Identifying Muscles of the Trunk, Core, and Head

The occipitofrontalis covers the skull from the occipital bone to the frontal bone. It is a muscle of facial expression.

The splenius capitus muscle will be stretched most effectively by rotation of the head and neck. The splenius capitus is a broad, strap like muscle found at the posterior neck. It attaches at the base of the skull and inserts at the vertebrae of the cervical and upper thoracic spine. It is involved in head extension, in addition to lateral flexion and rotation of the cervical spine.

Because it attaches superior to the hyoid bone, the geniohyoid does not depress the hyoid rather, it elevates it. The omohyoid (both inferior and superior bellies), the thyrohyoid, sternothyroid, and the sternohyoid muscles are collectively called the infrahyoid muscles, due to their attachments being inferior to the hyoid bone. Because of this attachment site, they depress the hyoid bone.

The risorius muscle is primarily responsible for drawing the angle of the mouth laterally as in smiling. Major functions of the orbicularis oris and orbicularis oculi muscles include the shaping of the lips during speech and closing the eyes, respectively. The masseter muscle is the main muscle for mastication.

There are many structures that aid in the process of inspiration and expiration. Inspiration involves enlarging the thoracic cavity for the lungs to expand, while expiration compresses the cavity to allow carbon dioxide to leave. Inspiration will always be active, while expiration can be either passive or active. One of the most important structures for inspiration is the diaphragm. The diaphragm will contract and move downwards into the abdominal cavity, allowing the lungs to expand and draw in air. Muscles of forced expiration are the anterior abdominals, the internal intercostals, and the serratus posterior inferior muscles. They help to relax the diaphragm, which increases the abdominal pressure and pushes the ribs inwards.

The procerus is the muscle that wrinkles the nose. It originates from the fascia covering the inferior aspect of the nasal bone and inserts into the skin on the forehead, in between the eyebrows. The frontalis raises the eyebrows and wrinkles the forehead. The buccinator pulls in the cheeks against the teeth. The risorius pulls the mouth into a grimace.

A spike in the concentration of luteinizing hormone (LH) leads to ovulation on day 14 of the menstrual cycle. This spike is known as the "LH surge" and is initiated by a positive feedback mechanism involving estrogen.

Follicle-stimulating hormone (FSH) is involved in the maturation of the follicle, but not ovulation. Progesterone functions in maintaining the endometrial tissue after implantation has occurred. Testosterone is not involved in the female reproductive cycle.

Ipsilateral rotation of the trunk at the spinal joints is an action of the internal abdominal obliques, not the rectus abdominis.

The platysma is a broad, flat muscle that lowers the jaw and stretches the lips down or to the sides. When the entire sheet is contracted it can increase the diameter of the neck. This is the muscle men flex when trying to shave under the chin and along the superior and anterior portion of the neck.

The masseter is in the jaw and is involved in the chewing motion. The mentalis muscle furrows the chin, while the risorius and orbicularis oris muscles pull the corners of the mouth out and in respectively.

The muscle that surrounds the opening of the mouth is known as the Obicularis Oris. The Obicularis Oclui surrounds the eye. The Masseter is connected to the mandible and responsible for chewing.The Glossus muscles are found inside the mouth and responsible for tongue movement. The Buccinator is found deep to the Masseter located on the cheek.