- Email: [email protected]
Observing accommodation in the cat
V&fm Res. Vol. 13, pp. 481482.
Pergamon Press 1973. Prints4 in Great Britain
RESEARCH OBSERVING
NOTE
ACCOMMODATION
IN THE CAT
A. HUGHES’ UniversityL&o*ltory of Physiology,ParksRoad, Oxford (Receioed 6 June1972)
DURINGa recent study of vergence in the cat (HUGHES,1972) numerous photographs were made of the eyes of animals that were looking at objects either 10 or 100 cm distant. Illumination was provided by an electronic flash set on the horizontal and 20” lateral to the midline. It was noticed, in photographs of some of the animals looking at the distant stimulus, that a combination of shadow and displacement of iris markings delineated a bulging of the iris around the pupil within a circle of radius about 2 mm (Fig. la). The radius of this bulge increased as much as three-fold in photographs of the same cats looking at a 10 cm distant object (Fig. lb). The bulge can be seen only when the pupil is relatively constricted but, within the range of sizes compatible with the observation, the variation of the pupil size appears to have no effect on its change of appearance in passing from a distant to a near look photo~aph. Under favourable conditions, side lighting and a relaxed, playful cat, the change may be directly observed; when an object is rapidly withdrawn from the animal’s close observation into the distance the iris may appear to become completely flattened. Clearly these changes are linked with the process of accommodation. Similar forward bulging of the cat iris and lens during sympathetic stimulation was described by ARNALY(1959) but the phenomenon has not previously been described during voluntary accommodation in conscious cats. Direct evidence of accommodation in conscious restrained cats has, of course, been obtained by infra-red optometry (ELUL and MARCHIAFAVA, 1964). The mechanism of accommodation in the cat has been uncertain since VAKKUR and BISHOP(1963) concluded from their schematic eye that a change in lens power would have only a minor effect on the refractive state of the cat eye. An alternative source of ~ommodative power was suggested to be the forward movement of the lens principle planes consequent upon either a change in the anterior surface curvature or the forward translation of the lens without change of shape. Any hypothesis of accommodatory mechanisms based upon a change of lens shape in the cat has been made unacceptable for mechanical reasons by FISHER’S (1971) recent discovery of a very high value of Young’s Modulus of polar elasticity for this animal relative to that of the human being. The forward bulging of the cat iris during accommodation and sympathetic stimulation must now be interpreted as resulting from lens translation and not from a change in surface curvature. VAKKTJR and BISHOP(1963) have described the pupillary margin of the iris as very thin and as lying closely applied to the anterior surface of the lens. The slight bulge in the distant-look photograph suggests that the remaining iris beyond 2 mm is not following ’ Presentaddress:Depa~rn~ntof Physiology,John Cm-tin School for Medical Research, A.N.V. Canberra* Australia. 481 V.R.1312-s
482
RESEARCH NOTE
the lens anterior surface and only does so when the lens moves forward during accommodation. If we regard the circumference of the bulge as defining the intersection of the iris and lens anterior surface then simple trigonometry now permits an estimate of the change in refractive power between the distant and near look photographs from measurements of the bulge radius. If the margin of the bulge remains in the same plane as it moves out from the visual axis then measurements of the bulge radius made on photographs at unit magnification must be corrected by the same value of cornea1 magnification; according to VAKKUR and BISHOP (1963) this would be 1.2 for the plane of the margin, which would nearly correspond with that of the lens anterior vertex in the emmetropic state. When Rl is the radius of the bulge margin corrected from a me~urement on a distant look photog~ph, R2 is that for the near look photog~ph and the radius of curvature of the anterior surface of the lens is 7.2 mm (VAKWR and BISHOP, 1963) then the forward movement of the lens, X, is given by, X=
arc “’sm
tan
!!)
