pectinate line
The pectinate line is the line of transition between the mucosal lining of the anal canal and the skin lining of the anal canal. So, this is the point where the mucosal zone ends and the skin begins. The white line is a transitional zone between the pectinate line and "regular" skin where there are some more subtle changes in the epithelial lining of the anal canal. But the pectinate line is the line that demarcates the major transition from mucosal lining to skin.