1iswn Rcs VoI
15. pp. 1353-1356. Pergamon Press 1975. Printed in Cheat Bntam
AN EVALUATION OF THE “RAMP” OF THE HORSE EYE
RETINA
J. G. SIVAK School of Optometry, University of Waterloo. Waterloo. Ontario, Canada and
D. B. ALLEN Department of Psychology, University of Guelph. Ontario, Canada (Received 16 March 1975) Abatraet-Using a rapid freezing and sectioning technique, the distance between the lens and retina of the horse eye was measured. There is no indication of a ramp retina that could serve accommodation. The pupil axis of the eye coincides with the maximum lens to retina distana. The changes in the lens-retina distance are greater below the axis than above it. Calculations were made of refractive power of the horse eye from measurements of curvature and refractive indices of the ocular tissues. These calculations agree both qualitatively and quantitatively with retinoscopic measurements on live horses. Both show that the refractive state shifts in the direction of hyperopia above and below the axis and that this shii is greater below the axis than above it. Some dynamic accommodative ability in the living eye was observed.
INTRODU~ION
It is common for reviewers of the subject of comparative accommodative mechanisms to classify such mechanisms as being of a dynamic or static nature (Walls, 1942; Duke-Elder, 19%; Prince, Diesem, Eglitis and Ruskell, 1960; Tansley, 1965). Included in the latter category is the concept of the ?arnp retina”, said to exist in horses and skates. This mechanism is based on a variation in distance between the lens and the retina. Ideally, an animal possessing such an eye would accommodate for the viewing of near objects by moving the eyes or bending the head so as to produce a new visual axis which would include a longer lens to retina distance. The description of such a mechanism for the horse
coincide with an area of retinal organization necessary for good resolution. The present study is an effort to establish with greater certainty whether in fact the ramp retina is a plausible accommodative device for the horse eye, by measuring the variation in lens to retina distance. METHODS
Rapid freezing and sectioning Eight horse eyes (right and left eyes of four horses) were removed from the heads of animals killed a few minutes earlier in a local feed company slaughterhouse. Upon
(e.g. Walls, 1942) derives from the work of Nicolas (1930). Reference to the report by Nicolas reveals that the above description is, to a major extent, conjecture. Nicolas did indicate that the horse eye is asymmetrical and that the lens to retina distance varies, being maximum at the top of a sagittal section (Fig. 1). He further noted that such variation could produce refractive changes amounting to several dioptres. However, Nicolas himself did not suggest that these
A
c
B : C
@ (a)
(b)
factors were indicative of a static accommodative system. Rather, he pointed them out as a caution to
refraction& of horses and as an explanation for the differences in refractive state reported by a number of authors. Nicolas considered that the true refractive state of a horse eye would be found with a measurement made along the pupil axis since this direction coincides or nearly coincides with the position of an area centralis. The existence of an area centralis would seem to concept of a ramp t-&m. The oPti=) advantage obtained by placing the image at a more appropriate distance from the lens would be negated by the fact that the image position would no longer Contradict
the
s
N @ (cl
(d)
Fig. I. The relationship between the lens and the retina of the horse eye as portrayed by the following authors: (a) Nicolas (1930); (b) Walls (1942); (c) Duke-Elder (1958); (d) Prince et al. (1960). 1353
1354
J G. SIVAKand D. B. ALLEN
removal. each eye was immediately frozen m a mtxture of dry ice and acetone. The large size of the frozen eyes and adnexa (> 50 mm) precluded the use of available mrcrotomes. However. another means of sectioning the eyes proved to be ideal. This consisted of the following procedure: The frozen eye was mounted on a microtome freezing attachment with the help of a tissue mount compound. and kept frozen by the release of carbon dioxide. Each eye was oriented for sagittal sectioning with the help of prior labeling during enucleation. The attachment was then mounted on the vice of a small milling machine. Thin sections (0.05 mm) of each eye were milled away using a Rywheel bit. As each section was removed a photograph was taken of the remaining block of tissue by means of a single lens reflex camera and close-up lens mounted above the preparation. This procedure was in line with a previous study (Sivak. 1974) in which it was felt that photographs of large blocks of tissue were less apt to be distorted than photographs of thin microtome sections. A series of photographic slides, corresponding to a series of sections, were made for each eye. Two of the eight eye preparations had to be discarded due to a malfunctioning CO, cylinder. The slides of the remaining six eyes were projected and lens thicknesses were measured. The slide. in the case of each eye, indicating the greatest lens thickness was assumed to represent a sagittal section along the geometric axis of the lens. Measurements of lens to retina distances were made from the centre of the equatorial chord of the lens to the choroid-retina border. These measurements were made along a line perpendicular to the lens chord and at three 15” intervals on either side of this line (Fig. 3B). In addition, chord and sagittal depth measurements were made of the anterior cornea and of both lens surfaces. Radii of curvature were calculated from these according to the r=‘+-
vz s 2s
2
where r = radius, S = sagittal depth of a chord and y = i of that chord. Refractive state sagitral plane
measurements
along
various
in the
axes
The refractive states of the eyes of two living horses were determined with a retinoscope and trial lenses, as
Fig. 2. Schematic representation indicating the directions along which retinoscopic measurements of r&active state were made.
