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
In vitro muscle testing appropriate magnification level is required due to the dexterity required for isolation of muscles. An external, fibre optic light source is also beneficial to provide sufficient light without the emission of heat. There is no correct approach for the preparation of muscles for testing, as long as the muscle is not damaged during preparation, the muscle-tendon unit is intact and there is something that can be used to anchor the muscle within the testing rig. Pieces of bone can be left at the proximal and/or distal end of skeletal muscles to allow for anchoring. In addition, silk sutures or aluminium