Which of the following movements does NOT increase or decrease the angle between bones?

Choice A reason: The buccinator muscle compresses the cheek, aiding in actions like blowing and chewing. While it contributes to facial expression, it is not the primary muscle responsible for smiling.

Choice B reason: The zygomaticus muscle is the correct answer. It originates from the zygomatic bone and inserts into the corner of the mouth. Its contraction pulls the mouth upward and outward, producing a smile. It is the principal muscle involved in smiling.

Choice C reason: The mentalis muscle elevates and wrinkles the skin of the chin and protrudes the lower lip. It is involved in expressions of doubt or displeasure, not smiling.

Choice D reason: The orbicularis oris encircles the mouth and controls movements of the lips, such as puckering or closing. It is essential for speech and kissing but not for smiling.

Choice E reason: The masseter is a major muscle of mastication (chewing). It elevates the mandible to close the jaw and has no role in facial expressions like smiling.

Choice A reason: While coordination is important in highly mobile joints, this is not the primary reason for the vulnerability of the glenohumeral joint.

Choice B reason: Ligaments around the shoulder joint are indeed more flexible to allow movement, but this alone does not fully explain the joint’s instability.

Choice C reason: Power generation is not significantly reduced due to mobility. Muscles around the shoulder can generate substantial force.

Choice D reason: Blood vessels are generally well protected and not easily compromised by joint movement. This is not a common consequence of shoulder mobility.

Choice E reason: This is the correct answer. The glenohumeral joint is a ball-and-socket joint with a shallow socket, allowing extensive range of motion. However, this anatomical configuration makes it relatively unstable and prone to dislocation, especially during forceful or awkward movements.