an alternative means of evaluating variation in lens to retina distance. Measurements were made along tire pupil axis (as estimated by the presentation of greatest pupil area) and approximately 30” above and 30” b&w this axis (Fig. 2). Retinoscopy was carried out at a distance of 66 cm. Each determination was verified by changing the test distance so as to produce motion of the retinoscopic reflex (“with” motion at distances 66 cm). This procedure was carried out on three separate occasions for each of the two horses. Measurement
of refractive
indices
Along with lens and corneal curvatures, the refractive indices of the ocular media must be known to c&u&e the refractive equivalent of variations in lens to retina distances. An additional six fresh horse eyes were used for this purpose. Refractive indices of the cornea, lens, aqueous and vitreous hurnours were measured with an Abbe refractometer. Measurements were repeated 3 or 4 times for each optical medium of each eye.Sincether&active index of the lens varies, it was necessary to mearums successive layers of the lens from the cortex to the core.
Table 1. Distances between centre of equatorial lens chord to retina-vitreous border in six horse eyes. Measurements are made perpendicular to the chord and at three 15” intervals above and below the perpendicular. All values are to the nearest @I mm
l5.O
45O
3o"
1.
29.1
31.1
31.7
31.6
31.0
30.2
20.8
2.
29.1
30.0
30.7
30.4
29.7
28.6
26.9
3.
27.4
20.9
26.6
28.6
21.7
26.9
23.4
4.
26.9
27.7
28.3
26.4
27.7
26.4
24.9
5.
27.5
28.9
29.6
29.6
29.3
27.9
26.6
6.
26.9
29.1
29.9
29.9
29.7
26.4
26.7
29.3
29.8
29.a
29.2
28.1
26.6
0.0
0.0
2.0
1.6
l5O
3o"
3.7
43"
10.7
The “ramp” retina of the horse eye RESULTS AND DlSCIJ!&lON
t355
Table 2, k&active states of the eyes of two horses measured along the geometric axis and approximately 30”
Rup~ freezing and sectioni~
above and 30” below
that
axis
The results obtained by rapid freezing and section-
ing rule out the possibility of a ramp retina, in the accommodative sense, for the horse eye (Table 1).The following statements apply to each of six eyes studied. The distance of the perpendicular line from the centre of the lens chord to the retina is the longest measured (Figs. 3A and 3). Mourns made 15” above the perpendicular indicate virtually no change. However, all measurements made above that 15’ line and all measurements made below the perpendicular indicate a gradual decrease in lens to retina distance. Admittedly, a slight asymmetry exists in that this decrease occurs at a faster rate below the perpendicular to the lens chord. Nevertheless, a shor~ning of the axial length of the eye will produce a refractive change in the direction of hyperopia whereas an accommodative mechanism (dynamic or static) should produce a refractive change in the myopic direction. Furthermore+the variation in lens to retina distance is small, particularly through a 45” angle subtended by the line 15” below the perpendicular and 30” above it. Little difference was observed for sagittai sections 2 or 3 mm on either side of the geometric axis of the lens. Rejhctive state measurements There is close agreement between the retinoscopic
