- Email: [email protected]
A revision of the rat schematic eye
Vision Res. Vol. 12, pp. 793-796. Pergamon Press 1972. Printed in Great Britain.
A REVISION
OF THE RAT SCHEMATIC
EYE
ROBERTW. MASSOPand FRED W. CHANG Indiana University, Division of Optometry, 800, East Atwater, Bloomington, Indiana 47401, U.S.A. (Receiued 19 May 1971)
BLOCK(1969) constructed a schematic eye for the rat in an attempt to resolve the conflict concerning the refractive state of the rat’s eye. To determine the power of the lens, he employed the procedure of measuring inverted aerial images. Using this technique, he calculated the index of refraction of the crystalline lens to be l-61, an abnormally high index for a mammal. It was found that Block’s calculated index does not agree with the measured index. PHILIPSON(1969) reported that the index of refraction for the rat’s lens (135 days old), as measured by refractometry, is l-390 in the outer cortex, l-455 in the inner cortex, and l-495 in the nucleus (mean index = 1.447). These figures agree quite well with the indices determined by microradiography (mean index = 1.420). It is readily apparent that many other values based on lens index (e.g. lens power) in Block’s schematic eye must be in error. Due to the necessity of understanding the refractive state of the rat’s eye in order to control retinal images, particularly in receptive field studies, it was decided to revise the schematic eye of the rat incorporating Philipson’s mean value for lens index. METHOD
Subjects Eight hooded rats, all over 120 days old, were usedin the present study. The animals were sacrificed with an overdose of pentobarbitat sodium and the eyes were enucleated in order for measurements to be taken.
Procedure Refractive indicies of the cornea, the aqueous humor, and the vitreous humor were determined with an AbW refractometer. The average index of the lens was taken from the mean of Philipson’s calculated index based on microradiogmphical data and the index measured with refractometry. Depth of the anterior chamber, lens dia., lens thickness, depth of the vitreous chamber, and comeal thickness were determined from frozen sections. The radius of curvature of the anterior surface of the cornea was measured directly with a radiuscope and the radius of the posterior surface was determined from thickness measurements. The radii of the anterior and posterior lens curvatures were calculated by the formula
(r = radius of curvature; y = semidiameter of the lens; s = sagitta of the half lens). The pupils of four hooded rats, two over 120 days old and two under 120 days old, were dilated with cyclogyl and their refractive errors were determined by streak retinoscopy and by ophthalmoscopy. RESULTS
AND
DISCUSSION
Table 1 compares the data of the present study with the data of LMHLJS (1932) and BLOCK (1969). Discrepancies between the present results and Block’s are most apparent 793
ROBERTW. M~ssop AND FRED W. CIUNG
794
TABLE1. MEASUREMENTSFORSCHEMATKEYE
Dimension Cornea index Aqueous index Vitrous index Lens index Cornea thickness Anterior cornea radius Posterior corena radius Lens thickness Lens diameter Anterior lens radius Posterior lens radius Anterior chamber depth Vitreous chamber depth Axial length
Present study Mean (SD.)
Block
I.374 1.344 1.347 1.4332 0.25 2.78 2.53 3.87 4.32 1.11 1.42 0.87 1.51 5.98
1.38 1.34 1.34 1.61 0.25 2,75 2.50 2.80 1.85 1.72 0.88 1.71 5.68
(0006) (0.005) (0.005) (0.22) (0.22) (0.19) (0.05)
(0.15) (0.29) (0.47)
LASHLEY
1.3325’ 1.3325’ 1*452r 0.25 3.95 4.50 0;6 1.11 6.35
’ Assumed from rabbit refractive index. ’ Based on Philipson’s data. All dimensions other than indicies in mm.
