Alimentary Canal

The Mouth and the Teeth

The alimentary canal opens to the outside at the mouth, leading into the buccal (oral) cavity, which houses the teeth and a muscular tongue. Each tooth is fixed into its own bony socket in the jaw -- an arrangement called thecodont. Like most mammals, humans grow two successive sets of teeth over a lifetime: a temporary set of milk (deciduous) teeth in childhood, later replaced by a permanent set -- this is called diphyodont dentition. An adult has 32 permanent teeth of four different shapes, each doing a different job -- this mix of tooth types is called heterodont dentition: incisors (I) for cutting, canines (C) for tearing, premolars (PM) and molars (M) for grinding. One half of a jaw, upper over lower, is written as a dental formula: 2123/2123 -- meaning 2 incisors, 1 canine, 2 premolars and 3 molars on each side, top and bottom. The tongue is a freely movable muscular organ, anchored to the floor of the mouth by a fold called the frenulum; its upper surface carries small projections (papillae), some of which bear taste buds.

From Mouth to Stomach

The oral cavity leads into a short pharynx, a passage shared by both food and air, since the oesophagus and the trachea (windpipe) both open into it. During swallowing, a flap of cartilage called the epiglottis folds down over the glottis (the opening of the windpipe) so that food is guided into the oesophagus and does not enter the airway. The oesophagus is a long, thin tube running down through the neck, chest and diaphragm to the stomach; a muscular gastro-oesophageal sphincter at its lower end controls the entry of food into the stomach.

The Stomach

The stomach is a J-shaped bag lying in the upper-left part of the abdomen, with four regions: the cardiac region (where the oesophagus opens in), the fundic region, the body (its main central part), and the pyloric region, which opens into the small intestine through the pyloric sphincter.

The Small and Large Intestine

The small intestine has three parts in sequence -- a C-shaped duodenum, a long coiled jejunum, and a highly coiled final part, the ileum -- which opens into the large intestine. The large intestine consists of the caecum, colon and rectum. The caecum is a small blind pouch harbouring symbiotic microorganisms, with a narrow, finger-like, vestigial vermiform appendix attached to it. The caecum leads into the colon, which runs in four parts -- ascending, transverse, descending and sigmoid -- before opening into the rectum, which finally opens to the outside through the anus.

The Four-Layered Gut Wall

From the oesophagus right down to the rectum, the wall of the alimentary canal is built of four concentric layers, from outside in:

• Serosa -- the outermost thin covering (mesothelium) with some connective tissue.
• Muscularis -- smooth muscle, usually an inner circular layer and an outer longitudinal layer (some regions also have an oblique layer).
• Sub-mucosa -- loose connective tissue carrying nerves and blood/lymph vessels; in the duodenum it also contains glands.
• Mucosa -- the innermost layer, lining the lumen (the central cavity of the tube). …