The Anal Sphincters

Abstract

The classical description of the external sphincter has been of a group of muscle fibres surrounding the anal canal and continuing with the pelvic floor at the edge, puborectalis portion, of the levator ani muscle. Studies have been made of this region by histological sections and by various methods of direct dissection. There has been considerable debate about the sub-divisions comprising the external sphincter. Initially it was described as being in three separate parts—subcutaneous, superficial and deep. Many authors supported this arrangement, which found its way into standard anatomy textbooks. The sphincter has also been described as consisting only of two parts—subcutaneous and deep (Goligher et al., 1955). This dispute has been largely resolved by two studies using careful dissection together with meticulous histological examination of the structures in this region (Oh and Kark, 1972: Shafik, 1975). They show that part of the reason for the discrepancy in the previous findings is that the relationships of the various parts of the sphincter muscles are not the same around the circumference of the anus. Thus, while in the mid-line posteriorly three layers are not difficult to distinguish, in the lateral aspects of the anus the deep and superficial parts may quite well be fused, and unless they are traced either anteriorly or posteriorly, they could not clearly be separated. Even these studies do differ in some detail and this, again, may be due to different interpretations of the structures dissected out.