D’YOUVILLE COLLEGE
BIOLOGY 659 - INTERMEDIATE PHYSIOLOGY I
VISUAL SYSTEM, OPTICS
Lecture 13: Structure of the Eye, Lenses
1.
Lenses and Refraction of Light:
• refraction of light (fig. 49 – 1 & ppts. 1 & 2): light travels at different
velocities in different transparent media; the ratio of light’s velocity in air to the
velocity in a medium (e.g., glass) is the refractive index (= 1.00 for air)
- due to these differing velocities, light striking a boundary between two
media of different refractive indices, at a non-perpendicular angle, undergoes a
change of direction (refraction)
- light in air entering a medium with refractive index > 1 has its path bent toward
a plane perpendicular to the surface, whereas the path is bent away from the
perpendicular in the opposite situation (light entering air from medium with refractive
index > 1); principle is exploited in lens construction
• convex lenses: constructed so as to converge light rays to a focal point (fig.
49 - 2 & ppt. 3); greater convex curvature gives stronger convergence, bringing rays
to focus nearer the lens; the distance to center of lens from focal point is the focal
length
- the greater the convergence, the greater the refractive power of the lens
(measured in diopters)
- the effect of convergence is to form an image that is inverted (fig. 49 – 7 &
ppt. 4)
• concave lenses: divergent lenses (due to reversal of curvature); no image
formation; compensates for convergence (fig. 49 - 3 & ppt. 5)
Bio 659 lec. 13
2.
- p. 2 -
Structure of the Eye: (consult any A & P text)
• layers: fibrous tunic (external) consists of sclera and cornea; vascular tunic
(intermediate) consists of choroid coat, ciliary body & iris, neural tunic (internal)
consists of retina (ppt. 6)
• lenses: cornea (anterior part of fibrous tunic) is convex & lens (suspended by
suspensory ligaments of ciliary body) is also convex
- lens may vary its curvature (accommodation); aqueous humor and vitreous
humor also contribute to focusing (fig. 49 – 9 & ppt, 7)
• cavities & humors (ppt. 6): anterior cavity is in front of lens and filled with
aqueous humor; subdivided by iris into anterior and posterior chamber; posterior
cavity filled with gelatinous vitreous humor (fixed from birth)
• ciliary body (fig. 49 – 10 & ppts. 8 & 9): processes extend inward (toward
lens) and anchor suspensory ligaments attached to lens
- ciliary muscles – contraction releases tension on suspensory ligaments,
permitting lens (naturally elastic) to become more spherical (increased convergence =
accommodation); relaxation restores ligament tension and flattens lens (ppt. 10)
- intraocular pressure (figs. 49 –19, 49 – 20, 49 – 21 & ppts. 11 to 13):
aqueous humor is produced by epithelium of ciliary body and circulates through anterior
cavity, passing through pupil into anterior chamber where it is drained to bloodstream
by modified veins (canal of Schlemm)
- if rate of formation exceeds rate of drainage, intraocular pressure rises
and glaucoma may result, causing destruction of fibers in optic nerve & or compromise
of retinal blood flow, which culminates in blindness
Bio 659 lec. 13
- p. 3 -
• iris: pigmented anterior part of vascular tunic; features central opening
(pupil) that admits light to posterior part of eye: two sets of muscles cause dilation
(radial mm.) or constriction (circular mm.) of pupil; adjustments of pupil in response
to changes in light intensity constitute pupillary reflex (see below)
• retina: several layers of neurons & photoreceptors (rods & cones)
3.
Optics of the Eye:
• distant vs. near vision: light rays reaching the eye from a distant source in
the visual field are nearly parallel and require less convergence to be focused on the
retina; light rays from a near source in the visual field are divergent and require
more convergence (stronger focal power) to be focused on retina (ppt. 14)
- ciliary muscles that promote accommodation (described above) are
controlled by parasympathetic nerve fibers carried by cranial n. III (oculomotor) to
synapse with second order neurons in ciliary ganglion & continue to ciliary mm.; loss
of lens elasticity with age results in loss of accommodation, a condition known as
presbyopia
- pupillary reflex controls amount of light admitted to eye; iris muscles
regulate dilation (sympathetic n. signals permitting more light to enter – response to
poor level of illumination of visual field) and constriction (parasympathetic n.
signals restricting light access – response to intense illumination of visual field); (see
fig. 51 – 11 & ppts. 15 & 16)
Bio 659 lec. 13
- p. 4 -
• normal & abnormal focal properties (figs. 49 – 12, 49 – 13 & ppts. 17 & 18):
emmetropia is the term for normal optical properties – ciliary mm. relaxed for distant
viewing, accommodation effective for near viewing
- hyperopia (farsightedness): eyeball too short (or lens not convergent
enough); parallel light rays brought to focus beyond retina; may be corrected with
convergent (convex) corrective lenses
- myopia (nearsightedness): eyeball too long (or over convergent lens);
parallel light rays brought to focus in front of retina; may be corrected with
divergent (concave) corrective lenses
- astigmatism results from differing curvature for different planes (of
cornea) – cannot be corrected by accommodation; corrected with cylindrical
corrective lens (as opposed to usual spherical lenses) (figs. 49 – 14, 49 – 15 & ppts. 19
& 20)
• visual acuity: ability to discriminate detail in visual field; contingent upon
diameter of fovea, a small central region of retina, populated exclusively by cones,
which are responsible for resolving detail in an image on the retina
• depth perception: ability to determine how near (or far) an object is in the
visual field; cues relied upon by brain include mainly motion parallax, relative size
occupied in visual field & stereopsis (overlap of slightly disparate images on both
retinas)