findings (Table 2) and the results obtained by rapid freezing and sectioning although retinoscopic refractive state measurements must be considered to be approximate due to diiculty in maintaining precise directions of measurement. Measurements made along the geometric axis indicate the least hyperopia or greatest myopia in each eye. A shift towards hyperopia, as found when rn~u~~n~ are made above or beiow that axis may be interpreted as a decrease in lens to retina distance. The shift toward hyperopia is greatest in above axis measurements in agreement with the corresponding below-axis measurements of the let&retina distance reported above. However. the refractive variation found in a single eye is never greater than l*OOD for any direction. It is of additional interest to note moderate changes (< 1QOD) in refractive state for a particular direction when com-
paring results obtained on different occasions. This finding suggests the existence of some dynamic accommodative ability. Calculated refractive equivalent of variations in lens to retina distance The refractive constants measured in the present
study are in good agreement with those noted by Nicolas (Table 3), with two exceptions: The value Nicolas gives for the index of refraction of the cornea (1.337)is very slightly above that of water and equal to his index for aqueous humour. This value is lower than the value reported here (1,369)and lower than the cornea1 indices reported for other mammals. Aurell and Holmgran (1953) found indices of l-382 and 1-373for the ox and pig respectively. That of the rat is 1.374(Massof and Chang, 1972)while the refractive index for the human cornea is given as i-376 (Bennett and Francis, 1962). The index cited by Nicolas for the horse lens (l-49) appears to be too high. Massof and Chang (1972) used a lens index of 1.433 for the calculation of the schematic eye of the rat. The index of the human lens varies from l-386 at the cortex to 1406 at the core while the effective lens index used for Gullstrand’s simplified human eye (No. 2) is 1,413 (Bennett and Francis, 1962).In the present study, the index varies from 1.383at the cortex to 1.429 at the core. An index of I-430 for the horse lens will be assumed for the calculations which follow.
Table 3. Refractiveconstants of the horse eye, as reported by Nicolas (1930)and as found in the present study. The tatter data represent averages of measurement from six eyes. Refractive indices do not refer to the same six eyes used to obtain linear measu~ments. Ranges are included in brackets mt*n WuI-LNl
tr)
mo-ul-ap
RafrWXi~=cer
s&U tr) v*rreieal-E~ camu
&t. Leaa Poe.smu
&mu
(runrpocrt.)
4qunau v1tnau Buour 6-u
-
w 22.6
43.66
47.63
16.57
17.0
11.0
1.337 1.337
1.333
12.6
46.6
43.1
x7.2
33.0
In.5
1.369 l.335
1.335
w
1.4%
cona
(3l.6-13.5) ~~~~ a7.6-43.0)
(=-to r*tipr) (il.6 -36.9)
(23.1 (13.h -19.9) 17.91
(9.~1r.41 '::z%
con
1.363 1.429 (1.369 a.422 -1.363)-1.436)
J. G.
1356
SIVAK
and D. B. ALLEY
The refractive powers of the anterior and posterior cornea and lens surfaces were calculated. using indices
and radii found in the present study, according the formula
to
where; F = refractive power, in dioptres; 11’= the refractive index of the medium into which light is passing; ~1= the refractive index of the medium from which light is passing; and I’ = the radius of curvature of the surface concerned. in metres. The refractive power of the anterior surface of the cornea was found to be 21.45 D. Assuming that the two cornea1 surfaces are parallel, the posterior surface will have a negative effect of about 2OOD, resulting in a total cornea1 F of 19.45 D. Taking into account the two lens surfaces and its thickness, an equivalent lens power of 14-88 D is found in accordance with the formula F
equiv. = F ant. surf. + F post. surf. d
- ;(F
ant. surf.)(F post. surf.)
