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The role of the spectacle in the visual optics of the snake eye
Vision &s. Vol. 17, pp. 293 to 298. Pergamon Press 1977. Printed in
Great Britain.
THE ROLE OF THE SPECTACLE IN THE VISUAL OPTICS OF THE SNAKE EYE J. G.
SIVAK
Laboratory of Comparative Optometry, University of Waterloo, Waterloo, Ontario (Received 5 April 1976)
Abstract-The atypical morphology and physiology of the snake eye is commonly attributed to a subterranean evolutionary period. It is suggested that many characteristics of this eye are due to the optical effects of the spectacle. Study of the gross morphology of the optical media of three species was undertaken by means of a rapid freezing and sectioning method. Refractive indices were measured. The refractive contribution of the spectacle is significant and almost equal to that of the lens. However, the spherical shape of the lens and its relatively high refractive index indicate an enhanced refractive function for this structure. Study of more primitive species is required.
INTRODUCTION
The eye lids of the snake are fused to form a trans-
parent covering over the cornea. This covering is known as the spectacle (Walls, 1934; Bellairs and Underwood, 1951). In contrast to the lizard, the snake eye lacks a ciliary muscle. Variations in focal power are achieved through movement of a spherical lens in response to changes in intraocular pressure induced by contraction of the iris musculature (Michel, 1932). The atypical morphology and physiology of the snake eye led Walls (1940, 1942) to conclude that this eye had evolved secondarily from a rudimentary state induced by an ancestral subterranean way of life. While objections exist (Armstrong, 1950; Bellairs, 1950), this view has been widely accepted (DukeElder, 1958; Romer, 1970). The effect of the spectacle on the optics of the snake eye is not known. Walls (1942) indicated that sebaceous material from the gland of Harder may enter the space between the spectacle and cornea and act as a lubricant. He also suggested that a spectacle of flattened curvature would reduce the refractive contribution of the cornea. In a situation in which the lens is the major refractive element (e.g. the fish eye), a spherical lens and an accommodative mechanism involving lens motion might be expected (Beer, 1894; Sivak, 1975). The present study will examine the relative refractive importance of the spectacle, cornea and lens of the snake eye in order to determine whether evolutionary pressures of an optical nature may have been exerted on this eye during its development.
METHODS
The study was carried, for the most part, at Mote Marine Laboratory, Sarasota, Fla. The species studied included four specimens of Coluber constrictor priapus (black racer), four specimens of Elaphae qundrauittata quadrauittata (yellow rat snake), and one specimen of EIaphae guttata (red rat snake). Each specimen was anaesthetized by being placed in an ether chamber. Refractive state was deter-
mined for each eye with a retinoscope and trial lenses before and after anaesthesia. Differences in the pre- and post-anaesthesia refractive error were assumed to indicate accommodative facility. Following anaesthesia, each specimen was sacrificed by decapitation. The eyes and surrounding orbital adnexa were removed with the spectacle intact. Intraocular dimensions
One eye was used to determine linear dimensions and radii of curvature of the ocular media in accordance with the following procedure: Immediately following its enucleation, the eye was rapidly frozen in a mixture of acetone and dry ice. S&e the &era of the snake eye lacks scleral ossicles or cartilage (Walls. 1940: Sivak. 19761 care was taken to avoid diitoiting the overall shape oithe globe. A thin nylon mesh was used to move the eye to and from the freezing mixture. The frozen eye was mounted for sagittal sectioning on a freezing microtome, and kept frozen through the release of carbon dioxide. A single lens reflex camera and bellows was mounted above the preparation. As thin sections of the eye were removed (lOtim over the middle third of the eye) photographs were taken of the remaining block of tissue. This procedure is in line with previous studies (Sivak, 1974, 1976) in which it was felt that photographs of large blocks of tissue were less apt to be distorted than photographs of thin microtome sections. The data of two of the nine eyes studied were discarded due to improper orientation on the microtome stage. Lens thicknesses were measured with a travelling microscope from the resulting photographic negatives. The negative, in the case of each eye, indicating the greatest lens thickness was assumed to represent a sagittal section along the geometric axis of the eye. Intraocular axial measurements were made along the line joining the center of the pupil to the center of the lens. Measurements were made of the distance between the posterior lens surface and the vitreous and choroid surfaces of the retina, lens thickness and the distance between the anterior lens surface and the cornea and spectacle. In addition, chord and sagittal depth measurements were made of the spectacle and cornea. Radii of curvature were calculated according to the formula : YZ s ‘=zs+T where S = sagittal depth of a chord and y = l/2 of that chord. Linear measurements were considered accurate to within 0.01 mm.
