) .
Clearly, active secretion of oxygen (the second case known in animals)
is associated with the choroid gland. Those fishes that have lost the
pseudobranch invariably lack a choroid gland also.
Only a general description of the retina is given here; additional information may be found in the section on visual electrophysiology. Fish
retinae are organized according to the ordinary vertebrate plan. Innermost
are the various neuronal and glial elements, which are relatively transparent. Light passes through these to the photoreceptor (“visual”) cells.