Structure of the Human Alimentary Canal

The human alimentary canal is a single, continuous, muscular tube some eight to ten metres long in the adult when fully stretched out, though it is coiled compactly to fit inside the abdominal and thoracic cavities. It begins at the mouth and ends at the anus, and along its length it passes through a series of clearly distinguishable regions, each structurally adapted to carry out a particular stage of digestion, so that food entering at one end is progressively broken down, absorbed from, and finally voided at the other.

The buccal cavity (mouth cavity) is the first region, bounded by the lips, cheeks, hard and soft palate above, and the floor of the mouth below; it houses the teeth (taken up in the next section), the muscular tongue, which manipulates food and carries the taste buds, and the openings of the ducts of the salivary glands. Food is mechanically broken down here by chewing (mastication) and mixed with saliva to form a soft, moist mass called a bolus, which is then swallowed. The bolus passes briefly through the pharynx, a short muscular passage common to both the digestive and respiratory pathways, and then enters the oesophagus, a narrow, muscular tube roughly twenty-five centimetres long that runs down through the thorax, behind the trachea and heart, and pierces the diaphragm to open into the stomach; the oesophagus has no digestive function of its own beyond actively propelling the bolus onward by peristalsis (Section 14.5).

The stomach is a wide, J-shaped, highly muscular and distensible bag lying just below the diaphragm on the left side of the abdomen, connected to the oesophagus at its upper end via the cardiac sphincter (which prevents the acidic stomach contents from regurgitating upward) and to the small intestine at its lower end via the pyloric sphincter (which regulates the slow, controlled release of stomach contents into the duodenum). Anatomically the stomach is divided into a small cardiac region near the oesophageal opening, a large, dome-shaped fundic region (the body of the stomach) where most gastric secretion and mixing occurs, and a narrower pyloric region leading to the pyloric sphincter. Its highly folded internal lining (rugae) allows it to expand considerably to store a meal, and its thick, three-layered muscular wall churns the food vigorously with gastric juice to form a semi-liquid mass called chyme.

The small intestine, the longest part of the canal at some six to seven metres, is where the bulk of digestion is completed and where almost all absorption takes place. It has three successive parts: the short, C-shaped duodenum (about 25 cm), which curves around the head of the pancreas and receives the ducts carrying bile from the liver and pancreatic juice from the pancreas; the jejunum, the next two-fifths or so of the tube; and the ileum, the remaining and longest part, which joins the large intestine at the ileo-caecal valve. The internal lining of the small intestine is thrown into countless finger-like projections called villi (and, at a still smaller scale, microvilli on each absorptive cell, forming the "brush border"), which enormously increase the surface area available for absorption.

The large intestine, shorter and wider than the small intestine, begins at the caecum, a small pouch bearing the vestigial vermiform appendix, and continues as the colon (ascending, transverse, descending and sigmoid portions), the rectum, and finally the short anal canal, which opens to the exterior at the anus, guarded by an internal (involuntary) and an external (voluntary) anal sphincter. The large intestine has essentially no digestive function of its own; its main roles are the reabsorption of water and some electrolytes from the residual material and the temporary storage and eventual elimination of faeces (Section 14.10). …