293
‘94
J.
Rg$ractice
G.
SIVAK
indices
KESCLTS
The determination of refractive indices of the refractive media of the second eye were made with an Abbe refractometer The procedure conformed to that used to measure the refractive index of liquids (Monk, 1963). With the exception of the spectacle, all measurements were carried out by placing a sample of either cornea, lens, aqueous or vitreous humors between the two refractometer prisms. Since the refractive index of the lens varies from the cortex to the core, the index measured in this manner was assumed to represent the equivalent index (Citron and Pinto. 1973). Measurements were repeated three or four times for each type of tissue. The fluid between the spectacle and the cornea was obtained by piercing the spectacle near its edge with a syringe needle. The fluid which welled up through the point of penetration was removed with a micro-pipette and placed on the measuring prism of the refractometer. In five of nine eyes studied, the quantity of fluid was insufficient to permit refractometer readings. Aqueous humor was obtained with a syringe following penetration of the cornea.
The refractive index of the spectacle was measured in accordance with the refractometer procedure used to measure the index of solids (Monk, 1963). Due to the curved nature of the spectacle, only two of the nine studied were large enough to permit adequate prism contact for measurement. The indices of three additional spectacles were found by measuring the actual spectacle thickness with a micrometer and the apparent thickness with a microscope. The refractive index is equal to the real thickness over the apparent.
The comparison of refractive states before and after anaesthesia indicates that an accommodative ability of considerable magnitude (up to 13 D) exists in the species studied (Table 1). Furthermore, the direction of refractive change agrees with the accommodative mechanism described by Michel (1932) and Walls (1942). Relaxation of the intraocular musculature with anaesthesia produces movement of the lens toward the retina and results in a refractive state shift toward hyperopia. Intraocular dimensions and refractive indices are very similar in the species studied (Tables 2 and 3). In contrast to statements by Walls (1942) and Bellairs and Underwood (19.51), the overall shape of the eye is slightly flattened and not spherical or slightly elongated. The average equatorial diameters of the eyes studied are approx lo”:, larger than that measured along the pupil axis. As noted by Walls (1940), the spectacle is somewhat flattened in shape. Its radius of curvature is always less than the radius of the scleral portion of the eye. However, this factor is compensated for by the high refractive index of the spectacle (1.50). According to Pumphrey (1961), the maximum index for biological material is about 1.53 (e.g. chitin, keratin). The refractive index of the fluid between the spectacle and cornea was found to be equal to or very
Table 1. Refractive states of the eyes of three species of snakes before and after ether anaesthesia. Measurements were made with a retinoscope along the pupil axis. The notations (+) and (-) indicate hyperopia and myopia respectively
Unanaesthettred Left c)e Rl$ht eye 0 00 + I.IX1 Jr 1.M) + 2.25
+I 25 + 2 25 i 50 * 2.00
-0.25 - 2.00 -0.50 + 2.25
~0.50 -200 -0.75 + 2.00
0.00
Table
2. Refractive
Snakes C. coluber
indices
priapus
of ocular
media
I
+025
of three species of snakes. Values 1973) are provided for comparison
of two lizard
I 365 I 36b
1.515 I.480 1.50 I 50
1.335 1.336 1.3355
I358 S61 1.364
1.495 I.510
1.338 1.337
1.50
1.336 1.337
E. wtmta
1.370 1.375 1.36X 1.373 ,371 1.362
Lizards Gecko gecko Iguana iguana
1.36 35
s E. qudravittafa
I
quadravittato
x
1
species (Citron
I I
, 440 I 433 I 446
,
1.453 1.443
I.735 33b 335 1.336 3355
I 335
I 455
and Pinto,
1.?3( ,31b I335 I336 13355
1.33b 1.336 1.336 336 1.335
I
I.458 I.455 I.464
1.335 1.335 1.335 I.336 I.335 1336
,336 336
1.415 1.4
I ?Ih I336
1
1.450
I 458
295
Fig. 1. Negative print of a vertical section of the eye of a red rat snake (Elaphae gurtata) following rapid freezing. The axial diameter of the lens is 2.57 mm.
