Hemroids Structure 

Description: The anal canal, a slit in the A-P dimension, about 2.5-4.0 cm in length, is the terminal part of the large intestine. Its upper boundry is at the U-shaped Puborectalis m. which delineates it from the rectum. The canal descends postero-inferiorly between the levator m. laterally, the anococcygeal ligament and the central perineal tendon, and is surrounded by internal and external sphincters. The int. sph. circumscribs the upper 2/3 of the canal, and the ext. sph. circumscribs the lower 2/3 is. The ext. sph. has three parts; the subcutaneous, the superficial and the deep. The superfical part is attached to the coccyx posteriorly, and the central perineal tendon, anteriorly.

 The superior part of the canal internally is characterized by a circumferential series of vertical folds or anal columns, about 20 in numberabout, and 2cm long. Distally, they are joined to each other by a fold of epithelium, creating an anal sinus above. The lower part of this series of columns is called the pectinate line. The circulation above this line is from the sup. rectal a. & v., and below the line and for the surrounding muscles, from the inferior rectal a. & v. The innervation of the canal above the line is via the hypogastric plexus, and below, from the pudendal n.

Function: The anal canal serves to transmit and lubricate stool as it passes externally from the rectum, and the anorectal junction maintains continence, until by relaxation, the anorectal angle straightens, allowing defecation. The upper part of the canal is sensitive to stretch, but the lower part is sensitive to pain, touch and temperature, as well as stretch. The contral of the int. sph. m. is involuntary, and the contral of the ext. sph. is voluntary.