with the lens. The lens thickness and diameter measurements of the present study agree quite well with Lashley’s measurements, therefore it is fairly certain that the sagitta relationships are accurate in determining lens curvatures. Figure 1 illustrates the schematic eye for the rat calculated from the values in Table 1. Table 2 compares the dimensions of the new eye to Block’s schematic eye. The large difference between the total dioptric powers of the two eyes can be attributed to Block’s error in the calculation of the lens power. Block estimated the rat’s refractive error at the plane of the cornea to be +9*3D. With retinoscopy he observed an equivocal range extending from +lOD to +20D (working distance 1 m). He stated that the correcting lens was “placed at the eye”, but as can be seen in Fig. 2 the vertex distance is critical. For Block’s own schematic eye, a range of 3 cm in RETINA F
N N’ I I
-0.75
mm
FIG. 1.Schematic representation of the rat’s eye based on values in Table 2. The cornea (C) is represented by a single thin lens. The crystalline lens (L1 to LJ is treated as a thick lens with principal planes at PL and PL’. The cardinal planes of the total eye (F, F’, P, P’, N, N’) are determined by treating P,_’ as the back vertex of the system.
795
A Revision of the Rat Schematic Eye TABLE2.
VALUES FOR SCHEMATIC EYE
Dimension Anterior cornea power Posterior cornea power Anterior lens power Posterior lens power Total wmea power Total lens power Total power of the eye e ;’ f’
05
15 2-o VERTEX
Block
Present study +134-5D -12.OD +84*3D +605D +122_8D + 132.OD +2157D 1.17 mm 1.76 mm 467 mm 6.27 mm 350 mm 451 mm
2.5 30 3-5 DISTANCE. cm
+138D -16D +146D +157D -+122D +263D +324D 1.54 mm 166mm 3.09 mm 414 mm 1.55 mm 5.80 mm
4-o
45
50
2. The two curves, generated by the formula FA = (F. - 2$)/I1+ d(F.- F.)] = total refractive power of the rat eye; FA = degree of ammetropia; and d = vertex distance),predict the refractive error of the rat’s eye as a function of vertex distance. The ordinate values give the necemary dioptric power of a single thin lens placed in some piane anterior to the cornea which will bring the final focus of the system (I?) to the plane of the retina. The top curve is based on the present schematic eye and the bottom curve is based on Block’s schematic eye. The arrow points out the predicted value for a vertex distance of 1.5 cm. which is compared to the measured value in Table 3. FIG.
vertex distance would allow for an error in measurement ranging from +7D to +lOD, however, his results were opposite the predicted direction. This discrepancy may be accounted for in the excess refractive power in Block’s system. At a vertex distance of l-5 cm the present schematic eye would predict a refractive error of +17D (see arrow in Fig. 2). The predicted errors agree very closely with the measured
ROBERTW. M~ssop and FRED W. CIUNG
196
TABLE3. RKFRACIT~EERROROF THE RAT
Rat A : D
200 200
Vertex distance (cm)
Streak retinoscope
1.5 1.5
f 16 Diopters f-I-13 16 Diopters
1.5
+ 13 Diopters
Ophthalmoscope + 17 Diopters + 13 15 Diopters Diopters + 10 Diopters
Predicted + 17 Diopters + 17 Diopters Diopters + 17 Diopters
errors (see Table 3). The discrepancy between the predicted and measured values of rats C and D may be attributed to either age differences or individual variation. Acknowledgement authors wish to express their thanks to Dr. R. W. EMERSON for his helpful criticisms and review of this report.
REFERENCES BLOCK,M. T. (1969). A note on the refraction and image formation of the rat’s eye. Vision Res. 9,705-712. F~CHAM, W. H. A. (1969). Optics. Hatton Press, London. LASHLEY,K. S. (1932). The mechanism of vision. V. The structure and image-forming power of the rats’ eye. J. camp. Psychol. 13, 173-200. PHIL~PSON,B. (1969). Distribution of protein within the normal rat lens. Znvestve Ophthal. 8, 258-270.