Visual optics of the snake eye
297
Table 3. Intraocular dimensions, radii of curvature and refractive powers of the spectacle and lens of the eyes of three species of snakes. All linear measurements are to the nearest 0.01 mm. Spectacle values include front and rear surfaces. Lens values represent the equivalent lens power as calculated for a spherical lens. The refractive indices
used in the calculations were the averages reported in Table 2. Dimensions involving the retina refer to the retina choroid border Length (overall, cm)
Sp&Wleretina (mm)
(mm)
Post. lensretina (mm)
Spectacle radius of curvature (mm)
Spectaclerelr.
lens surf. (mm)
Spectacle-ant.
1.46 12.0
4.92 4.55
3.93 3.40
5.87 5.36
57.2 62.6
53.8 58.1
0.99 0.85 0.82 0.67
3.18 2.97 3.04 2.61
2.38 2.09 1.94 1.78
3.79 3.31 3.71 3.34
88.9 101.8 90.9 1cQ.s
104.2 110.5 108.0 125.6
0.68
2.57
1.75
2.72
117.9
140.0
C. coluber priepm 130.0 10.32 128.5 9.15 E. quadravirtata qdrauittata 137.5 6.55 135.0 5.91 133.0 5.80 132.5 5.06 E. quffofa 135.0
5.0
Lens diameter
slightly higher to that of the aqueous and vitreous humors of the eye. This finding casts doubt on the view that this space is filled with a sebaceous secretion of Harderian gland origin (Walls, 1942). Corneal refraction need not be considered since its index is not significantly greater than that of the fluid in front of it. Since the indices of the pre-cornea1 fluid and aqueous humor are virtually identical, and since the anterior and posterior cornea1 surfaces are parallel or nearly parallel to one another, the index of refraction of the cornea is largely irrelevant. In contrast to the smooth and glossy appearance of the spectacle, the cornea was very wrinkled once the spectacle was removed. Calculations of the refractive power of the spectacle were made in accordance with the formula: F=_
n1 - n r
where n’ = the refractive index of the medium into which light is passing, n = the refractive index of the medium from which light is passing and I = radius of curvature. The spectacle is a thin lens (approx 0.08 mm thick). Both anterior and posterior surfaces (assumed to be parallel) were considered when calculating spectacle refractive power (Table 3). A spherical or near-spherical lens shape was noted in each of the eyes studied (Fig. 1). In two instances (E. quadruvittata quadravittatu) the axial diameter of the lens was slightly larger (2%) than the equatorial diameter. A comparison of lens refractive indices indicates a signific+ntly higher index in snakes than in lizards (Table 2), confirming a prediction by Walls (1940). The radius of curvature of the lens was assumed to be one half the measured axial diameter. Lens refractive powers were calculated in accordance with the formula: F
=
power
(D)
Lens refr. power (D)
DISCUSSION The above results clearly indicate that while the spectacle has virtually eliminated the cornea as a refractive element of the eye, the spectacle itself has assumed an important refractive function. However, it is also apparent that the refractive role of the lens has been enhanced. The latter point is made clearer by comparing the snake eye to the eye of a lizard (Fig. 2). Citron and Pinto (1973) determined the optical constants of the eye’of Iguana iguana, a diurnal lizard. An iguana eye having an axial length of 8.7mm possesses a cornea with a refractive power of 129 D while the equivalent refractive power of the lens is only 41 D. Mahendra (1938) noted wide variations in spectacle size relative to the tize of the eyes of snakes. In the majority (including the three species studied here), the spectacle is about the same size as the cornea which it covers. However, in a number of genera the spectacle is much larger and covers a considerable portion of the head around the eye. Mahendra considered the large spectacle type to represent the primitive condition. If the spectacle surface is assumed to be a portion of a sphere, then one which is much larger than the eye will have a large radius of curvature and little refractive ability. In this instance, the lens would assume most, if not all of the refractive burden of the eye. The need to study additional species of snakes is indicated.
