The muscles of the neck which lie beneath it are surrounded by a layer of deep fascia, called the cervical fascia. The platysma myoides has been already described with the muscles of expression. This is a strong tubular membrane, attached above to the lower border of the mandible, the parotid fascia, the upper part of the mastoid process, and the superior nuchal line. At the back of the neck it is continuous with the deep fascia which gives a thin investment to the trapezius muscle. In front, it is attached to the lower border of the body and great cornu of the hyoid bone. Below, it ends upon the front surface of the presternum and the clavicle. The deep layer of the cervical fascia separates from it just below the hyoid bone, and runs downwards in close proximity to the sterno-hyoid muscles and the other depressors of the hyoid bone, to the upper part of the posterior surface of the sternum, and the posterior surface of the clavicle. Laterally, it blends with and completes the sheath of the sterno-mastoid muscle, which is partly formed by the superficial layer of the cervical fascia ; it also binds down the posterior belly of the omo-hyoid to the clavicle and first rib. Below its attachment to the sternum and clavicle it is continued downwards in front of the trachea and great vessels of the root of the neck into the superior mediastinum, and it finally joins the pericardium. Behind, it gives an investment to the depressors of the hyoid bone, between this deep layer of the cervical fascia and the superficial layer is a small pace containing a part of the course of the anterior jugular vein, a lymphatic gland, some fat and loose connective tissue.
A still deeper fibrous layer, the prevertebral fascia, stretches across the neck and divides the cylindrical tube formed by the cervical fascia into a posterior and interior compartment. Its surfaces are directed forwards and backwards. Behind the rests in the middle line upon the ligaments covering the front of the bodies of le cervical vertebrae. Laterally, it covers in the prevertebral muscles, and is attached to the deep surface of the superficial layer of the cervical fascia between the sterno-mastoid and trapezius muscles.
In the compartment formed between the deep layer of the cervical fascia as it invests the depressors of the hyoid bone, and the prevertebral fascia, processes are given off which form the sheath of the great vessels of the neck, and invest the tyroid gland, the trachea, and pharynx.
The muscles of the front of the neck may be divided into three groups : the fist group consisting of one muscle which ascends from the sternum and clavicle to the head, the sterno-cleido-mastoideus ; the second, of those which ascend from the sternum, clavicle, and shoulder blade to the hyoid bone and thyroid cartilage ; the third, of those which are attached to the hyoid bone below, and the skull and lower jaw above.
The sterno-cleido-mastoid (French: muscle sterno-cléido-mastoïdien), or sterno-mastoid muscle, named from its attachments, is a strong ribbon-shaped and, bifurcated below, and somewhat constricted in its middle third.
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