Final answer:
The lingual surface of the tooth, also known as the palatal surface for upper teeth, is the side of the tooth adjacent to the tongue and is part of the oral cavity's complex structure and functions.
Step-by-step explanation:
At times, the lingual surface of the tooth may also be referred to as the palatal surface when it concerns the upper teeth, or simply the tongue side of the teeth. This surface is adjacent to the tongue and is covered by various structures and tissues that play different roles in the oral cavity, from mechanical functions to sensory and secretory activities. For instance, the tongue's upper surface features different types of lingual papillae, such as fungiform and filiform, important for taste and manipulating food, while the lingual glands secrete essential fluids like mucus and a watery serous fluid containing lingual lipase.