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The epicranium includes all the tissues that are found to the exterior of the cranium, the section of the skull that encloses the brain. These include the muscles covering this portion of the skull, the frontalis and occipitalis, collectively referred to as the epicranius muscle. In addition, the epicranium includes the galea aponeurosis, connective tissue blanketing most of the top of the skull, and the skin. Collectively, these tissues are often referred to as the scalp.
Corresponding with the bones of the cranium they cover — the frontal and occipital bones — the muscles of the epicranium include the frontalis and occipitalis. Shaped like curved plates with jagged edges that fit together to form the rounded cranium, the bones these muscles cover are found in the forehead region, in the case of the frontalis, and the lower posterior skull, as with the occipitalis. The frontalis muscle covers most of the forehead, with fibers stretching vertically from the skin behind the eyebrows to the forward border of the galea aponeurosis, and is the muscle that raises the eyebrows. On the lower posterior skull, the occipitalis muscle arises from a curved ridge known as the superior nuchal line, and its fibers ascend a short distance before attaching to the rear border of the galea aponeurosis. This muscle assists the frontalis in raising the eyebrows.
Though these muscles of the epicranium are located on the opposite sides of the skull, they are able to share a function because they are joined by the galea aponeurosis. Also known as the epicranial aponeurosis or galea aponeurotica, this layer of tissue, made of dense connective fibers like collagen and elastin, originates on a ridge above the superior nuchal line called the highest nuchal line. Stretching across the top of the skull and extending toward the temporalis muscle behind the ears on either side of the head, it attaches to the top of the frontalis muscle at the top of the forehead. When the epicranial muscles contract, they shorten, drawing the skin above the eyebrows upward and backward toward the aponeurosis.
Beneath the galea aponeurosis in the epicranium is another thin layer of vascular connective tissue called areolar tissue, and below that is the pericranium, the membrane covering the cranial bones that is partly made of fibrous tissue and partly of nourishing progenitor cells. Above the aponeurosis is a layer of subcutaneous or stored fatty tissue, and directly above that is where the skin begins. The skin here is dense with hair follicles as well as sebaceous, or oil-producing, glands.
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