Mammoth Memory

Antagonistic muscles – opposing muscles (radial and circular)

Antagonistic muscles are the pair of opposing muscles in the iris that control the size of the pupil. One is a circular muscle layer and the other a radial muscle layer.


The antagonistic muscles can be found in the iris and are responsible for changing the size of the pupil

The two mussels (muscles) were antagonistic towards the pupil, first stretching him, then squeezing him to a pinpoint. 

Radial and circular muscle

Located in the iris, the radial muscle opens the pupil wider, and the circular muscle reduces it, but only one type can work at any one time, while the other relaxes. They get the name “antagonistic” because they oppose each other, working in opposite directions.

The image shows there are 2 antagonistic muscles in the iris, the radial, close to the pupil and the radial a more broader muscle that is the coloured part of the iris

When the circular muscle contracts and the radial muscle relaxes, the pupil reduces in size.
When the radial muscle contracts and the circular muscle relaxes, the pupil opens wider.

Diagram showing that first the circular and radial muscles contract for bright light, in normal light the radial muscle contracts and circular muscle relaxes and in dark light both muscles relax to give a large aperture

How the antagonistic muscles work to constrict and dilate the pupil.

Side ward view of the antagonistic muscles also showing the ciliary muscles


Sectional side view showing position of iris in relation to lens. 


These muscles are normally hidden by the pigmented top layer of the iris (which gives your eyes their colour).

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