The perineum is diamond shape.it is surrounded by anteriorly pubic symphysis, posteriorly by tips of coccyx and laterally by ischial tuberosity.

Between the two ischial tuberosity there is a horizontal line which devide the perineum into two triangle part .

The upper triangle part is called urogenital triangle.( In which the urethral opening and vaginal opening present)

The lower triangle part is called anal triangle ( in which anus opening is present)

The anal triangle area bounded  anteriorly by the horizontal line made  between two ischial tuberosity and posteriorly by coccyx.and laterally by sacrotuberous ligaments.

anal opening is present in anal triangle the lateral sides of anus opening there is situated pair of ischioanal fossae.

External anal sphincter muscle which is skeletal muscle ( voluntary muscle)is responsible for anus opening at the time of defecation which is present around the anus.