Abstract-Based on measurements of excised eyes and Prn~mso~‘s (1969) measured index of refraction of the rat lens, a revision of BLOCK’S(1969) schematic rat eye was made. The new eye accurately predicts the measured ammetropia of about + 17 diopters (vertex distance = l&m.) as determined by retinoscopy and ophthalmoscopy.
R&um&CIn prodde a une revision de l’oeil schematique du rat selon BLOCK (1969) a partir des mesures de F%m.msO~ (1969) sur l’indice de refraction du cristallin du rat, effect&es sur des yeux &nucl&s. Ce nouvel oeil pr&dit avec pr&cision I’ametropie de +17 dioptries environ (distance frontale 1,5 cm), determinee par r&inoscopie et ophtalmoscopie.
Zusammenfassung-Es wurde eine Uberprtifung von BLOCK’S(1969) schematischem auge vorgenommen. Dabei dienten Messungen an herauspraparierten Augen und PHIfJPSON (1969) beziiglich der Brechkraft der Rattenlinse als Grundlage. Das neue schema fiihrt genau zu der gemessenen Ammetropie von etwa +17 dpt (Schnittweite dis sich durch Retinoskopie und mit dem Ophtahlmometer ergeben hatte.
Rattendie von Augen1,5 cm),
Pesrorut+Ha oc~onamm nsMepem& sriywreapoBannor0 rnasa II n3Mepennoro PnnIpsoN, OM (1969) nOKa3aTeKK npeJIoMJrerina xpycraJmKa Kpbrcbr, npoE3neneEa pe~nsrirr cxeMaTmrecKoro rnasa rcpbrcbr, npe.Kno~ennoro BLXIK, OM(1969). HOE& CxeMaTHyecIcattrnas TOKHO npenc~aard6ae.~ ruMepemin aMerpomim rtpn6mun’renb~o nonyraehlyro ripu -I- 17 IniOlIl@i (BepTeKC-;sHCTV = 1,5 CM.), KaK 3Ta aMeTpOIIIDl OIIpelWIWTCSi C llOMOlJ&IK) permrocKorxmfE Ot$TiiJIbMOCKOlTEK.
A REVISION
OF THE RAT SCHEMATIC
EYE
ROBERTW. MASSOPand FRED W. CHANG Indiana University, Division of Optometry, 800, East Atwater, Bloomington, Indiana 47401, U.S.A. (Receiued 19 May 1971)
BLOCK(1969) constructed a schematic eye for the rat in an attempt to resolve the conflict concerning the refractive state of the rat’s eye. To determine the power of the lens, he employed the procedure of measuring inverted aerial images. Using this technique, he calculated the index of refraction of the crystalline lens to be l-61, an abnormally high index for a mammal. It was found that Block’s calculated index does not agree with the measured index. PHILIPSON(1969) reported that the index of refraction for the rat’s lens (135 days old), as measured by refractometry, is l-390 in the outer cortex, l-455 in the inner cortex, and l-495 in the nucleus (mean index = 1.447). These figures agree quite well with the indices determined by microradiography (mean index = 1.420). It is readily apparent that many other values based on lens index (e.g. lens power) in Block’s schematic eye must be in error. Due to the necessity of understanding the refractive state of the rat’s eye in order to control retinal images, particularly in receptive field studies, it was decided to revise the schematic eye of the rat incorporating Philipson’s mean value for lens index. METHOD
Subjects Eight hooded rats, all over 120 days old, were usedin the present study. The animals were sacrificed with an overdose of pentobarbitat sodium and the eyes were enucleated in order for measurements to be taken.
