Contribution of the External Anal Sphincter to the Pressure Zone in the Anal Canal.

It is well known both by clinical examination and by objective measurement that a high pressure zone is to be found in the anal canal, due to the action of the anal sphincteric muscles. Division of part of the internal anal sphincter in the operation of internal sphincterotomy resulted in a diminution in this pressure zone (Bennett and Duthie, 1964). The pressure remaining after internal sphincterotomy was thought to be due to the action of the external anal sphincter since this muscle has been shown by electromyography to be in a constant state of tonic activity (Floyd and Walls, 1953; Taverner and Smiddy, 1959). In the present study, the contribution of the external anal sphincter to the pressure zone in the anal canal has been investigated.