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fovea pterygoidea on the anterior. aspect of the neck of the mandible (Figs. 403 and 404, p. 455), and (2) the articular disc and capsule of the mandibular articulation.
This muscle is covered by the insertion of the temporal muscle and the coronoid process of the mandible, and is usually crossed by the internal maxillary artery. It conceals the mandibular branch of the trigeminal nerve, and the pterygoid origin of the internal pterygoid muscle.
M. Pterygoideus Internus.-The internal pterygoid muscle, placed beneath the external pterygoid muscle and the ramus of the mandible, has likewise a double origin-(1) from the medial surface of the lateral pterygoid lamina and the pyramidal process of the palate bone, and (2) by a stout tendon from the tuberosity
of the maxilla. Its two heads of origin embrace the inferior fibres of the external pterygoid muscle.
It is quadrilateral in form, and is directed downwards, laterally, and backwards lateral to the auditory tube and the tensor and levator muscles of the palate, to be inserted into a triangular impression on the medial surface of the mandible, between the mylo-hyoid groove and the angle of the bone (Fig. 404, p. 455).
This muscle is covered by the ramus of the mandible and temporal muscle, and partially by the external pterygoid muscle. In contact with its superficial surface are the spheno-mandibular ligament, and the inferior alveolar and lingual nerves and their accompanying vessels. The muscle conceals the tensor veli palatini and the wall of the pharynx (superior constrictor).
Nerve-Supply. The mandibular division of the trigeminal nerve supplies all the muscles of mastication except the buccinator, which is supplied by the facial nerve. The internal pterygoid muscle is supplied by the nerve before its division into anterior and posterior parts; the other muscles are innervated by the anterior trunk.
Actions. The above muscles, assisted by others in the neck, produce the various movements of the mandible as follows:
THE MUSCLES OF THE NECK.
In addition to those included among the muscles of the back (p. 438), the following series of muscles occur in the neck: (1) sterno-cleido-mastoid; (2) the muscles of the hyoid bone (supra-hyoid and infra-hyoid); (3) the muscles of the tongue (extrinsic and intrinsic); (4) the muscles of the pharynx and soft palate; and (5) the prævertebral muscles.
M. Sternocleidomastoideus.-The sterno-mastoid muscle is the prominent muscle projecting on the side of the neck, and separating the anterior from the posterior triangle. It arises by two heads (1) a narrow, tendinous, sternal head, from the anterior surface of the manubrium sterni (Fig. 330, p. 370), and (2) a broader clavicular origin, partly tendinous, partly fleshy, from the superior surface of the clavicle in its medial third (Fig. 327, p. 366). The muscle is inserted into the lateral surface of the mastoid portion of the temporal bone and into the superior nuchal line of the occipital bone (Fig. 396, p. 444).
The muscle passes obliquely over the side of the neck, separating the anterior from the posterior triangle. It is almost superficial in its whole extent, but is overlapped superiorly by the parotid gland and is covered in its inferior part by the platysma. It is crossed by the external jugular vein, and by superficial branches of the cervical plexus. Its deep surface is in contact with: (a) in its lower third, the infra-hyoid muscles, which separate it from the common carotid artery, and the subclavian artery and the internal jugular vein; (b) in its middle third, with the cervical nerves which emerge between the transverse processes of the cervical vertebræ to form the cervical plexus; and (c) in its superior third, with the splenius capitis muscle, and the accessory nerve, which there pierces the deep surface of the muscle. Near its insertion the muscle is related to the splenius capitis, longissimus capitis, the posterior belly of the digastric, and the occipital artery.
The sterno-cleido-mastoid muscle is properly divisible into three parts: (1) sterno-mastoid, placed superficially, and passing obliquely from the sternum to the mastoid process; (2) cleidomastoid, placed more deeply, and directed vertically upwards from the clavicle to the mastoid process; and (3) cleido-occipitalis, passing obliquely upwards and backwards behind the cleidomastoid to the superior nuchal line of the occipital bone.
Nerve-Supply. The sterno-mastoid muscle is innervated by the accessory nerve, joined by a branch from the cervical plexus (C. 2.).
Actions. When one muscle acts alone, it flexes the head laterally, and rotates it to the opposite side. The two muscles acting together (1) flex the head in a forward direction, and (2) act as extraordinary muscles of inspiration, by raising the sternum and clavicles.
The Muscles of the Hyoid Bone.
The muscles attached to the hyoid bone are in three series: (1) infra-hyoid muscles, connecting the hyoid bone to the scapula, the wall of the thorax, and
thyreoid cartilage; (2) supra-hyoid muscles, connecting it to the mandible, cranium, and tongue; and (3) the middle constrictor muscle of the pharynx (p. 464).
The infra-hyoid muscles comprise the omo-hyoid, sterno-hyoid, sterno-thyreoid, and thyreo-hyoid muscles.
M. Omohyoideus.-The omo-hyoid is a muscle with two bellies, anterior and posterior. The posterior belly arises from the superior margin of the scapula and the superior transverse scapular ligament (Fig. 333, p. 372). It forms a narrow muscular band, which passes obliquely forwards and upwards, and ends in an intermediate tendon beneath the sterno-mastoid muscle.
From this tendon the anterior belly proceeds upwards, to be inserted into the lateral part of the inferior border of the body of the hyoid bone.
The posterior belly of the muscle separates the posterior triangle into occipital and subclavian parts; the anterior belly crosses the common carotid artery at the
FIG. 408.-THE MUSCLES OF THE TONGUE AND HYOID BONE (right side).
level of the cricoid cartilage, and in the anterior triangle forms the boundary between the muscular and carotid triangles. A process of the deep cervical fascia binds down the tendon and the posterior belly to the clavicle and the first rib.
