Читать книгу Pathy's Principles and Practice of Geriatric Medicine - Группа авторов - Страница 482

The rectum forms the final part of the alimentary tract. It is a tubular structure 12–15 cm in length, extending the sigmoid colon to the anal canal, which extends around 4 cm from the anal verge to the anorectal ring. The rectum follows the shape of the sacrum and, unlike the rest of the colon, lacks teniae coli, as these fuse in the sigmoid and are continuous in the rectum, forming a longitudinal muscle layer that surrounds the rectum along its length. The anal canal is separated by a dentate line into an upper mucosal lining and lower cutaneous segment. The area above the dentate line is supplied by the sympathetic and parasympathetic systems, whereas below the dentate line, the somatic nervous system provides innervation.¹⁰

The internal anal sphincter is a circular muscle layer that starts from the rectum. The internal anal sphincter is contracted at rest, preventing the involuntary loss of stool and gas. The pressure at the anus is increased by the presence of anal cushions, along with the muscular tone of the internal and external anal sphincters and puborectalis. The internal anal sphincter contributes 50–85% of the resting tone of the sphincter complex, with the external anal sphincter contributing 25–30% and the remaining 15% coming from the anal cushions.¹¹ The puborectalis forms a sling around the rectum and, when contracted, causes the anorectal angle to form. The external sphincter comprises striated muscle and is under voluntary control; the internal sphincter is formed from smooth muscle and is under the control of the autonomic system. At rest, the anus forms an angle of approximately 90° to the rectum. In the normal state, the rectum is empty¹²; it is an organ of expulsion, not of storage.