where II = index of refraction of the lens and d its thickness, in metres. A similar calculation results in an equivalent refractive power for the entire eye of 31.6 D. The focal length of such an eye is 31-6 mm. Reference to Table 1 indicates an average shortening in the lens to retina distance of 05mm along a line 30” above the perpendicular to the lens chord. This is equivalent to a shift toward hyperopia of approximately 0.5 D. The equivalent shift along a line
30’ below the perpendicular is 1.8 D. These MILKS agree approximately with the refractive shifts found retinoscopically (Table 2). Acknow/edge~n2erlrs-The authors thank the Ontario Veterinary College, Guelph, Ontario and Barton Feeder Co.. Orillia, Ontario for supplying eyes used in this study. This research was supported by a grant from the National Research Council of Canada to J. G. Sivak. The authors also thank Prof. J. D. Moreland for reviewing the manuscript. REFERENCES
Aurell G. and Holmgren H. (1953) On the metachromatic staining of cornea1 tissue and some observations on its transparency. Acra ophthal. 31, l-27. Bennett A. G. and Francis J. L. (1962) The eye as an optical system. In The Eye, Vol. 4: Visual Optics and the Optical Space Sence (Edited by Davson H.), pp. 101-l 3 I. Academic Press, New York. Duke-Elder S. (1958) System of Ophthalmology, Vol. 1: The Eye in Evolution. Kimpton, London. Massof R. W. and Chang F. W. (1972) A revision of the rat schematic eye. Vision Res. 12, 793-796. Nicolas E. (1930) Vererinary and Comparative Ophrhalmology. H. & W. Brown, London. Prince J. H., Diesem C. D., Eglitis I. and Ruskell G. L. (1960) Anatomy and Histology of the Eye and Orbit in Domestic Animals. Thomas. Sorimdield. Ill. Sivak J. G. (1973)The refractive erro; of the fish eye. Visiorr Res. 14. 2Oe213. Tansley K. (1965) Vision in Vertebrates. Chapman & Hall, London. Walls G. L. (1942) The Vertebrate Eye and its Adaptive Radiation. Cranbrook Institute of Science, Bloomfield Hills, Mich.
m
[facing page 13 56
15. pp. 1353-1356. Pergamon Press 1975. Printed in Cheat Bntam
AN EVALUATION OF THE “RAMP” OF THE HORSE EYE
RETINA
J. G. SIVAK School of Optometry, University of Waterloo. Waterloo. Ontario, Canada and
D. B. ALLEN Department of Psychology, University of Guelph. Ontario, Canada (Received 16 March 1975) Abatraet-Using a rapid freezing and sectioning technique, the distance between the lens and retina of the horse eye was measured. There is no indication of a ramp retina that could serve accommodation. The pupil axis of the eye coincides with the maximum lens to retina distana. The changes in the lens-retina distance are greater below the axis than above it. Calculations were made of refractive power of the horse eye from measurements of curvature and refractive indices of the ocular tissues. These calculations agree both qualitatively and quantitatively with retinoscopic measurements on live horses. Both show that the refractive state shifts in the direction of hyperopia above and below the axis and that this shii is greater below the axis than above it. Some dynamic accommodative ability in the living eye was observed.
