Final answer:
The masseter muscle is injected with arterial fluid during embalming to keep the mouth closed. It is a powerful muscle responsible for elevating the mandible and closing the mouth.
Step-by-step explanation:
The muscle injected with arterial fluid to facilitate the closure of the mouth during embalming is the masseter muscle. This muscle's primary function is to elevate the mandible (lower jaw) and close the mouth, which is essential in the mastication process. When preparing a body for a funeral, embalmers will often manipulate the masseter to ensure the mouth remains closed and gives a natural appearance.
During embalming, arterial fluid is injected into the masseter to stiffen it and maintain the position of the jaw postmortem. The masseter is one of the most powerful muscles of mastication, and this is why it is targeted to maintain the mouth in a closed state. In conjunction with the temporalis muscle, which retracts the mandible, the masseter provides the necessary force to perform the closure of the mouth.