Large Intestines of the Dog
4. Anal canal
Anal Canal is the very short terminal part of the large intestine, and it continues the rectum to the anus. The mucous membrane of the anal canal is divided into three zones: from cranial to caudal, the columnar zone, intermediate zone, and the cutaneous zone. The columnar zone is covered by columnar epithelium, and its mucosa is arranged in longitudinal folds or ridges that are called the anal columns. Between the columns at the anocutaneous line, grooves or small pockets, anal sinuses, are formed. The anocutaneous line, or the intermediate zone separates the columnar zone from the cutaneous zone. The cutaneous zone is covered with cornified stratified squamous epithelium and fine hairs, and contains the microscopic circumanal glands. On each side of the cutaneous zone open the ducts of the anal sacs (paranal sinus), which are small reservoir for the secretions of the sebaceous glands that are found in the wall of the sacs. The anal sacs are located ventrolateral to the anus, between the internal and external anal sphincters. The secretion of the anal sacs serves as a marker of identification.