M. Sternohyoideus.-The sterno-hyoid muscle arises from the posterior surface of the manubrium, from the back of the first costal cartilage, and from the clavicle (Fig. 327, p. 366).
It passes vertically upwards in the neck, medial to the omo-hyoid and anterior to the sterno-thyreoid muscle, to be inserted into the medial part of the body of the hyoid bone. Except near its origin, which is covered by the sternum, clavicle, and sternal head of the sterno-mastoid, the muscle is superficially placed.
M. Sternothyreoideus.-The sterno-thyreoid muscle arises beneath the sterno-hyoid from the back of the manubrium and first costal cartilage.
Broader than the preceding muscle, it passes upwards, and slightly in a lateral direction in the neck, in front of the trachea and thyreoid gland, and deep to the sterno-mastoid, omo-hyoid, and sterno-hyoid muscles. It is inserted into the oblique
line of the thyreoid cartilage. The muscle is marked by an oblique tendinous intersection in the middle of its length.
M. Thyreohyoideus.-The thyreo-hyoid muscle continues the line of the preceding muscle to the hyoid bone. Short and quadrilateral, it arises from the oblique line of the thyreoid cartilage.
Passing over the thyreo-hyoid membrane, deep to the omo-hyoid and sternohyoid, it is inserted into the body and great cornu of the hyoid bone.
The levator glandulæ thyreoidea is an occasional slip stretching between the hyoid bone and the isthmus or pyramid of the thyreoid gland.
FIG. 409.-THE MUSCLES OF THE SIDE OF THE NECK (anterior and posterior triangles).
Nerve-Supply.-The sterno-hyoid, sterno-thyreoid, and omo-hyoid are supplied by the ansa hypoglossi; the thyreo-hyoid, by a special branch from the hypoglossal nerve. Through the ansa hypoglossi the muscles are innervated by nerves which are ultimately derived from the first three cervical nerves. The descendens hypoglossi is derived from the first two cervical nerves, the descendens cervicis by the second and third; and these two trunks combine to form the ansa. The thyreo-hyoid muscle is innervated (through the hypoglossal) from the loop between the first and second cervical nerves.
Actions. The sterno-hyoid, sterno-thyreoid, and omo-hyoid are depressors of the hyoid bone. The two former muscles are also accessory muscles of inspiration. The omo-hyoid is a feeble elevator of the scapula. The thyreo-hyoid is, on the one hand, an elevator of the thyreoid cartilage, and acting with the previous muscles, on the other hand, it is a depressor of the hyoid bone.
The supra-hyoid muscles comprise the digastric, stylo-hyoid, mylo-hyoid, and
genio-hyoid muscles; and also two muscles, the genioglossus and hyoglossus, which will be described along with the extrinsic muscles of the tongue.
M. Digastricus.-The digastric muscle, as its name implies, possesses two anterior and posterior.
The posterior belly arises, under cover of the sterno-mastoid muscle, from the mastoid notch medial to the mastoid process. It is directed forwards and downwards, in company with the stylo-hyoid muscle, to end in an intermediate tendon, which is connected by a pulley-like band of cervical fascia to the body of the hyoid bone.
The anterior belly of the muscle is directed forwards and upwards, over the mylo-hyoid muscle, to the chin, and is inserted into the oval digastric fossa on the inferior border of the mandible close to the symphysis (Fig. 410).
The muscle forms the inferior boundary of the submaxillary division of the anterior triangle, containing the submaxillary gland. The posterior belly in company with the stylo-hyoid crosses the carotid arteries and internal jugular vein. The occipital artery extends posteriorly along its inferior margin, and the parotid gland covers its superior border. The hypoglossal nerve emerges from under cover of the muscle. The anterior belly, as it passes to its insertion, lies upon the mylohyoid muscle.
Nerve Supply. - The posterior belly is supplied by the facial nerve; the anterior belly by the nerve to the mylo-hyoid, a branch of the inferior alveolar
M. Stylohyoideus. -The stylo-hyoid muscle arises from the posterior border of the styloid process of the temporal bone.
Crossing the anterior triangle obliquely, along with the posterior belly of the digastric muscle, it is inserted into the body of the hyoid bone, by two slips which enclose the tendon of the digastric muscle.
FIG. 410.-MUSCLE-ATTACHMENTS ON THE MEDIAL SIDE OF THE
M. Mylohyoideus.-The mylo-hyoid muscle forms with its fellow a diaphragm in the floor of the mouth. It arises from the inferior three-fourths of the mylo-hyoid ridge of the mandible (Fig. 410).
It is directed downwards and medially, to be inserted into (1) the superior border of the body of the hyoid bone, and more anteriorly (along with the opposite muscle) into (2) a median raphe extending from the hyoid bone nearly to the chin.
The muscle is in contact, on its superficial or lateral surface, with the digastric muscle and the submaxillary gland. Its deep or medial surface is partially covered by the mucous membrane of the floor of the mouth, and is separated from the muscles of the tongue by the deep part of the submaxillary gland, the sublingual gland, the submaxillary duct, and the lingual and hypoglossal nerves.
Nerve-Supply. The muscle is supplied by the nerve to the mylo-hyoid, a branch of the inferior alveolar nerve.
M. Geniohyoideus.-The genio-hyoid muscle arises from the inferior of the two mental spines on the posterior surface of the symphysis of the mandible (Fig. 410).