What is the ciliary body?

When we look at a distant object or a near object, we naturally adjust for focus. In other words, it has a mechanism that forms an image on the retina by thickening or thinning the lens.

The main things involved in accommodation are the "lens" that changes the refractive index by changing the thickness, the muscles of the "ciliary body" that are connected to it, and the "small chin" that pulls the lens out of the ciliary body. A connective tissue called an obi.

Specifically:

●When looking at near objects, the ciliary muscle contracts (tenses), the zonules loosen, and the lens thickens.

●When looking at a distance, the ciliary muscle relaxes, the zonules are stretched, and the lens becomes thin.

I think this is hard to understand, so let's write more concretely.

The ciliary muscle contracts and expands the ring-shaped muscle to adjust the focus.

This ciliary muscle is a muscle, but it is actually a circular muscle.

When we think of muscles, we often imagine straight muscles such as skeletal muscles, but such straight muscles shorten when contracted (tensed) and lengthen when relaxed.

So what happens when a circular muscle such as the ciliary muscle contracts and relaxes?

When the circular muscle contracts, the circumference corresponding to the length of the straight line shortens. In other words, the circle formed by the muscles becomes smaller. The state of the Chin frenulum that is connected to it also changes.

When the ciliary muscle contracts, the circle becomes smaller and the zonules are no longer stretched and loosened. The crystalline lens connected to the zonules is no longer pulled outward, and the elasticity of the crystalline lens increases the thickness. As a result, the focal length becomes shorter and you can see closer.

Conversely, when the ciliary muscle relaxes, the circumference lengthens and the circle formed also becomes larger. The zonules are pulled outwards, thinning the lens, increasing the focal length, and allowing distant vision.

Note) This site explains that ``When looking at a near object, the ciliary body contracts and becomes taller, and the zonules loosen, so the crystalline lens thickens due to its own elasticity.'' This is written for ease of understanding, and is exactly as described above. It is difficult to understand that the ciliary body is a circular muscle without specific explanation.

The eye is the most highly evolved precision machine in the human body.

The ciliary muscle continues to contract (tension) if you keep looking at a nearby object, and your eyes get tired. Conversely, when you look far away, you relax, so you don't get tired.

The eyes are made that way.

I have a question for you reading this. What do you think the size (diameter) of the lens of the eye is? Actually, the size (diameter) of the lens is about 9 mm. It's not even 1 cm. The thickness varies depending on the adjustment, but it is 4 to 5 mm. The thickness of such a small lens was adjusted by contraction and relaxation of the ciliary muscle.

The eye is the most highly evolved precision machine in the human body. Have you been straining your eyes lately? Take care!