LIZARD
SNAKE n.lYF-CoRNEel-” “=1336--IO”EO”S-“=,
x1 - hhll r
where I = radius of the lens, n, = refractive index of the ocular humors (1.3355) and n2 = refractive index of the lens. The refractive powers of the lens and spectacle are almost equal in the black racer while that of the lens is about 15% greater than that of the spectacle in the rat snakes (Table 3).
-
Inun -
Fig. 2. Comparison of the intraocular dimensions and refractive indices of a lizard eye (Iguana iguana; Citron and Pinto, 1973) and a snake eye. The subspectacle fluid of the snake eye (refractive index = 1.3355-1.337) has been omitted for clarity.
29x
J. G.
ilcknowledyements-This study was supported by a grant from the National Council of Canada. The author is grateful to the Mote Marine Laboratory, Sarasota. Florid;, for facilities and assistance. In particular. the author wishes to thank Ms. Donna Johnson and Dr. Lvnn Carter.
REFERENCES
Armstrong C. W. (1950) An experimental study of the visual pathways in a reptile (Lacertu uivipara). J. Anat. 84, 146-l 67. Bellairs A. D.‘A. (1950) Observations on the cranial anatomy of Annie/la, and a comparison with that of other burrowing lizards. Proc. zoo. Sot. Land. 119, 887-904. Bellairs A. D.‘A. and Underwood G. (1951) The origin of snakes. Biol. Ret;. 26, 193-237. Beer T. (1894) Die Accommodation des Fischauges. Pj7iigers Arch. yes. Physiol. 58, 523-650. Citron M. C. and Pinto L. H. (1973) Retinal image: larger and more illuminous for a diurnal lizard. vision Res. 13, 873-876.
SlVAK
Duke-Elder S. (1958) System qf Ophtkml,,~o/oy~. Vol. 1. I ii<, EL.~ in Eoolution. Henrv Kimnton. London. Maiendra B. C. (1938) some iemarks on the phvlo.zen\ of the Ophidia. Anclt. An:. 86, 347 3% Michel K. (19321 Die Akkommodatlon des Schl;~ngenauges. Jew. Z. b’at~vw. 66, 577 62X. Monk G. S. (1963) Light Princip/c,s md E.~pcwn~wr.~. Dover. New York. Pumphrey R. J. (1961) Concernmg {-ision. In The Cell ~lnt/ r/le Oryunisnz (Edited by Ramsay J. A. and Wigglesworth V. B.). pp. 193 208. Cambridge University Press. LendOtI.
Romer A. S. (19701 The Vertehrute Body. Saunders, Philadelphia. Sivak J. G. (1975) Accommodative mechanisms in aquatic vertebrates. In Vision in Fishes (Edited by Ali M. A.), pp. 289-297. Plenum. New York. Walls G. L. (1934) The significance of the reptilian “spectacle”. .Am. J. Ophthal. 17, 1045-1047. Walls G. L. (1940) Ophthalmological implications for the early history of snakes. Copeia l-8. Walls G. L. (1942) The Vertebrate eye und its Adaptive Rudiation. Cranbrook Institute of Science, Bloomfield Hills. Michigan.
Great Britain.
THE ROLE OF THE SPECTACLE IN THE VISUAL OPTICS OF THE SNAKE EYE J. G.
SIVAK
Laboratory of Comparative Optometry, University of Waterloo, Waterloo, Ontario (Received 5 April 1976)
Abstract-The atypical morphology and physiology of the snake eye is commonly attributed to a subterranean evolutionary period. It is suggested that many characteristics of this eye are due to the optical effects of the spectacle. Study of the gross morphology of the optical media of three species was undertaken by means of a rapid freezing and sectioning method. Refractive indices were measured. The refractive contribution of the spectacle is significant and almost equal to that of the lens. However, the spherical shape of the lens and its relatively high refractive index indicate an enhanced refractive function for this structure. Study of more primitive species is required.
