this lecture covered the anatomical features of the eye which relate to its refraction and focusing of incoming light; namely, the lens and the muscles that control the convexity of the lens. a bit more of the embryology of the eyes: the eyeballs are derived from the same rostral portion of the neural tube as the cerebrum, and as such have outer layers that are analogous to those in the CNS. the eyes are surrounded by an outer fibrous layer, homologous to the dura mater, followed by a vascular layer composed of anastamosing "choroid" blood vessels which is homologous to the arachnoid and pia mater, followed by a nervous layer which is homologous to the neurons in the CNS.
in order to form focused images the eye must focus beams of light that are diverging from single point sources: in other words, any light source will emit light that diverges and needs to be converged back into a single point in the back of the retina. the lens is a transparent component of the eye (derived from ectoderm) which accomplishes this refraction through its convexity. the convexity of the lens is maintained internally by elastic fibers and externally through the suspensory ligaments, which are attached to the ciliary bodies that surround the lens radially. when these ciliary muscles contract, this relaxes the suspensory ligaments, allowing the internal elastic fibers to create a more convex shape in the lens; and the opposite effect for the relaxation of the ciliary muscles.
the general rule with convexity of lens vs. distance of light source: the further away the light source, the less the beams of light are diverging, and thus less refraction is necessary, and thus less convexity is necessary. the closer the light source, the more divergence is happening, and more refraction is necessary to refocus the light beam at the back of the retina. hyperopia is far sightedness, resulting from an eyeball that is too short- in this case, the light is being focused on a point beyond the retina. myopia is the opposite, resulting from an eyeball that is too long and light being focused in front of the retina. hyperopia is the age related loss of convexity of the lens caused by the loss of elasticity.
a few more anatomical notes: the sclera is the tough white connective tissue that covers the eyeball, from which the extrinsic muscles of the eye insert. the cornea is the translucent, avascular continuation of the sclera that covers the lens and lets light into the eyeball. the vitreous body is the gel like fluid in the eyeball itself which is made largely of water, hyaluronic acid, and type II collagen. the anterior and posterior chambers are deep to the cornea, separated by the iris, and are the site of secretion of the aqueous humor by ciliary processes, which then drains into Schlemm's canal into the vascular system.
questions
embryology...
1. what is the eyeball derived from?
2. what are the neural and pigmented retinal layers derived from?
3. what are the sclera and choroid derived from?
4. what is the lens derived from?
5. optic nerve is surrounded by...
eyeball anatomy...
6. what is the vitreous body and what is it made of?
7. what are "floaters"?
8. what is the hyaloid canal and how is it related to floaters?
9. what are the three outer layers of the eyeball and what CNS structures are they homologous to?
10. what is the sclera?
11. describe the cornea.
12. what does the vascular coat consist of?
refraction and the lens...
13. why are the cornea and lens curved?
14. describe the lens.
15. describe how the lens adjusts the refraction of light according to the distance from the light source.
16. what is hyperopia?
17. what is myopia?
18. what is the resting convexity of the lens maintained by?
19. internal elastic fibers in the lens produce...
20. what is the capsule of the lens made of?
21. describe the production of new lens cells.
22. what is presbyopia and what is it caused by?
23. what is cataracts?
24. describe the suspensory ligaments that attach to the lens.
25. what do suspensory ligaments do?
26. convexity of the lens is altered by...
27. what happens when the ciliary muscles contract?
28. where are the anterior and posterior chambers? what are they partitioned by?
29. where does aqueous humor form and travel to?
30. what does Schlemm's canal do?
31. what is glaucoma caused by?
answers
1. same part of the neural tube that produces the cerebral hemispheres.
2. optic vesicle.
3. embryonic meningeal tissues.
4. ectoderm.
5. dura, arachnoid, pia, subarachnoid space.
6. the gel-like fluid inside the eyeball made of water, hyaluronic acid, type II collagen fibers.
7. tiny clumps of gel in the vitreous body that are too large to be phagocytosed during development.
8. the hyaloid canal is the vestigial remains of the hyaloid artery, remnants of which can turn into floaters.
9. outer fibrous coat homologous to dura mater, vascular coat homologous to arachnoid/pia mater, and nervous coat homologous to CNS layer.
10. dense, white CT, which is the point of insertion for the extrinsic muscles of the eye.
11. the cornea is a transparent, avascular continuation of the sclera in the center of the eyeball which allows light into the eye.
12. a layer of anastamosing blood vessels (choroid) and the ciliary body, which regulates the refraction of light.
13. to refract light from a point source in order to converge back into one point.
14. clear, avascular, and depends on diffusion of nutrients from vitreous humor, like the cornea.
15. for close light sources, the lens becomes more convex and for far away light sources the lens becomes less convex.
16. far-sightedness: eyeball is too short and light converges behind the retina.
17. near-sightedness: eyeball is too long and light converges in front of the retina.
18. internal elastic fibers and suspensory ciliary ligaments.
19. an inherent tendency to bulge.
20. collagen IV and glycoprotein.
21. lens cells are produced from the germinal center on the side of the lens, which then migrate from the germinal zone, lose their nucleus, and become transparent.
22. age-related loss of lens resting convexity caused by loss of elasticity.
23. reduction of vision due to opacity of lens.
24. zonule fibers that extend from ciliary body to the equatorial perimeter of the lens.
25. maintain resting tension and decrease convexity of the lens via outward tension.
26. ciliary muscle.
27. the tension in the suspensory ligaments decreases, which allows the lens to increase its elasticity and convexity.
28. deep to the cornea, partitioned by the iris.
29. formed by the ciliary processes, secreted into the anterior and posterior chambers.
30. absorbs the aqueous humor into veins.
31. a buildup of fluid pressure due to inadequate drainage into Schlemm's canal.
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