Imagine the anal canal as a circular room with pink walls. Now imagine that this circular room has short curtains all the way around--pink curtains that hang in columns and reach about halfway down the wall.
You're standing in the center of the room and have a white crayon in your hand. You walk over to the wall and place the tip of the crayon where the curtain bottom meets the wall. Next, you continue that line all the way around the room. You've just drawn the pectinate line--a thin white line that runs along the bottom edges of anal columns in the anal canal.
What does the pectinate line do?The pectinate line serves as a dividing line. For example, hemorrhoids above the pectinate line are considered internal hemorrhoids; they don't tend to hurt because the area above the pectinate line isn't very sensitive to pain. Hemorrhoids below the pectinate line are considered external hemorrhoids; they tend to hurt and itch because the area below the pectinate line is very sensitive to pain.
Pain sensation differs above and below the pectinate line because the tissue is different. Above the pectinate line, the anal canal is made of tissue much like that found in the intestines. This tissue is designed to respond to changes in pressure. Below the pectinate line, the anal canal is basically comprised of skin, which is why it's so sensitive to pain.
- Hemorrhoids. MedicineNet. 30 Aug. 2006 [http://www.medicinenet.com/hemorrhoids/article.htm].
- Goldberg, Stephen. Clinical Anatomy Made Ridiculously Simple. MedMaster, 1999.