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  1. Many muscles are used to smile. The levator anguli oris, zygomaticus major, and zygomaticus minor elevate the corners of the mouth. The risorius contracts as well if you make a closed-mouth smile. The depressor anguli oris muscle contracts when you frown, pulling the corners of the mouth inferiorly. If you frown and pout as well, that also requires contraction of the depressor labii inferioris, mentalis, and platysma.

  2. The extrinsic muscles of the tongue originate on head and neck structures and insert on the tongue.
    (1) The left and right genioglossus muscles originate on the mandible and protract when you stick out your tongue. (2) The left and right styloglossus muscles originate on the styloid processes of the temporal bone. These muscles elevate and retract the tongue (pull the tongue back into the mouth). (3) The left and right hyoglossus muscles originate at the hyoid bone and depress and flatten the tongue. (4) The left and right palatoglossus muscles originate on the soft palate and elevate the posterior portion of the tongue.

  3. Contraction of the superior or inferior rectus muscle moves the eye in the direction indicated by the muscle name. However, neither the superior nor the inferior rectus muscle pulls directly parallel to the long axis of the eyeball; thus, both muscles also move the eye slightly in the medial direction. So, the superior rectus muscles move the eyes superiorly and medially, while the inferior rectus muscles move the eyes inferiorly and medially.

  4. When food enters the pharynx, the three pharyngeal constrictors (superior, middle, and inferior) contract sequentially to initiate swallowing and force the bolus inferiorly into the esophagus.

  5. The suprahyoid muscles are superior to the hyoid bone, and the infrahyoid muscles are inferior to the hyoid bone. The suprahyoid muscles are associated with the floor of the mouth. In general, these muscles elevate the hyoid bone during swallowing and speaking. The infrahyoid muscles either depress the hyoid bone or depress the thyroid cartilage of the larynx.

  6. Bilateral contraction of both sternocleidomastoid muscles flexes the head and neck, whereas unilateral contraction of just one sternocleidomastoid muscle results in lateral flexion and rotation of the head to the opposite side.

  7. The muscle fibers of the diaphragm converge from its margins toward a fibrous central tendon. This strong aponeurosis functions as the insertion tendon for all peripheral muscle fibers of the diaphragm. When the diaphragm contracts, the central tendon is pulled inferiorly, toward the abdominal cavity, increasing the vertical dimensions of the thoracic cavity.

  8. The external and internal abdominal oblique muscles can laterally flex the vertebral column (bend the body laterally) if they contract unilaterally or compress the abdominal wall if they are flexed bilaterally.

  9. Each rectus sheath is formed from the aponeuroses of the external oblique, internal oblique, and transversus abdominis muscles. The linea alba serves as a connecting region between the left and right rectus sheaths.

  10. The pelvic diaphragm muscles form the pelvic floor and support the pelvic viscera. These muscles include the coccygeus, the external anal sphincter, and the levator ani (iliococcygeus, pubococcygeus, and puborectalis).

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