-
tan (SrcZL
g)
mm
( which corresponds in the illustrated case to an increase in power of 3D, 043 mm forward movement, from an assumed 1D for 100 cm viewing distance to a consequent 4D for a IO-cm p~~n~tion distance. In view of the limited acco~~ation available to the cat (VAKKUR and BISHOP, 1963; ELUL and MARCHIFAVA, 1964) the discrepancy from the expected 10D is not improbable. This estimate of @8 mm for the forward translation of the lens in the conscious, voluntarily accommodating cat well exceeds the maximum value of 0.3 mm (1D) reported by O’NEILL and BRODKEY (1970) during ciliary ganglion stimulation in the anaesthetized animal. With suitable animals the method provides a simple qualitative index of the presence of accommodation without the need for complex apparatus or excessive restraint of the cat. Acknowlealpnrenl-I am indebted to the M.R.C. for support.
REFERENCES M. F. (1959). Studies on intraocular effects of the orbital pa~~pathet~c pathway. Arc& Opkrh~I. 61,14-29. ELUL,R. and MARCHIAFAVA, P. L. (1964). Accommodation of the eye as related to behaviour in the cat.
ARMALY,
Arch ital. Biol. 102, 616-644.
From, R. F. (1971). The elastic constants of the human lens. J. Physiol., Lond. 212,147-180. HUOHBS,A. (1972). Vergence in the cat. Vision Res. 12, 1961-1994. O’NEILL.W. D. and BRODKEY, J. S. (1970). A nonlinear analysis of the mechanics of accommodation. Vision
m. io, 375-392.
VAKKUR, G.
J. and
BISHOP,
P. 0. (1963). The schematic eye in the cat. Vision Res. 3, 357-381.
FIG. l(a). The eyes of a cat that is looking at an object 100 cm away. Note the appearance of bulging of the iris adjacent to the pupil. This is not the result of pigmentation. FIG. l(b). The eyes of the same animal now looking at a IO-cm distant object. Note the near11 three-fold increase in the diameter of the bulge as indicated by shadowing and wrinkling of the peripheral iris.
Lfbcing page 482
Pergamon Press 1973. Prints4 in Great Britain
RESEARCH OBSERVING
NOTE
ACCOMMODATION
IN THE CAT
A. HUGHES’ UniversityL&o*ltory of Physiology,ParksRoad, Oxford (Receioed 6 June1972)
DURINGa recent study of vergence in the cat (HUGHES,1972) numerous photographs were made of the eyes of animals that were looking at objects either 10 or 100 cm distant. Illumination was provided by an electronic flash set on the horizontal and 20” lateral to the midline. It was noticed, in photographs of some of the animals looking at the distant stimulus, that a combination of shadow and displacement of iris markings delineated a bulging of the iris around the pupil within a circle of radius about 2 mm (Fig. la). The radius of this bulge increased as much as three-fold in photographs of the same cats looking at a 10 cm distant object (Fig. lb). The bulge can be seen only when the pupil is relatively constricted but, within the range of sizes compatible with the observation, the variation of the pupil size appears to have no effect on its change of appearance in passing from a distant to a near look photo~aph. Under favourable conditions, side lighting and a relaxed, playful cat, the change may be directly observed; when an object is rapidly withdrawn from the animal’s close observation into the distance the iris may appear to become completely flattened. Clearly these changes are linked with the process of accommodation. Similar forward bulging of the cat iris and lens during sympathetic stimulation was described by ARNALY(1959) but the phenomenon has not previously been described during voluntary accommodation in conscious cats. Direct evidence of accommodation in conscious restrained cats has, of course, been obtained by infra-red optometry (ELUL and MARCHIAFAVA, 1964). The mechanism of accommodation in the cat has been uncertain since VAKKUR and BISHOP(1963) concluded from their schematic eye that a change in lens power would have only a minor effect on the refractive state of the cat eye. An alternative source of ~ommodative power was suggested to be the forward movement of the lens principle