Mouth. Historical illustration of a human mouth (centre), displaying the teeth, the tongue and the tonsils. The tongue has been lifted to show the sublingual salivary ducts. The top diagram shows the structure of one of the salivary glands, the parotid gland. The salivary glands secrete saliva, an alkaline fluid that aids swallowing. It contains enzymes that begin the breakdown of starch in food. The bottom diagram shows the labial glands of the lips. Colour lithograph by Fairland from The Viscera of the Human Body, 1840. Based on drawings by Bagg.