Procedure Refractive indicies of the cornea, the aqueous humor, and the vitreous humor were determined with an AbW refractometer. The average index of the lens was taken from the mean of Philipson’s calculated index based on microradiogmphical data and the index measured with refractometry. Depth of the anterior chamber, lens dia., lens thickness, depth of the vitreous chamber, and comeal thickness were determined from frozen sections. The radius of curvature of the anterior surface of the cornea was measured directly with a radiuscope and the radius of the posterior surface was determined from thickness measurements. The radii of the anterior and posterior lens curvatures were calculated by the formula
(r = radius of curvature; y = semidiameter of the lens; s = sagitta of the half lens). The pupils of four hooded rats, two over 120 days old and two under 120 days old, were dilated with cyclogyl and their refractive errors were determined by streak retinoscopy and by ophthalmoscopy. RESULTS
AND
DISCUSSION
Table 1 compares the data of the present study with the data of LMHLJS (1932) and BLOCK (1969). Discrepancies between the present results and Block’s are most apparent 793
ROBERTW. M~ssop AND FRED W. CIUNG
794
TABLE1. MEASUREMENTSFORSCHEMATKEYE
Dimension Cornea index Aqueous index Vitrous index Lens index Cornea thickness Anterior cornea radius Posterior corena radius Lens thickness Lens diameter Anterior lens radius Posterior lens radius Anterior chamber depth Vitreous chamber depth Axial length
Present study Mean (SD.)
Block
I.374 1.344 1.347 1.4332 0.25 2.78 2.53 3.87 4.32 1.11 1.42 0.87 1.51 5.98
1.38 1.34 1.34 1.61 0.25 2,75 2.50 2.80 1.85 1.72 0.88 1.71 5.68
(0006) (0.005) (0.005) (0.22) (0.22) (0.19) (0.05)
(0.15) (0.29) (0.47)
LASHLEY
1.3325’ 1.3325’ 1*452r 0.25 3.95 4.50 0;6 1.11 6.35
’ Assumed from rabbit refractive index. ’ Based on Philipson’s data. All dimensions other than indicies in mm.
with the lens. The lens thickness and diameter measurements of the present study agree quite well with Lashley’s measurements, therefore it is fairly certain that the sagitta relationships are accurate in determining lens curvatures. Figure 1 illustrates the schematic eye for the rat calculated from the values in Table 1. Table 2 compares the dimensions of the new eye to Block’s schematic eye. The large difference between the total dioptric powers of the two eyes can be attributed to Block’s error in the calculation of the lens power. Block estimated the rat’s refractive error at the plane of the cornea to be +9*3D. With retinoscopy he observed an equivocal range extending from +lOD to +20D (working distance 1 m). He stated that the correcting lens was “placed at the eye”, but as can be seen in Fig. 2 the vertex distance is critical. For Block’s own schematic eye, a range of 3 cm in RETINA F
N N’ I I
-0.75
mm
FIG. 1.Schematic representation of the rat’s eye based on values in Table 2. The cornea (C) is represented by a single thin lens. The crystalline lens (L1 to LJ is treated as a thick lens with principal planes at PL and PL’. The cardinal planes of the total eye (F, F’, P, P’, N, N’) are determined by treating P,_’ as the back vertex of the system.
795
A Revision of the Rat Schematic Eye TABLE2.
VALUES FOR SCHEMATIC EYE
Dimension Anterior cornea power Posterior cornea power Anterior lens power Posterior lens power Total wmea power Total lens power Total power of the eye e ;’ f’
05
15 2-o VERTEX
Block
Present study +134-5D -12.OD +84*3D +605D +122_8D + 132.OD +2157D 1.17 mm 1.76 mm 467 mm 6.27 mm 350 mm 451 mm
2.5 30 3-5 DISTANCE. cm
+138D -16D +146D +157D -+122D +263D +324D 1.54 mm 166mm 3.09 mm 414 mm 1.55 mm 5.80 mm
4-o
45
50
2. The two curves, generated by the formula FA = (F. - 2$)/I1+ d(F.- F.)] = total refractive power of the rat eye; FA = degree of ammetropia; and d = vertex distance),predict the refractive error of the rat’s eye as a function of vertex distance. The ordinate values give the necemary dioptric power of a single thin lens placed in some piane anterior to the cornea which will bring the final focus of the system (I?) to the plane of the retina. The top curve is based on the present schematic eye and the bottom curve is based on Block’s schematic eye. The arrow points out the predicted value for a vertex distance of 1.5 cm. which is compared to the measured value in Table 3. FIG.