INTRODU~ION
It is common for reviewers of the subject of comparative accommodative mechanisms to classify such mechanisms as being of a dynamic or static nature (Walls, 1942; Duke-Elder, 19%; Prince, Diesem, Eglitis and Ruskell, 1960; Tansley, 1965). Included in the latter category is the concept of the ?arnp retina”, said to exist in horses and skates. This mechanism is based on a variation in distance between the lens and the retina. Ideally, an animal possessing such an eye would accommodate for the viewing of near objects by moving the eyes or bending the head so as to produce a new visual axis which would include a longer lens to retina distance. The description of such a mechanism for the horse
coincide with an area of retinal organization necessary for good resolution. The present study is an effort to establish with greater certainty whether in fact the ramp retina is a plausible accommodative device for the horse eye, by measuring the variation in lens to retina distance. METHODS
Rapid freezing and sectioning Eight horse eyes (right and left eyes of four horses) were removed from the heads of animals killed a few minutes earlier in a local feed company slaughterhouse. Upon
(e.g. Walls, 1942) derives from the work of Nicolas (1930). Reference to the report by Nicolas reveals that the above description is, to a major extent, conjecture. Nicolas did indicate that the horse eye is asymmetrical and that the lens to retina distance varies, being maximum at the top of a sagittal section (Fig. 1). He further noted that such variation could produce refractive changes amounting to several dioptres. However, Nicolas himself did not suggest that these
A
c
B : C
@ (a)
(b)
factors were indicative of a static accommodative system. Rather, he pointed them out as a caution to
refraction& of horses and as an explanation for the differences in refractive state reported by a number of authors. Nicolas considered that the true refractive state of a horse eye would be found with a measurement made along the pupil axis since this direction coincides or nearly coincides with the position of an area centralis. The existence of an area centralis would seem to concept of a ramp t-&m. The oPti=) advantage obtained by placing the image at a more appropriate distance from the lens would be negated by the fact that the image position would no longer Contradict
the
s
N @ (cl
(d)
Fig. I. The relationship between the lens and the retina of the horse eye as portrayed by the following authors: (a) Nicolas (1930); (b) Walls (1942); (c) Duke-Elder (1958); (d) Prince et al. (1960). 1353
1354
J G. SIVAKand D. B. ALLEN
removal. each eye was immediately frozen m a mtxture of dry ice and acetone. The large size of the frozen eyes and adnexa (> 50 mm) precluded the use of available mrcrotomes. However. another means of sectioning the eyes proved to be ideal. This consisted of the following procedure: The frozen eye was mounted on a microtome freezing attachment with the help of a tissue mount compound. and kept frozen by the release of carbon dioxide. Each eye was oriented for sagittal sectioning with the help of prior labeling during enucleation. The attachment was then mounted on the vice of a small milling machine. Thin sections (0.05 mm) of each eye were milled away using a Rywheel bit. As each section was removed a photograph was taken of the remaining block of tissue by means of a single lens reflex camera and close-up lens mounted above the preparation. This procedure was in line with a previous study (Sivak. 1974) in which it was felt that photographs of large blocks of tissue were less apt to be distorted than photographs of thin microtome sections. A series of photographic slides, corresponding to a series of sections, were made for each eye. Two of the eight eye preparations had to be discarded due to a malfunctioning CO, cylinder. The slides of the remaining six eyes were projected and lens thicknesses were measured. The slide. in the case of each eye, indicating the greatest lens thickness was assumed to represent a sagittal section along the geometric axis of the lens. Measurements of lens to retina distances were made from the centre of the equatorial chord of the lens to the choroid-retina border. These measurements were made along a line perpendicular to the lens chord and at three 15” intervals on either side of this line (Fig. 3B). In addition, chord and sagittal depth measurements were made of the anterior cornea and of both lens surfaces. Radii of curvature were calculated from these according to the r=‘+-
vz s 2s
2
where r = radius, S = sagittal depth of a chord and y = i of that chord. Refractive state sagitral plane
measurements
along
various
in the
axes
The refractive states of the eyes of two living horses were determined with a retinoscope and trial lenses, as
Fig. 2. Schematic representation indicating the directions along which retinoscopic measurements of r&active state were made.
an alternative means of evaluating variation in lens to retina distance. Measurements were made along tire pupil axis (as estimated by the presentation of greatest pupil area) and approximately 30” above and 30” b&w this axis (Fig. 2). Retinoscopy was carried out at a distance of 66 cm. Each determination was verified by changing the test distance so as to produce motion of the retinoscopic reflex (“with” motion at distances 66 cm). This procedure was carried out on three separate occasions for each of the two horses. Measurement
of refractive
indices
Along with lens and corneal curvatures, the refractive indices of the ocular media must be known to c&u&e the refractive equivalent of variations in lens to retina distances. An additional six fresh horse eyes were used for this purpose. Refractive indices of the cornea, lens, aqueous and vitreous hurnours were measured with an Abbe refractometer. Measurements were repeated 3 or 4 times for each optical medium of each eye.Sincether&active index of the lens varies, it was necessary to mearums successive layers of the lens from the cortex to the core.
