Final answer:
The surface towards the midline of the mouth on the left side is typically known as the gingiva, also called the gums, but the buccal mucosa is another noteworthy surface, as it lines the cheeks of the oral vestibule.
Step-by-step explanation:
The surface towards the midline of the mouth on the left side would typically be referred to as the gingiva, which is the gum tissue surrounding the teeth. The cheek area, which is part of the oral vestibule bounded externally by the cheeks and internally by the gums and teeth, is lined with a type of mucous membrane known as the buccal mucosa. This membrane is made up of non-keratinized, stratified squamous epithelium, relevant to the overall health and function of the oral cavity.