"Anal fissures occur in the specialized tissue that lines the anus and anal canal, called anoderm. At a line just inside the anus--referred to as the anal verge or intersphincteric groove--the skin (dermis) of the inner buttocks changes to anoderm. Unlike skin, anoderm has no hairs, sweat glands, or sebaceous (oil) glands and contains a larger number of somatic sensory nerves that sense light touch and pain. (The abundance of nerves explains why anal fissures are so painful.) The hairless, gland-less, extremely sensitive anoderm continues for the entire length of the anal canal until it meets the demarcating line for the rectum, called the dentate line. (The rectum is the distal 15 cm of the colon that lies just above the anus and just below the sigmoid colon.)"
http://www.medicinenet.com/anal_fissure/article.htm
Bummer!!