The eye and its parts · Biology Class 11–12 | StemPy Lab
Course 17.3·Lesson 1 of 3
The eye and its parts
The eye is a ball with three layers. The sclera and the cornea form the outer layer, the choroid with the ciliary body and the iris form the middle layer, and the retina forms the inner layer. The lens focuses light, and the iris sets the size of the pupil. Two chambers hold the aqueous humour and the vitreous humour.
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Sense organs
Your senses tell you about the world. The eye and the ear are two of the sense organs. This chapter of NCERT describes them. The NEET syllabus of the National Testing Agency does not list the structure of the eye and the ear, but the boards, A-Level, IB and AP ask them. So the lessons of this course are taught for those exams, and the NEET style tests of this chapter do not ask them.
The eyeball
The paired eyes lie in sockets of the skull called orbits. The adult eyeball is a sphere. Its wall has three layers.
The outer layer is the sclera, made of dense connective tissue. Its front part is the transparent cornea.
The middle layer is the choroid. It has many blood vessels, and it looks bluish.
The choroid is thin over the back two thirds of the eyeball.
It is thick at the front, where it forms the ciliary body.
The ciliary body continues forward as the iris, the pigmented and coloured part that we see.
The inner layer is the retina. It holds the cells that sense light.
A labelled section of the human eye seen from the side, with the front on the left. The outer layer is the sclera, which becomes the transparent cornea at the front. Under the sclera lies the middle layer, the choroid, which is thick at the front where it forms the ciliary body and continues as the coloured iris. The iris has an opening in the middle, the pupil. Behind the pupil lies the crystalline lens, held in place by ligaments attached to the ciliary body. The space between the cornea and the lens is the aqueous chamber, with aqueous humour, and the space between the lens and the retina is the vitreous chamber, with vitreous humour. The inner layer is the retina. At the back, the optic nerve leaves the eye at the blind spot, and the yellowish macula lutea with its central pit, the fovea, lies beside it.
The lens, the pupil and the two chambers
The lens is a transparent, crystalline structure. Ligaments attached to the ciliary body hold it in place.
The pupil is the opening in the iris, in front of the lens. The muscles of the iris regulate its size.
The aqueous chamber is the space between the cornea and the lens. It holds a thin watery fluid, the aqueous humour.
The vitreous chamber is the space between the lens and the retina. It holds a transparent gel, the vitreous humour.
You should find that a wider pupil lets in more light, and that the area changes as the square of the width. So a pupil that is twice as wide lets in four times as much light.
The retina
The retina has three layers of cells. From the inside to the outside they are the ganglion cells, the bipolar cells and the photoreceptor cells. There are two kinds of photoreceptor cells, the rods and the cones. The next lesson describes them.
Two panels. On the left, a section of the retina with light entering from the vitreous side at the top. The layers from the inside to the outside are the ganglion cells, the bipolar cells and the photoreceptor cells, which are rods and cones, resting on the choroid at the bottom. Arrows show light passing through the first two layers to reach the photoreceptors, and the signal passing back from the photoreceptors to the bipolar cells, the ganglion cells and the optic nerve. On the right, a graph of the approximate number of rods and cones against the angle from the fovea. Cones are crowded at the fovea and fall away quickly. Rods are absent at the fovea and most numerous about 20 degrees from it. A gap marks the blind spot. A note says that the graph shows the approximate shape only.
Light must pass through the ganglion and bipolar layers to reach the photoreceptors. The signal then travels back the other way. This is why the retina is often called inverted. NCERT does not use the word.
Two special places on the retina
The blind spot is the place where the optic nerve leaves the eye and the blood vessels of the retina enter it. It lies medial to the posterior pole of the eyeball and a little above it. It has no photoreceptor cells, so it cannot sense light.
The macula lutea is a yellowish pigmented spot at the posterior pole, lateral to the blind spot. In its middle there is a pit, the fovea. The fovea is a thinned out part of the retina where only cones are densely packed. Here the visual acuity, the power to see fine detail, is the greatest.
Try it in Python
The program works out the area of the pupil and the light that enters.
Practise with new numbers
Common mistakes
Saying that the cornea is a separate layer. It is the transparent front part of the sclera.
Mixing up the two humours. The aqueous humour is a watery fluid in front of the lens. The vitreous humour is a gel behind the lens.
Saying that the blind spot has many rods. It has no photoreceptors at all.
Saying that the fovea has rods and cones in equal number. The fovea has only cones.
Saying that the iris is the opening. The iris is the coloured part. The opening in it is the pupil.
Remember these
Outer layer: sclera with the cornea in front. Middle layer: choroid, ciliary body, iris. Inner layer: retina.
Lens: held by ligaments of the ciliary body. Pupil: regulated by the iris muscles.
Aqueous humour (watery, in front of the lens). Vitreous humour (gel, behind the lens).
Retina layers from inside to outside: ganglion cells, bipolar cells, photoreceptor cells.
Blind spot: no photoreceptors, the optic nerve leaves. Fovea: only cones, the greatest acuity.
Check yourself
Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.
Name the three layers of the wall of the eyeball from the outside inwards.
What holds the lens in place, and which structure regulates the size of the pupil?
Where is the aqueous humour, and where is the vitreous humour?
Why can you not see an object whose image falls on the blind spot?
Where is the visual acuity the greatest, and why?
References
Clark M. A., Douglas M. and Choi J. (2018). Biology 2e, OpenStax, chapter 36, Sensory Systems, section 36.5, Vision.
Betts J. G. et al. (2022). Anatomy and Physiology 2e, OpenStax, chapter 14, The Somatic Nervous System.
NCERT (2023). Biology, Textbook for Class XI, chapter 18.
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