BIOLOGY Volume 1 - A Guide to General Biology - 2004

8. HETEROTROPHIC NUTRITION

8.3. The human alimentary canal

8.3.3. The mouth cavity

The Mouth cavity is a chamber located immediately behind the oral opening where food is chewed. During mastication, the muscular Tongue moves food around the mouth, mixing it and moistening it with saliva. The surface of the tongue bears taste buds (Fig. 8.16) containing receptors that detect sweet, salty, bitter, and sour substances. Simple (innate) and conditioned (acquired) Reflexes stimulate the secretion of saliva by the Salivary Glands. Visual and olfactory receptors play an important role in triggering salivation reflexes (sec. 8.4.1).

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Fig. 8.16. A. Scanning electron micrograph of the tongue surface of a three-week-old puppy. Taste buds are located in the groove surrounding the papilla. B. Vertical section of the tongue showing taste buds.

Per day, the human salivary glands secrete about 1.5 l of saliva. Saliva is an aqueous secretion containing the Enzymes amylase and Lysozyme, as well as mucus and various mineral salts, including chlorides, which enhance enzyme activity. Mucus moistens and lubricates the food, making it easier to swallow. Salivary amylase initiates the Digestion of starch, breaking it down first into shorter polysaccharide chains and then into the disaccharide maltose. Lysozyme destroys pathogenic Bacteria by catalysing The breakdown of their Cell walls (sec. 5.10.6). Eventually, the semi-solid, partially digested food fragments are bound together into a bolus, which is pushed towards the Pharynx. From there, a reflex swallowing action propels it into the oesophagus.