Structure of the alimentary canal

The alimentary canal is a single, continuous, muscular tube that begins at the mouth and ends at the anus, and each stretch of it is shaped for a specific task. The mouth opens into the buccal (oral) cavity, where teeth and tongue carry out mechanical digestion. Human teeth are set in individual sockets in the jawbone (a thecodont arrangement) and come in two generations across a lifetime — 20 temporary milk teeth followed by 32 permanent teeth (diphyodont) — and in four distinct shapes suited to different jobs (heterodont): chisel-like incisors for cutting, dagger-like canines for tearing, and premolars and molars for grinding and crushing, summarised for one half-jaw by the dental formula. The tongue is a mobile muscular organ, anchored at the back by the frenulum, that mixes food with saliva, helps form it into a bolus, aids swallowing and carries taste buds on its papillae. Food then passes through the pharynx, a short shared passage for food and air (guarded during swallowing by the epiglottis, which stops food entering the windpipe, and flanked by the tonsils), and down the oesophagus, a muscular tube that carries food to the stomach by peristalsis, its entry controlled by the cardiac sphincter. The stomach is a temporary storage organ with three regions — cardiac, fundic and pyloric — whose folded rugae let it expand for a large meal, and whose pyloric sphincter releases partially digested food into the duodenum in controlled amounts. The small intestine, the longest part of the canal, has three regions — the short, U-shaped duodenum, the long coiled jejunum, and the even longer, highly coiled ileum — and its lining is thrown into villi (each covered in even smaller microvilli, giving a 'brush border' that dramatically increases the absorptive surface area) interspersed with mucus-secreting goblet cells and lymphoid Peyer's patches. The **la …