Sphincter Muscles

Sphincter Muscles (Gr. sphinkter, 'that which binds tight') surround an opening or canal which, by their action, they are able to close or diminish in size. They are found round the mouth of the eye, the pylorus of the stomach, the outlet of the urinary bladder, and the anus. It is to the presence of these muscles that the higher animals owe the power of retaining for a considerable period the excrementitious matters collected in the bladder and rectum, and of discharging them at intervals, the sphincter muscles being, like those engaged in the process of respiration, mainly, but not entirely, under the control of the will.