Anatomy Which animal has fixed eyes?


Our window to the world

We see the world with our eyes. The sense of sight gives us precise but also deceptive images of the outside world. The optical apparatus and the anatomy of the eyes, which are shown here, are responsible for this.

Here's the way to the sense of sight

The outside eye


The eye is a very important means of expression for our face. It not only serves as an organ of perception, but also shows other people a lot of our inner life, more than we are perhaps aware of.


The auxiliary apparatus of the eye

The eyelids protect the sensitive surface of the eye from bumps or heavy soiling due to closure. This closure works quickly and reflexively, without our being able to influence it (eye closing reflex due to bubbles on the cornea).

Above the eyelids are the lacrimal glands, which continuously secrete tear fluid. Every blink of the eye wets the entire surface of the eye with tear fluid. The cornea is nourished and disruptive dust particles are washed away. The frequent blinking of the eye can prevent the eye from drying out. Such drying up of the eyes occurs after death and is referred to as “breaking of the eyes”.

Figure auxiliary apparatus of the eye

1 lacrimal gland
2 upper eyelid
3 outline of the orbit (bony eye socket)
4 conjunctival sac
5 outer lid band
6 dermis
7 lower eyelid
8 tear point
9 tear tubes
10 bags under the eyes
11 inferior turbinate
12 lower nasal passage Fig. From Faller, The Human Body

Figure eyeball from the side and eye muscles

The eyes themselves are moved by 6 eye muscles each.

The structure of the eye

The eye is protected in the bony eye socket of the skull (orbit) and embedded in fatty tissue. At the side and back, the eye has a clear three-layer structure:

  • as the outermost firm capsule the sinewy dermis,
  • the black colored, strongly perfused choroid,
  • inside is the delicate, light-sensitive retina.

Figure horizontal section through the left eye (source Wikipedia)


The optic nerve leads from the retina to the brain (optic nerve junction, knee body, visual cortex).

Transformations of the three layers in the eye

The dermis, which is visible from the outside as the white of the eyes, is transformed into the transparent cornea in the front center of the eye.

The choroid also undergoes a transformation in the anterior eye area: it becomes the iris and the ciliary body immediately behind it.

Fibers that have grown together with the lens emanate from the ciliary body, the wrinkled ring around the lens. Inside the ciliary body is a ring-shaped muscle: the ciliary muscle. Its activity can change the shape of the lens.

The lens is a crystal clear body with a very strong refractive power (around 10 diopters). It lies between the iris and the vitreous humor. Its anterior surface is less curved than the posterior (unequal biconvex lens). Their thickness is on average 3.7 mm, with accommodation it increases to 4.4 mm.

Behind the lens is the gelatinous vitreous body. As its name suggests, it is crystal clear. It fills the entire back space of the eyeball. Its refractive power is low, the refractive power close to that of water.

The spaces in front of the vitreous are filled with clear aqueous humor. The iris and lens divide these spaces into two chambers: an anterior and a posterior eye chamber.

The fundus of the eye

How can I see the fundus of my own eye? Illumination (moving flashlight) from the side.

Illustration The fundus of the left eye

3 exit point of the optic nerve (blind spot)
5 branch of the central artery
6 branch of the central vein
7 yellow spot

We mainly recognize two places on the fundus that interest us:
- Exit point of the optic nerve and blood vessels: blind spot.

- vein-free zone: yellow spot.

Regarding the blind spot: on our retina we are blind at the point where the optic nerve pulls out of the eyeball.

About the yellow spot: In the visual axis, approx. 4 mm from the blind spot, the light sensory cells are particularly concentrated and numerous. This spot is called the yellow spot. On the fundus of the eye, this is exactly the place where we have our "fixation point".

The retina

The retina lines the inner wall of the eyeball up to the ciliary body as wafer-thin nerve tissue (approx. 0.1-0.4 mm). It is the actual light-absorbing organ of the eye and consists of over 100 million light- and color-sensitive sensory cells and a multiple of connecting nerve cells.

The retina in cross section


Light sensory cells (rods, slim and cones, thick)


Bipolar cells (switching cells)



nerve cells leading away (to the optic nerve)

There are two groups of sensory cells

  • Chopsticks for seeing through light and dark
  • Cones for color vision

Cones are about 18 times more common than rods. Each light sensory cell is connected with nerve fibers, near the yellow spot in a ratio of 1: 1, at the edge of the retina about 130 sensory cells are connected with a nerve fiber.
Compared to some animals, humans have relatively few visual cells:

animalPhotoreceptor cells per mm2
brown owl
Brown rat

The sensory cells are closest in the yellow spot. You can find cones there mainly.

Light process and visual purple

As soon as light hits the retina, a substance in the rods, the so-called visual purple (rhodopsin), breaks down and releases a small amount of energy. Visual purple consists of a protein (opsin) and the retinal, the aldehyde of vitamin A. This vitamin is found in carrots, with vitamin A deficiency we can suffer from night blindness.

When the retina is illuminated, opsin and retinal fall apart, creating a stimulus in the sensory cells that is sent to the brain via the nerve fibers of the optic nerve.