Iris sphincter muscle Iris sphincter muscle The iris sphincter muscle (pupillary sphincter, pupillary constrictor, circular muscle of iris, circular fibers) is a muscle in the part of the eye called the iris. It encircles the pupil of the iris, appropriate to its function as a constrictor of the pupil. It is found in vertebrates and some cephalopods. Initially, all the myocytes are of the smooth muscle type but, later in life, most cells are of the striated muscle type. Its dimensions are about 0.75 mm wide by 0.15 mm thick. In humans, it functions to constrict the pupil in bright light (pupillary light
Skeletal muscle in the rectus femoris. Convergent arrangements are triangle or fan shaped, with wide origins and more narrow insertions. The wide variation of pennation angles in this architecture can actually allow for multiple functions. For instance, the trapezius, a prototypical convergent muscle, can aid in both shoulder elevation and depression. Multipennate arrangements are not limited to a particular arrangement, but—when used specifically—commonly refer to what is essentially a combination of bipennate or unipennate arrangements with convergent arrangements. An example of this architecture would be the human deltoid muscle. Many muscles are named by the action the muscle performs. These include: The