6
F. W. MUNZ
retina and bringing more distant objects into focus. In elasmobranchs
there are smooth muscle fibers in the ciliary body that pull the lens closer
to the cornea. (The ciliary body is a continuation of the choroid that
separates the anterior and posterior chambers and from which the iris
projects. ) Thus, accommodation would bring near objects into focus. This
mechanism has not been demonstrated unequivocally in elasmobranchs
( Nicol, 1963).
Pumphrey (1961) has stated very clearly what he believes is the
method of accommodation in trout and, probably, in other teleosts. When
a distant object to the side of the animal is viewed by the unaccommodated eye, its image is focused on the retina (Fig. 2A). Because of the
short focal length of the lens, closer objects are imaged only a few microns
farther from the lens. Therefore, even quite nearby objects should also
be in good focus. During accommodation, the retractor lentis muscle (of
ectodermal origin) moves the lens posteriorly, rather than toward the
fundus (back) of the eye (Fig. 2A). Laterally placed objects should still
be imaged on the retina fairly sharply. The view of objects directly in
front of the trout is very different. The retina is not a hemisphere concentric with the lens, but it is somewhat ellipsoidal. For such objects
viewed by the unaccommodated eye, the distance from lens to retina is
slightly greater than for laterally placed objects. To the front, therefore,
the trout is near-sighted (myopic); Pumphrey stated that objects between 10 and 20 cm away would be seen clearly (Fig. 2B). At rest, the
eyes are well adjusted for binocular viewing of any prey at close range.
The effect of accommodation is to decrease the distance from lens to
retina, bringing more distant objects into focus. In this connection, recall
the temporal location of fish foveae and specialized retinal areas.
Certain data recently obtained by Baylor and Shaw (1962) are relevant to accommodation and Pumphrey’s interpretation. In the alewife,
Alosa, the retina is also ellipsoidal rather than spherical. Retinoscopic
measurements were made of refractive error in the eyes of living, immersed, marine fishes ( 5 elasmobranch and 17 teleost species). The eyes
were all far-sighted (hypermetropic). In the alewife and silversides,
Menidia, measurements were made from positions lateral and anterior
to the fish. From the anterior position, the average refractive error was
5 (alewife) or 8 (silversides) diopters less than from the lateral position.
These differences are consistent with the ellipsoidal shape of the retina
and are within the range of accommodation observed by Baylor and
Shaw. The difficulty seems to be whether the unaccommodated eye is
normal-sighted (emmetropic) as stated by Pumphrey ( 1961) or farsighted. A crucial question is what specific ocular structure is responsible
for the retinoscopic reflection. Baylor and Shaw stated that reflection
comes from behind the retina; Nicol (1963) has indicated that this point
F. W. MUNZ
retina and bringing more distant objects into focus. In elasmobranchs
there are smooth muscle fibers in the ciliary body that pull the lens closer
to the cornea. (The ciliary body is a continuation of the choroid that
separates the anterior and posterior chambers and from which the iris
projects. ) Thus, accommodation would bring near objects into focus. This
mechanism has not been demonstrated unequivocally in elasmobranchs
( Nicol, 1963).
Pumphrey (1961) has stated very clearly what he believes is the
method of accommodation in trout and, probably, in other teleosts. When
a distant object to the side of the animal is viewed by the unaccommodated eye, its image is focused on the retina (Fig. 2A). Because of the
short focal length of the lens, closer objects are imaged only a few microns
farther from the lens. Therefore, even quite nearby objects should also
be in good focus. During accommodation, the retractor lentis muscle (of
ectodermal origin) moves the lens posteriorly, rather than toward the
fundus (back) of the eye (Fig. 2A). Laterally placed objects should still
be imaged on the retina fairly sharply. The view of objects directly in
front of the trout is very different. The retina is not a hemisphere concentric with the lens, but it is somewhat ellipsoidal. For such objects
viewed by the unaccommodated eye, the distance from lens to retina is
slightly greater than for laterally placed objects. To the front, therefore,
the trout is near-sighted (myopic); Pumphrey stated that objects between 10 and 20 cm away would be seen clearly (Fig. 2B). At rest, the
eyes are well adjusted for binocular viewing of any prey at close range.
The effect of accommodation is to decrease the distance from lens to
retina, bringing more distant objects into focus. In this connection, recall
the temporal location of fish foveae and specialized retinal areas.
Certain data recently obtained by Baylor and Shaw (1962) are relevant to accommodation and Pumphrey’s interpretation. In the alewife,
Alosa, the retina is also ellipsoidal rather than spherical. Retinoscopic
measurements were made of refractive error in the eyes of living, immersed, marine fishes ( 5 elasmobranch and 17 teleost species). The eyes
were all far-sighted (hypermetropic). In the alewife and silversides,
Menidia, measurements were made from positions lateral and anterior
to the fish. From the anterior position, the average refractive error was
5 (alewife) or 8 (silversides) diopters less than from the lateral position.
These differences are consistent with the ellipsoidal shape of the retina
and are within the range of accommodation observed by Baylor and
Shaw. The difficulty seems to be whether the unaccommodated eye is
normal-sighted (emmetropic) as stated by Pumphrey ( 1961) or farsighted. A crucial question is what specific ocular structure is responsible
for the retinoscopic reflection. Baylor and Shaw stated that reflection
comes from behind the retina; Nicol (1963) has indicated that this point