INTRODUCTION
The eye lids of the snake are fused to form a trans-
parent covering over the cornea. This covering is known as the spectacle (Walls, 1934; Bellairs and Underwood, 1951). In contrast to the lizard, the snake eye lacks a ciliary muscle. Variations in focal power are achieved through movement of a spherical lens in response to changes in intraocular pressure induced by contraction of the iris musculature (Michel, 1932). The atypical morphology and physiology of the snake eye led Walls (1940, 1942) to conclude that this eye had evolved secondarily from a rudimentary state induced by an ancestral subterranean way of life. While objections exist (Armstrong, 1950; Bellairs, 1950), this view has been widely accepted (DukeElder, 1958; Romer, 1970). The effect of the spectacle on the optics of the snake eye is not known. Walls (1942) indicated that sebaceous material from the gland of Harder may enter the space between the spectacle and cornea and act as a lubricant. He also suggested that a spectacle of flattened curvature would reduce the refractive contribution of the cornea. In a situation in which the lens is the major refractive element (e.g. the fish eye), a spherical lens and an accommodative mechanism involving lens motion might be expected (Beer, 1894; Sivak, 1975). The present study will examine the relative refractive importance of the spectacle, cornea and lens of the snake eye in order to determine whether evolutionary pressures of an optical nature may have been exerted on this eye during its development.
METHODS
The study was carried, for the most part, at Mote Marine Laboratory, Sarasota, Fla. The species studied included four specimens of Coluber constrictor priapus (black racer), four specimens of Elaphae qundrauittata quadrauittata (yellow rat snake), and one specimen of EIaphae guttata (red rat snake). Each specimen was anaesthetized by being placed in an ether chamber. Refractive state was deter-
mined for each eye with a retinoscope and trial lenses before and after anaesthesia. Differences in the pre- and post-anaesthesia refractive error were assumed to indicate accommodative facility. Following anaesthesia, each specimen was sacrificed by decapitation. The eyes and surrounding orbital adnexa were removed with the spectacle intact. Intraocular dimensions
One eye was used to determine linear dimensions and radii of curvature of the ocular media in accordance with the following procedure: Immediately following its enucleation, the eye was rapidly frozen in a mixture of acetone and dry ice. S&e the &era of the snake eye lacks scleral ossicles or cartilage (Walls. 1940: Sivak. 19761 care was taken to avoid diitoiting the overall shape oithe globe. A thin nylon mesh was used to move the eye to and from the freezing mixture. The frozen eye was mounted for sagittal sectioning on a freezing microtome, and kept frozen through the release of carbon dioxide. A single lens reflex camera and bellows was mounted above the preparation. As thin sections of the eye were removed (lOtim over the middle third of the eye) photographs were taken of the remaining block of tissue. This procedure is in line with previous studies (Sivak, 1974, 1976) in which it was felt that photographs of large blocks of tissue were less apt to be distorted than photographs of thin microtome sections. The data of two of the nine eyes studied were discarded due to improper orientation on the microtome stage. Lens thicknesses were measured with a travelling microscope from the resulting photographic negatives. The negative, in the case of each eye, indicating the greatest lens thickness was assumed to represent a sagittal section along the geometric axis of the eye. Intraocular axial measurements were made along the line joining the center of the pupil to the center of the lens. Measurements were made of the distance between the posterior lens surface and the vitreous and choroid surfaces of the retina, lens thickness and the distance between the anterior lens surface and the cornea and spectacle. In addition, chord and sagittal depth measurements were made of the spectacle and cornea. Radii of curvature were calculated according to the formula : YZ s ‘=zs+T where S = sagittal depth of a chord and y = l/2 of that chord. Linear measurements were considered accurate to within 0.01 mm.
293
‘94
J.
Rg$ractice
G.
SIVAK
indices
KESCLTS
The determination of refractive indices of the refractive media of the second eye were made with an Abbe refractometer The procedure conformed to that used to measure the refractive index of liquids (Monk, 1963). With the exception of the spectacle, all measurements were carried out by placing a sample of either cornea, lens, aqueous or vitreous humors between the two refractometer prisms. Since the refractive index of the lens varies from the cortex to the core, the index measured in this manner was assumed to represent the equivalent index (Citron and Pinto. 1973). Measurements were repeated three or four times for each type of tissue. The fluid between the spectacle and the cornea was obtained by piercing the spectacle near its edge with a syringe needle. The fluid which welled up through the point of penetration was removed with a micro-pipette and placed on the measuring prism of the refractometer. In five of nine eyes studied, the quantity of fluid was insufficient to permit refractometer readings. Aqueous humor was obtained with a syringe following penetration of the cornea.