vertex distance would allow for an error in measurement ranging from +7D to +lOD, however, his results were opposite the predicted direction. This discrepancy may be accounted for in the excess refractive power in Block’s system. At a vertex distance of l-5 cm the present schematic eye would predict a refractive error of +17D (see arrow in Fig. 2). The predicted errors agree very closely with the measured
ROBERTW. M~ssop and FRED W. CIUNG
196
TABLE3. RKFRACIT~EERROROF THE RAT
Rat A : D
200 200
Vertex distance (cm)
Streak retinoscope
1.5 1.5
f 16 Diopters f-I-13 16 Diopters
1.5
+ 13 Diopters
Ophthalmoscope + 17 Diopters + 13 15 Diopters Diopters + 10 Diopters
Predicted + 17 Diopters + 17 Diopters Diopters + 17 Diopters
errors (see Table 3). The discrepancy between the predicted and measured values of rats C and D may be attributed to either age differences or individual variation. Acknowledgement authors wish to express their thanks to Dr. R. W. EMERSON for his helpful criticisms and review of this report.
REFERENCES BLOCK,M. T. (1969). A note on the refraction and image formation of the rat’s eye. Vision Res. 9,705-712. F~CHAM, W. H. A. (1969). Optics. Hatton Press, London. LASHLEY,K. S. (1932). The mechanism of vision. V. The structure and image-forming power of the rats’ eye. J. camp. Psychol. 13, 173-200. PHIL~PSON,B. (1969). Distribution of protein within the normal rat lens. Znvestve Ophthal. 8, 258-270.
Abstract-Based on measurements of excised eyes and Prn~mso~‘s (1969) measured index of refraction of the rat lens, a revision of BLOCK’S(1969) schematic rat eye was made. The new eye accurately predicts the measured ammetropia of about + 17 diopters (vertex distance = l&m.) as determined by retinoscopy and ophthalmoscopy.
R&um&CIn prodde a une revision de l’oeil schematique du rat selon BLOCK (1969) a partir des mesures de F%m.msO~ (1969) sur l’indice de refraction du cristallin du rat, effect&es sur des yeux &nucl&s. Ce nouvel oeil pr&dit avec pr&cision I’ametropie de +17 dioptries environ (distance frontale 1,5 cm), determinee par r&inoscopie et ophtalmoscopie.
Zusammenfassung-Es wurde eine Uberprtifung von BLOCK’S(1969) schematischem auge vorgenommen. Dabei dienten Messungen an herauspraparierten Augen und PHIfJPSON (1969) beziiglich der Brechkraft der Rattenlinse als Grundlage. Das neue schema fiihrt genau zu der gemessenen Ammetropie von etwa +17 dpt (Schnittweite dis sich durch Retinoskopie und mit dem Ophtahlmometer ergeben hatte.
Rattendie von Augen1,5 cm),
Pesrorut+Ha oc~onamm nsMepem& sriywreapoBannor0 rnasa II n3Mepennoro PnnIpsoN, OM (1969) nOKa3aTeKK npeJIoMJrerina xpycraJmKa Kpbrcbr, npoE3neneEa pe~nsrirr cxeMaTmrecKoro rnasa rcpbrcbr, npe.Kno~ennoro BLXIK, OM(1969). HOE& CxeMaTHyecIcattrnas TOKHO npenc~aard6ae.~ ruMepemin aMerpomim rtpn6mun’renb~o nonyraehlyro ripu -I- 17 IniOlIl@i (BepTeKC-;sHCTV = 1,5 CM.), KaK 3Ta aMeTpOIIIDl OIIpelWIWTCSi C llOMOlJ&IK) permrocKorxmfE Ot$TiiJIbMOCKOlTEK.