Identify the muscle of the eye indicated by the arrow in the image below.

Sensory receptors are specialized structures that detect specific environmental or internal stimuli and convert them into electrical signals for the central nervous system. They are classified based on the type of stimulus they respond to, such as chemical, thermal, light, or mechanical changes. Mechanoreceptors are particularly important in detecting physical deformation such as stretch, pressure, vibration, and tension. Stretch receptors and baroreceptors fall within this category because they respond to mechanical distortion of tissues and blood vessels.

A. Chemoreceptors: Chemoreceptors detect chemical changes in the internal or external environment, such as oxygen, carbon dioxide, pH levels, and specific dissolved substances. They are found in structures like the carotid bodies, aortic bodies, and taste buds. Their function is essential in regulating respiration and maintaining acid-base balance. Stretch receptors and baroreceptors do not respond to chemical changes.

B. Thermoreceptors: Thermoreceptors are sensory receptors that detect changes in temperature, both heat and cold. They are primarily located in the skin and hypothalamus and help regulate body temperature through autonomic responses such as sweating or shivering. Their function is related to thermal homeostasis rather than pressure or stretch detection.

C. Photoreceptors: Photoreceptors are specialized sensory cells located in the retina that respond to light stimuli. They include rods, which detect low light intensity, and cones, which are responsible for color vision and visual acuity. Their function is essential for vision and image formation. Since they respond to electromagnetic light waves rather than mechanical deformation, they are not related to stretch receptors or baroreceptors.

D. Mechanoreceptors: Mechanoreceptors are sensory receptors that respond to mechanical stimuli such as pressure, stretch, vibration, and tension. Stretch receptors located in muscles and lungs detect changes in length, while baroreceptors in the carotid sinus and aortic arch detect changes in blood vessel wall pressure. These receptors play a key role in regulating blood pressure, proprioception, and visceral reflexes. Because they respond specifically to mechanical deformation, stretch receptors and baroreceptors are correctly classified as mechanoreceptors.

The marked structure is the mandible, which is the largest and strongest facial bone forming the lower jaw. It is the only movable bone of the skull, articulating with the temporal bone at the temporomandibular joint (TMJ). The mandible supports the lower teeth and plays a critical role in mastication, speech, and facial structure. It is shaped like a horseshoe and consists of the body and two rami.

A. Maxilla: The maxilla forms the upper jaw and is a fixed bone of the facial skeleton. It contributes to the hard palate, the floor of the orbit, and the upper dental arch. Unlike the mandible, it is immovable and does not form a joint for chewing motion.

B. Zygomatic bone: The zygomatic bone forms the prominence of the cheek and part of the lateral wall of the orbit. It contributes to facial contour and protection of the eye. Compared to the mandible, it is a non-movable facial bone and does not participate in jaw movement or mastication.

C. Mandible: The mandible is the lower jawbone and the only movable bone of the skull. It articulates with the temporal bone at the TMJ, allowing chewing, speaking, and mouth opening. It supports the lower teeth and provides attachment for muscles of mastication such as the masseter and temporalis. Since the marked structure is the movable lower jaw, it corresponds to the mandible.

D. Temporal bone: The temporal bone forms part of the lateral skull and houses structures of the ear. It contributes to the cranial base and forms the socket for the mandibular articulation (TMJ). However, it is not the jawbone itself but rather the bone that articulates with the mandible.