Table 1. Distances between centre of equatorial lens chord to retina-vitreous border in six horse eyes. Measurements are made perpendicular to the chord and at three 15” intervals above and below the perpendicular. All values are to the nearest @I mm
l5.O
45O
3o"
1.
29.1
31.1
31.7
31.6
31.0
30.2
20.8
2.
29.1
30.0
30.7
30.4
29.7
28.6
26.9
3.
27.4
20.9
26.6
28.6
21.7
26.9
23.4
4.
26.9
27.7
28.3
26.4
27.7
26.4
24.9
5.
27.5
28.9
29.6
29.6
29.3
27.9
26.6
6.
26.9
29.1
29.9
29.9
29.7
26.4
26.7
29.3
29.8
29.a
29.2
28.1
26.6
0.0
0.0
2.0
1.6
l5O
3o"
3.7
43"
10.7
The “ramp” retina of the horse eye RESULTS AND DlSCIJ!&lON
t355
Table 2, k&active states of the eyes of two horses measured along the geometric axis and approximately 30”
Rup~ freezing and sectioni~
above and 30” below
that
axis
The results obtained by rapid freezing and section-
ing rule out the possibility of a ramp retina, in the accommodative sense, for the horse eye (Table 1).The following statements apply to each of six eyes studied. The distance of the perpendicular line from the centre of the lens chord to the retina is the longest measured (Figs. 3A and 3). Mourns made 15” above the perpendicular indicate virtually no change. However, all measurements made above that 15’ line and all measurements made below the perpendicular indicate a gradual decrease in lens to retina distance. Admittedly, a slight asymmetry exists in that this decrease occurs at a faster rate below the perpendicular to the lens chord. Nevertheless, a shor~ning of the axial length of the eye will produce a refractive change in the direction of hyperopia whereas an accommodative mechanism (dynamic or static) should produce a refractive change in the myopic direction. Furthermore+the variation in lens to retina distance is small, particularly through a 45” angle subtended by the line 15” below the perpendicular and 30” above it. Little difference was observed for sagittai sections 2 or 3 mm on either side of the geometric axis of the lens. Rejhctive state measurements There is close agreement between the retinoscopic
findings (Table 2) and the results obtained by rapid freezing and sectioning although retinoscopic refractive state measurements must be considered to be approximate due to diiculty in maintaining precise directions of measurement. Measurements made along the geometric axis indicate the least hyperopia or greatest myopia in each eye. A shift towards hyperopia, as found when rn~u~~n~ are made above or beiow that axis may be interpreted as a decrease in lens to retina distance. The shift toward hyperopia is greatest in above axis measurements in agreement with the corresponding below-axis measurements of the let&retina distance reported above. However. the refractive variation found in a single eye is never greater than l*OOD for any direction. It is of additional interest to note moderate changes (< 1QOD) in refractive state for a particular direction when com-
paring results obtained on different occasions. This finding suggests the existence of some dynamic accommodative ability. Calculated refractive equivalent of variations in lens to retina distance The refractive constants measured in the present
study are in good agreement with those noted by Nicolas (Table 3), with two exceptions: The value Nicolas gives for the index of refraction of the cornea (1.337)is very slightly above that of water and equal to his index for aqueous humour. This value is lower than the value reported here (1,369)and lower than the cornea1 indices reported for other mammals. Aurell and Holmgran (1953) found indices of l-382 and 1-373for the ox and pig respectively. That of the rat is 1.374(Massof and Chang, 1972)while the refractive index for the human cornea is given as i-376 (Bennett and Francis, 1962). The index cited by Nicolas for the horse lens (l-49) appears to be too high. Massof and Chang (1972) used a lens index of 1.433 for the calculation of the schematic eye of the rat. The index of the human lens varies from l-386 at the cortex to 1406 at the core while the effective lens index used for Gullstrand’s simplified human eye (No. 2) is 1,413 (Bennett and Francis, 1962).In the present study, the index varies from 1.383at the cortex to 1.429 at the core. An index of I-430 for the horse lens will be assumed for the calculations which follow.
