A layer of muscle cells consisting of 1–3 rows is present on the inner part of the circular layer of mouse colon. These muscle cells are thinner, with a denser cytoplasm and more caveolae than those of the main portion of the circular layer. A connective interstice divides the two parts of the circular layer and is occupied by a nerve plexus rich in varicosities. Groups of cells, identified as interstitial cells of Cajal, and a nerve plexus rich in varicosities are located on the border between the innermost circular layer and the tela submucosa. The interstitial cells have many cytological features in common with muscle cells (caveolae, basal lamina, thin filaments, smooth endoplasmic reticulum, glycogen particles) and touch each other and the nerve endings frequently and the muscle cells of the innermost layer rarely.