Which muscle acts to open the jaw?

The lateral pterygoid muscle is the primary muscle responsible for opening the jaw. This muscle plays a crucial role in the movements of the mandible, particularly during the action of depression (lowering) of the jaw. When the lateral pterygoid contracts, it pulls the head of the mandible forward and downward, which facilitates the opening of the jaw.

The medial pterygoid muscle, while also involved in the movement of the mandible, primarily assists in closing the jaw and elevating the mandible. The temporalis muscle is mainly responsible for elevating the mandible as well, contributing to jaw closure rather than opening. The omohyoid muscle is involved in the movement of the hyoid bone and does not directly affect the opening of the jaw.

Thus, the lateral pterygoid's specific action in depressing and protruding the mandible makes it the correct answer regarding the muscle that opens the jaw.