Table 3. Refractiveconstants of the horse eye, as reported by Nicolas (1930)and as found in the present study. The tatter data represent averages of measurement from six eyes. Refractive indices do not refer to the same six eyes used to obtain linear measu~ments. Ranges are included in brackets mt*n WuI-LNl
tr)
mo-ul-ap
RafrWXi~=cer
s&U tr) v*rreieal-E~ camu
&t. Leaa Poe.smu
&mu
(runrpocrt.)
4qunau v1tnau Buour 6-u
-
w 22.6
43.66
47.63
16.57
17.0
11.0
1.337 1.337
1.333
12.6
46.6
43.1
x7.2
33.0
In.5
1.369 l.335
1.335
w
1.4%
cona
(3l.6-13.5) ~~~~ a7.6-43.0)
(=-to r*tipr) (il.6 -36.9)
(23.1 (13.h -19.9) 17.91
(9.~1r.41 '::z%
con
1.363 1.429 (1.369 a.422 -1.363)-1.436)
J. G.
1356
SIVAK
and D. B. ALLEY
The refractive powers of the anterior and posterior cornea and lens surfaces were calculated. using indices
and radii found in the present study, according the formula
to
where; F = refractive power, in dioptres; 11’= the refractive index of the medium into which light is passing; ~1= the refractive index of the medium from which light is passing; and I’ = the radius of curvature of the surface concerned. in metres. The refractive power of the anterior surface of the cornea was found to be 21.45 D. Assuming that the two cornea1 surfaces are parallel, the posterior surface will have a negative effect of about 2OOD, resulting in a total cornea1 F of 19.45 D. Taking into account the two lens surfaces and its thickness, an equivalent lens power of 14-88 D is found in accordance with the formula F
equiv. = F ant. surf. + F post. surf. d
- ;(F
ant. surf.)(F post. surf.)
where II = index of refraction of the lens and d its thickness, in metres. A similar calculation results in an equivalent refractive power for the entire eye of 31.6 D. The focal length of such an eye is 31-6 mm. Reference to Table 1 indicates an average shortening in the lens to retina distance of 05mm along a line 30” above the perpendicular to the lens chord. This is equivalent to a shift toward hyperopia of approximately 0.5 D. The equivalent shift along a line
30’ below the perpendicular is 1.8 D. These MILKS agree approximately with the refractive shifts found retinoscopically (Table 2). Acknow/edge~n2erlrs-The authors thank the Ontario Veterinary College, Guelph, Ontario and Barton Feeder Co.. Orillia, Ontario for supplying eyes used in this study. This research was supported by a grant from the National Research Council of Canada to J. G. Sivak. The authors also thank Prof. J. D. Moreland for reviewing the manuscript. REFERENCES
Aurell G. and Holmgren H. (1953) On the metachromatic staining of cornea1 tissue and some observations on its transparency. Acra ophthal. 31, l-27. Bennett A. G. and Francis J. L. (1962) The eye as an optical system. In The Eye, Vol. 4: Visual Optics and the Optical Space Sence (Edited by Davson H.), pp. 101-l 3 I. Academic Press, New York. Duke-Elder S. (1958) System of Ophthalmology, Vol. 1: The Eye in Evolution. Kimpton, London. Massof R. W. and Chang F. W. (1972) A revision of the rat schematic eye. Vision Res. 12, 793-796. Nicolas E. (1930) Vererinary and Comparative Ophrhalmology. H. & W. Brown, London. Prince J. H., Diesem C. D., Eglitis I. and Ruskell G. L. (1960) Anatomy and Histology of the Eye and Orbit in Domestic Animals. Thomas. Sorimdield. Ill. Sivak J. G. (1973)The refractive erro; of the fish eye. Visiorr Res. 14. 2Oe213. Tansley K. (1965) Vision in Vertebrates. Chapman & Hall, London. Walls G. L. (1942) The Vertebrate Eye and its Adaptive Radiation. Cranbrook Institute of Science, Bloomfield Hills, Mich.
m
[facing page 13 56
