According to Goodsall's rule, what happens if the external opening of an anal fistula is located below the transverse anal line?

Goodsall's rule provides a guideline for the behavior of anal fistulas based on the location of the external opening. When the external opening of a fistula is situated below the transverse anal line, the rule suggests that it typically communicates with the anal canal in a midline posteriorly.

This is due to the anatomy of the anal region and the development of the anal canal. The posterior midline area is predominantly drained by the anal canal structures in comparison to other regions, which helps in understanding the pathways that anal fistulas may take. Thus, when the external opening is located below the transverse anal line, it suggests a posterior trajectory aligning with the anatomy, leading to the fistula opening into the anal canal from that midline posterior position.

This anatomical guideline is crucial for surgical planning, as it helps predict the fistulous tract's course and informs the appropriate management of the fistula.