Final answer:
The mylohyoid muscle is situated below the submandibular space; it forms the oral cavity's floor and is involved in swallowing and speaking. Above this muscle are the submandibular glands, which secrete saliva.
Step-by-step explanation:
The muscle located below the submandibular space is the mylohyoid muscle. This muscle forms the floor of the oral cavity and attaches to the mylohyoid lines on both sides of the mandible. It serves to elevate the hyoid bone and the floor of the mouth during activities like swallowing and speaking. The submandibular glands are located just above this muscle and are responsible for secreting saliva into the oral cavity, playing an essential role in the process of digestion. The sublingual glands are found even deeper, below the tongue, and also contribute to saliva production through the lesser sublingual ducts.