Ciliary: Definition, Classification, Structure and Function

The ciliary muscle is found in the eye’s ciliary body. It is found in the anterior vascular choroid layer (middle layer) of the eye. It regulates the accommodation reflex of the eyes by modifying the curvature of the lens via zonular fibres. It is made up of smooth muscle fibres that are aligned in three directions: circular, longitudinal, and radial.

Ciliary Definition

The ciliary definition states that ciliary is that coloured portion and the circular structure of eyes that extends from the iris. Aqueous humour is the fluid that the ciliary body produces in the eye. It also houses the ciliary muscle, which alters the curvature of the lens as you concentrate your eyes on a close object.

Further, the ciliary definition mentions that the ciliary muscle has a  triangular form and it is found beneath the anterior sclera, just posterior to the limbus. The shortest side of the triangle form faces anterior-inward. This is the region of the ciliary body where the base of the iris inserts itself.

Ciliary Movement

Cilia are hair-like outgrowths that can be seen on the plasma membrane. Ciliary movement is the rhythmic movement of cilia that generates fluid or cell movement. Ciliary movement, for example, aids in the movement of the cell as well as the flow of food within the cell in Paramecium. Cilia are found in epithelial linings such as the fallopian tube and respiratory system, where they aid in fluid circulation and trap any foreign particles in the mucus.

Function of Ciliary Muscle 

Zonular fibres connect ciliary muscles to the lens of the eyes. They may contract and modify the curvature of the lens, allowing us to focus on close things. Ciliary muscle contraction makes the lens more spherical and boosts the focusing power. When it is relaxed, the zonular fibres stretch, flattening the lens and allowing the distant object to be focused on the retina.  This process of stretching and flattening is also referred to as the accommodation reflex.

The accommodation reflex is due to ciliary muscles. Ciliary muscles assist in adjusting the curvature of the lens to focus on a close object. It is also in charge of controlling the flow of aqueous humour into Schlemm’s canal.

When the ciliary muscles flex, the lens becomes more spherical. It is so because the stress on the zonular fibres is released. We can now plainly see close objects as our lenses can concentrate on them. This process is also referred to as accommodation.

Ciliary muscles are not voluntary. They are part of the ciliary body and are located between the sclera and the ligaments that hold the lens in place. They aid in the focus at distant and close objects by altering the curvature of the lens.

Ciliary Classification

The ciliary classification has the ciliary muscle made up of three distinct muscle fibres: longitudinal, circular, and oblique.

Longitudinal Fibres 

The most exterior fibres, the longitudinal fibres (meridional), link the ciliary body anteriorly to the scleral spur and trabecular meshwork at the limbus and posteriorly to the suprachoroidal lamina. It is the fibres connecting the choroid and sclera and reaches as far back as the equator of the eye. The contraction of the longitudinal muscle opens the trabecular meshwork and Schlemm’s canal.

Circular Fibres

The circular fibres (sphincteric) make up the more anterior and inner sections. They also run parallel to the limbus. This insertion occurs in the posterior iris. When these fibres flex, the zonules relax, increasing the lens’s axial diameter and convexity.

Oblique Fibres

It is the ciliary classification, where the oblique fibres (radial or intermediate) join the longitudinal and circular fibres. The constriction of these fibres may result in a widening of the uveal trabecular gaps.

Structure of Eye Lens and Role of Ciliary Muscle

The bi-convex lens of the eye is made of a translucent fibrous jelly-like substance. The ciliary muscles’ primary job is to modify the shape of the lens of the eye to aid with focusing. The ciliary muscles also serve to control the flow of aqueous fluid in the eye. In other words, the ciliary muscles are made up of circular layers of smooth muscle that assist the suspended lens.

External elements include:

Sclera, Conjunctiva, Cornea, Iris and Pupil.

Internal components include:

Retina, lens, and optic nerve, Vitreous Humour and Aqueous Humour