The refractive index of the spectacle was measured in accordance with the refractometer procedure used to measure the index of solids (Monk, 1963). Due to the curved nature of the spectacle, only two of the nine studied were large enough to permit adequate prism contact for measurement. The indices of three additional spectacles were found by measuring the actual spectacle thickness with a micrometer and the apparent thickness with a microscope. The refractive index is equal to the real thickness over the apparent.
The comparison of refractive states before and after anaesthesia indicates that an accommodative ability of considerable magnitude (up to 13 D) exists in the species studied (Table 1). Furthermore, the direction of refractive change agrees with the accommodative mechanism described by Michel (1932) and Walls (1942). Relaxation of the intraocular musculature with anaesthesia produces movement of the lens toward the retina and results in a refractive state shift toward hyperopia. Intraocular dimensions and refractive indices are very similar in the species studied (Tables 2 and 3). In contrast to statements by Walls (1942) and Bellairs and Underwood (19.51), the overall shape of the eye is slightly flattened and not spherical or slightly elongated. The average equatorial diameters of the eyes studied are approx lo”:, larger than that measured along the pupil axis. As noted by Walls (1940), the spectacle is somewhat flattened in shape. Its radius of curvature is always less than the radius of the scleral portion of the eye. However, this factor is compensated for by the high refractive index of the spectacle (1.50). According to Pumphrey (1961), the maximum index for biological material is about 1.53 (e.g. chitin, keratin). The refractive index of the fluid between the spectacle and cornea was found to be equal to or very
Table 1. Refractive states of the eyes of three species of snakes before and after ether anaesthesia. Measurements were made with a retinoscope along the pupil axis. The notations (+) and (-) indicate hyperopia and myopia respectively
Unanaesthettred Left c)e Rl$ht eye 0 00 + I.IX1 Jr 1.M) + 2.25
+I 25 + 2 25 i 50 * 2.00
-0.25 - 2.00 -0.50 + 2.25
~0.50 -200 -0.75 + 2.00
0.00
Table
2. Refractive
Snakes C. coluber
indices
priapus
of ocular
media
I
+025
of three species of snakes. Values 1973) are provided for comparison
of two lizard
I 365 I 36b
1.515 I.480 1.50 I 50
1.335 1.336 1.3355
I358 S61 1.364
1.495 I.510
1.338 1.337
1.50
1.336 1.337
E. wtmta
1.370 1.375 1.36X 1.373 ,371 1.362
Lizards Gecko gecko Iguana iguana
1.36 35
s E. qudravittafa
I
quadravittato
x
1
species (Citron
I I
, 440 I 433 I 446
,
1.453 1.443
I.735 33b 335 1.336 3355
I 335
I 455
and Pinto,
1.?3( ,31b I335 I336 13355
1.33b 1.336 1.336 336 1.335
I
I.458 I.455 I.464
1.335 1.335 1.335 I.336 I.335 1336
,336 336
1.415 1.4
I ?Ih I336
1
1.450
I 458
295
Fig. 1. Negative print of a vertical section of the eye of a red rat snake (Elaphae gurtata) following rapid freezing. The axial diameter of the lens is 2.57 mm.
Visual optics of the snake eye
297
Table 3. Intraocular dimensions, radii of curvature and refractive powers of the spectacle and lens of the eyes of three species of snakes. All linear measurements are to the nearest 0.01 mm. Spectacle values include front and rear surfaces. Lens values represent the equivalent lens power as calculated for a spherical lens. The refractive indices
used in the calculations were the averages reported in Table 2. Dimensions involving the retina refer to the retina choroid border Length (overall, cm)
Sp&Wleretina (mm)
(mm)
Post. lensretina (mm)
Spectacle radius of curvature (mm)
Spectaclerelr.
lens surf. (mm)
Spectacle-ant.