planes consequent upon either a change in the anterior surface curvature or the forward translation of the lens without change of shape. Any hypothesis of accommodatory mechanisms based upon a change of lens shape in the cat has been made unacceptable for mechanical reasons by FISHER’S (1971) recent discovery of a very high value of Young’s Modulus of polar elasticity for this animal relative to that of the human being. The forward bulging of the cat iris during accommodation and sympathetic stimulation must now be interpreted as resulting from lens translation and not from a change in surface curvature. VAKKTJR and BISHOP(1963) have described the pupillary margin of the iris as very thin and as lying closely applied to the anterior surface of the lens. The slight bulge in the distant-look photograph suggests that the remaining iris beyond 2 mm is not following ’ Presentaddress:Depa~rn~ntof Physiology,John Cm-tin School for Medical Research, A.N.V. Canberra* Australia. 481 V.R.1312-s
482
RESEARCH NOTE
the lens anterior surface and only does so when the lens moves forward during accommodation. If we regard the circumference of the bulge as defining the intersection of the iris and lens anterior surface then simple trigonometry now permits an estimate of the change in refractive power between the distant and near look photographs from measurements of the bulge radius. If the margin of the bulge remains in the same plane as it moves out from the visual axis then measurements of the bulge radius made on photographs at unit magnification must be corrected by the same value of cornea1 magnification; according to VAKKUR and BISHOP (1963) this would be 1.2 for the plane of the margin, which would nearly correspond with that of the lens anterior vertex in the emmetropic state. When Rl is the radius of the bulge margin corrected from a me~urement on a distant look photog~ph, R2 is that for the near look photog~ph and the radius of curvature of the anterior surface of the lens is 7.2 mm (VAKWR and BISHOP, 1963) then the forward movement of the lens, X, is given by, X=
arc “’sm
tan
!!)
-
tan (SrcZL
g)
mm
( which corresponds in the illustrated case to an increase in power of 3D, 043 mm forward movement, from an assumed 1D for 100 cm viewing distance to a consequent 4D for a IO-cm p~~n~tion distance. In view of the limited acco~~ation available to the cat (VAKKUR and BISHOP, 1963; ELUL and MARCHIFAVA, 1964) the discrepancy from the expected 10D is not improbable. This estimate of @8 mm for the forward translation of the lens in the conscious, voluntarily accommodating cat well exceeds the maximum value of 0.3 mm (1D) reported by O’NEILL and BRODKEY (1970) during ciliary ganglion stimulation in the anaesthetized animal. With suitable animals the method provides a simple qualitative index of the presence of accommodation without the need for complex apparatus or excessive restraint of the cat. Acknowlealpnrenl-I am indebted to the M.R.C. for support.
REFERENCES M. F. (1959). Studies on intraocular effects of the orbital pa~~pathet~c pathway. Arc& Opkrh~I. 61,14-29. ELUL,R. and MARCHIAFAVA, P. L. (1964). Accommodation of the eye as related to behaviour in the cat.
ARMALY,
Arch ital. Biol. 102, 616-644.
From, R. F. (1971). The elastic constants of the human lens. J. Physiol., Lond. 212,147-180. HUOHBS,A. (1972). Vergence in the cat. Vision Res. 12, 1961-1994. O’NEILL.W. D. and BRODKEY, J. S. (1970). A nonlinear analysis of the mechanics of accommodation. Vision
m. io, 375-392.
VAKKUR, G.
J. and
BISHOP,
P. 0. (1963). The schematic eye in the cat. Vision Res. 3, 357-381.
FIG. l(a). The eyes of a cat that is looking at an object 100 cm away. Note the appearance of bulging of the iris adjacent to the pupil. This is not the result of pigmentation. FIG. l(b). The eyes of the same animal now looking at a IO-cm distant object. Note the near11 three-fold increase in the diameter of the bulge as indicated by shadowing and wrinkling of the peripheral iris.
Lfbcing page 482