Which muscle is responsible for opening the mouth?

The digastric muscle is specifically responsible for the opening of the mouth. It consists of two muscle bellies (anterior and posterior) that work together to depress the mandible, which is essential for mouth opening. When the digastric contracts, it pulls down on the jaw, allowing for actions like speaking, chewing, and breathing to occur more freely.

The other muscles listed do not perform this function. The platysma primarily plays a role in facial expressions and helps to tense the skin of the neck, while the masseter is primarily known for its role in elevating the mandible during chewing. The zygomaticus muscles are involved in smiling and pulling the corners of the mouth upward, rather than opening it. Thus, the role of the digastric muscle in opening the mouth is distinct and crucial for various oral functions.