1.46 12.0
4.92 4.55
3.93 3.40
5.87 5.36
57.2 62.6
53.8 58.1
0.99 0.85 0.82 0.67
3.18 2.97 3.04 2.61
2.38 2.09 1.94 1.78
3.79 3.31 3.71 3.34
88.9 101.8 90.9 1cQ.s
104.2 110.5 108.0 125.6
0.68
2.57
1.75
2.72
117.9
140.0
C. coluber priepm 130.0 10.32 128.5 9.15 E. quadravirtata qdrauittata 137.5 6.55 135.0 5.91 133.0 5.80 132.5 5.06 E. quffofa 135.0
5.0
Lens diameter
slightly higher to that of the aqueous and vitreous humors of the eye. This finding casts doubt on the view that this space is filled with a sebaceous secretion of Harderian gland origin (Walls, 1942). Corneal refraction need not be considered since its index is not significantly greater than that of the fluid in front of it. Since the indices of the pre-cornea1 fluid and aqueous humor are virtually identical, and since the anterior and posterior cornea1 surfaces are parallel or nearly parallel to one another, the index of refraction of the cornea is largely irrelevant. In contrast to the smooth and glossy appearance of the spectacle, the cornea was very wrinkled once the spectacle was removed. Calculations of the refractive power of the spectacle were made in accordance with the formula: F=_
n1 - n r
where n’ = the refractive index of the medium into which light is passing, n = the refractive index of the medium from which light is passing and I = radius of curvature. The spectacle is a thin lens (approx 0.08 mm thick). Both anterior and posterior surfaces (assumed to be parallel) were considered when calculating spectacle refractive power (Table 3). A spherical or near-spherical lens shape was noted in each of the eyes studied (Fig. 1). In two instances (E. quadruvittata quadravittatu) the axial diameter of the lens was slightly larger (2%) than the equatorial diameter. A comparison of lens refractive indices indicates a signific+ntly higher index in snakes than in lizards (Table 2), confirming a prediction by Walls (1940). The radius of curvature of the lens was assumed to be one half the measured axial diameter. Lens refractive powers were calculated in accordance with the formula: F
=
power
(D)
Lens refr. power (D)
DISCUSSION The above results clearly indicate that while the spectacle has virtually eliminated the cornea as a refractive element of the eye, the spectacle itself has assumed an important refractive function. However, it is also apparent that the refractive role of the lens has been enhanced. The latter point is made clearer by comparing the snake eye to the eye of a lizard (Fig. 2). Citron and Pinto (1973) determined the optical constants of the eye’of Iguana iguana, a diurnal lizard. An iguana eye having an axial length of 8.7mm possesses a cornea with a refractive power of 129 D while the equivalent refractive power of the lens is only 41 D. Mahendra (1938) noted wide variations in spectacle size relative to the tize of the eyes of snakes. In the majority (including the three species studied here), the spectacle is about the same size as the cornea which it covers. However, in a number of genera the spectacle is much larger and covers a considerable portion of the head around the eye. Mahendra considered the large spectacle type to represent the primitive condition. If the spectacle surface is assumed to be a portion of a sphere, then one which is much larger than the eye will have a large radius of curvature and little refractive ability. In this instance, the lens would assume most, if not all of the refractive burden of the eye. The need to study additional species of snakes is indicated.
LIZARD
SNAKE n.lYF-CoRNEel-” “=1336--IO”EO”S-“=,
x1 - hhll r
where I = radius of the lens, n, = refractive index of the ocular humors (1.3355) and n2 = refractive index of the lens. The refractive powers of the lens and spectacle are almost equal in the black racer while that of the lens is about 15% greater than that of the spectacle in the rat snakes (Table 3).
-
Inun -
Fig. 2. Comparison of the intraocular dimensions and refractive indices of a lizard eye (Iguana iguana; Citron and Pinto, 1973) and a snake eye. The subspectacle fluid of the snake eye (refractive index = 1.3355-1.337) has been omitted for clarity.
29x
J. G.
ilcknowledyements-This study was supported by a grant from the National Council of Canada. The author is grateful to the Mote Marine Laboratory, Sarasota. Florid;, for facilities and assistance. In particular. the author wishes to thank Ms. Donna Johnson and Dr. Lvnn Carter.
REFERENCES
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