Page:EB1911 - Volume 19.djvu/68

 The buccinator muscle in the substance of the cheeks rises from the upper and lower jaws and runs forward to blend with the orbicularis oris. All the foregoing are known as muscles of expression and all are supplied by the seventh or facial nerve. The temporal muscle at the side of the cranium (fig. 3) and the masseter (fig. 2), which rises from the zygoma, close the mouth, since both are inserted into the ramus of the mandible; while, rising from the pterygoid plates, are the external and internal pterygoid muscles (fig. 3), the former of which pulls forward the condyle, and so the whole mandible, while the latter helps to close the mouth by acting on the angle of the lower jaw. This group of muscles forms the masticatory set, all of which are supplied by the third division of the fifth nerve. For the muscles of the orbit, see ; for those of the soft palate and pharynx, see ; and for those of the tongue, see. (fig. 4).—Just below the mandible is the digastric, which, as its name shows, has two bellies and a central tendon; the anterior belly, supplied by the fifth nerve, is attached to the mandible near the symphysis, the posterior supplied by the seventh of the mastoid process, while the central tendon is bound to the hyoid bone. Stretching across from one side of the lower jaw to the other and forming a floor to the mouth is the mylo-hyoid muscle; posteriorly this reaches the hyoid bone, and in the mid-line has a tendinous raphe separating the two halves of the muscle. Rising from the manubrium sterni and inner part of the clavicle is the sterno-cleido-mastoid, which is inserted into the mastoid process and superior curved lines of the occipital bone; when it contracts it makes the face look over the opposite shoulder, and it is supplied by the spinal accessory nerve as well as by branches from the cervical plexus. It is an important surgical landmark, and forms a diagonal across the quadrilateral outline of the side of the neck, dividing it into an anterior triangle with its apex downward and a posterior with its apex upward. In the anterior triangle the relative positions of the hyoid bone, thyroid cartilage and sternum should realized, and then the hyo-glossus, thyro-hyoid, sterno-hyoid and sterno-thyroid muscles are explained by their names. The omo-hyoid muscle rises from the upper border of the scapula and runs across both triangles to the hyoid bone. Where it passes deep to the sterno-mastoid it has a central tendon which is bound to the first rib by a loop of cervical fascia. Rising from the styloid process are three muscles, the stylo-glossus, stylo-hyoid and stylo-pharyrlgeus, the names of which indicate their attachments. Covering these muscles of the anterior triangle is a thin sheet, close to the skin, called the platysma, the upper fibres of which run back from the mouth over the cheek and are named the risorius (fig. 2); this sheet is one of the few remnants in man of the skin musculature or panniculus carnosus of lower Mammals. With regard to the nerve supply of the anterior triangle muscles, all those which go to the tongue are supplied by the hypoglossal or twelfth cranial nerve, while the muscles below the hyoid bone are apparently supplied from this nerve but really from the upper cervical nerves (see ; and ). The posterior triangle is formed by the sterno-mastoid in front, the trapezius behind, and the clavicle below; in its floor from above downward part of the following muscles are seen: complexus, splenius, levator anguli scapulae, scalenus medius and scalenus anticus. Sometimes a small piece of the scalenus posticus is caught sight of behind the scalenus medius. The splenius rotates the head to its own side, the levator anguli scapulae raises the upper angle of the scapula, while the three scalenes run from the transverse processes of the cervical vertebrae 'and fix or raise the upper ribs. The trapezius (fig. 5) arises from the spines of the thoracic vertebrae and the ligamentum nuchae, and is inserted into the outer third of the clavicle and the spine of the scapula; it is used in shrugging the shoulders and in drawing the upper part of the scapula toward the mid-dorsal line. Its nerve supply is the spinal accessory and third and fourth cervical nerves. When the superficial muscles and com plexus are removed from the back of the neck, the sub-occipital triangle is seen beneath the occipital bone. Externally it is bounded by the superior oblique, running from the transverse process of the atlas to the lateral part of the occipital bone, internally by the rectus capitis posticus major, passing from the spine of the axis to the lateral part of the occipital bone, and inferiorly by the inferior oblique joining the spine of the axis to the transverse