Afterstudying this section, you should be able to:
■ identify the different sections of the large intestine
■ describe the structure and functions of the large intestine, the rectum and the anal canal.
The large intestine.This is about 1.5 metres long, beginning at the caecum in the right iliac fossa and terminating at the rectum and anal canal deep in the pelvis.
Its lumen is about 6.5 cm in diameter, larger than that of the small intestine. It forms an arch round the coiled-up small intestine (Fig. 12.30).
For descriptive purposes the colon is divided into the caecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum and anal canal.
The caecum. This is the first part of the colon (Fig. 12.31). It is a dilated region which has a blind end interiorly and is continuous with the ascending colon superiorly. Just below the junction of the two the ileocaecal valve opens from the ileum. The vermiform appendix is a fine tube, closed at one end, which leads from the caecum. It is usually about 8 to 9 cm long and has the same structure as the walls of the colon but contains more lymphoid tissue.
The ascending colon. This passes upwards from the caecum to the level of the liver where it curves acutely to the left at the hepatic flexure to become the transverse colon.
The transverse colon. This is a loop of colon that extends across the abdominal cavity in front of the duodenum and the stomach to the area of the spleen where it forms the splenic flexure and curves acutely downwards to become the descending colon.
The descending colon. This passes down the left side of the abdominal cavity then curves towards the midline. After it enters the true pelvis it is known as the sigmoid colon.
The sigmoid colon. This part describes an S-shaped curve in the pelvis that continues downwards to become the rectum.
The rectum.This is a slightly dilated section of the colon about 13 cm long. It leads from the sigmoid colon and terminates in the anal canal.
The anal canal.This is a short passage about 3.8 cm long in the adult and leads from the rectum to the exterior. Two sphincter muscles control the anus; the internal sphincter, consisting of smooth muscle, is under the control of the autonomic nervous system and the external sphincter, formed by skeletal muscle, is under voluntary control (Fig. 12.32).
The four layers of tissue described in the basic structure of the gastrointestinal tract (Fig. 12.2) are present in the colon, the rectum and the anal canal. The arrangement of the longitudinal muscle fibres is modified in the colon. They do not form a smooth continuous layer of tissue but are instead collected into three bands, called taeniae coli, situated at regular intervals round the colon. They stop at the junction of the sigmoid colon and the rectum. As these bands of muscle tissue are slightly shorter than the total length of the colon they give it a sacculated or puckered appearance (Fig. 12.32).
The longitudinal muscle fibres spread out as in the basic structure and completely surround the rectum and the anal canal. The anal sphincters are formed by thickening of the circular muscle layer.
In the submucosal layer there is more lymphoid tissue than in any other part of the alimentary tract, providing
non-specific defence against invasion by resident and other microbes.
In the mucosal lining of the colon and the upper region of the rectum are large numbers of goblet cells forming simple tubular glands, which secrete mucus. They are not present beyond the junction between the rectum and the anal canal.
The lining membrane of the anal canal consists of stratified squamous epithelium continuous with the mucous membrane lining of the rectum above and which merges with the skin beyond the external anal sphincter. In the upper section of the anal canal the mucous membrane is arranged in 6 to 10 vertical folds, the anal columns. Each column contains a terminal branch of the superior rectal artery and vein.
Functions of the large intestine, rectum and anal canal:
The pancreas is a pale grey gland weighing about 60 grams. It is about 12 to 15 cm long and is situated in the epigastric and left hypochondriac regions of the abdominal cavity (see Figs 3.34 and 3.35, pp. 48 and 49). It consists of a broad head, a body and a narrow tail. The head lies in the curve of the duodenum, the body behind the stomach and the tail lies in front of the left kidney and just reaches the spleen. The abdominal aorta and the inferior vena cava lie behind the gland.
The pancreas is both an exocrine and endocrine gland.