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 bellies-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

MANDIBLE.

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.

Nerve-Supply.-Facial nerve.

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).

It is directed downwards and somewhat posteriorly, along the inferior border of the genioglossus, to be inserted into the anterior surface of the body of the hyoid bone. The muscles of opposite sides are often fused together.

The muscle is placed deeper than the anterior belly of the digastric muscle and the mylo-hyoid, and is in contact with the inferior border of the genioglossus muscle.

Nerve-Supply.-It is supplied by the hypoglossal nerve, but its nerve can be traced back to an origin from the communication between that nerve and the first and second cervical

nerves.

Actions. The digastric, stylo-hyoid, mylo-hyoid, and genio-hyoid muscles are all elevators of the hyoid bone. The posterior belly of the digastric and stylo-hyoid also retract, while the anterior belly of the digastric and the genio-hyoid protract it. The anterior belly of the digastric, mylo-hyoid, and genio-hyoid also assist in opening the mouth.

The Muscles of the Tongue.

The muscular substance of the tongue consists of two symmetrical series of muscles placed on either side of a membranous raphe in the median plane. The

series comprise (1) extrinsic muscles arising from the soft wwwmery Army palate, styloid process, hyoid bone and mandible, and (2) intrinsic muscles proper to the tongue itself. Each set consists of four series of muscles.

M. transversus

FIG. 411.-A, TRANSVERSE, AND B, LONGITUDINAL VERTICAL

SECTIONS THROUGH THE TONGUE (Krause).

A. The extrinsic muscles are four in number: (1) genioglossus, (2) hyoglossus, (3) styloglossus, and (4) glossopalatinus.

M. Genioglossus.The genioglossus muscle (O.T. geniohyoglossus) (Fig. 408, p. 459) is an extrinsic muscle of the tongue as well as a supra-hyoid muscle.

It is a fan-shaped muscle arising by its apex from

the superior of the two mental spines, behind the symphysis of the mandible (Fig. 410, p. 461).

From that origin the muscular fibres diverge; the lowest fibres are directed downwards and backwards, to be inserted into the body of the hyoid bone; the highest fibres curve forwards, to be attached to the tip of the tongue; the intermediate fibres are attached to the substance of the tongue in its whole length between the base and tip.

The muscles of opposite sides are separated by the median raphe of the tongue. On the lateral aspect, of each, are the hyoglossus and mylo-hyoid muscles.

M. Hyoglossus.-The hyoglossus muscle is also an extrinsic muscle of the tongue as well as a supra-hyoid muscle.

It arises from the body and great cornu of the hyoid bone.

It is directed upwards and forwards, to be inserted into the side of the tongue, its fibres interlacing with the fibres of the styloglossus.

The muscle is quadrilateral, and lies between the genioglossus and, mylo-hyoid muscles, separated from the latter by the mucous membrane of the floor of the mouth, the sublingual and part of the submaxillary glands, the lingual and hypoglossal nerves, and the submaxillary duct.

The chondroglossus is a small separated slip of the hyoglossus, not always present.

M. Styloglossus.-The styloglossus muscle arises from the anterior border of the styloid process near its tip, and from the stylo-hyoid ligament.

It sweeps forwards and medially, and is inserted into the side and inferior surface of the tongue, its fibres spreading out to decussate with those of the glossopalatinus and hyoglossus muscles beneath the submaxillary gland and the mucous membrane of the tongue.

M. Glossopalatinus.-The glossopalatinus (O.T. palatoglossus) is a thin sheet of muscular fibres arising from the inferior surface of the soft palate, where it is continuous with fibres of the opposite muscle.

It passes downwards, in the glosso-palatine arch, and spreads out, to be inserted into the sides of the tongue, blending with the styloglossus and the deep transverse fibres of the tongue.

The muscle is placed directly beneath the mucous membrane of the soft palate and tongue.

B. Intrinsic Muscles of the Tongue.-Besides receiving the fibres of insertion of the extrinsic muscles, the substance of the tongue is composed of four intrinsic muscles on either side-two in the sagittal plane, the superior and inferior longitudinal muscles; two in the frontal plane, the transverse and vertical muscles.

M. Longitudinalis Superior.-The superior longitudinal muscle extends from base to tip of the tongue. It is placed on its dorsum immediately under the mucous membrane, into which many of its fibres are inserted.

M. Longitudinalis Inferior.-The inferior longitudinal muscle is a cylindrical band of muscular fibres occupying the inferior part of the organ on each side, in the interval between the genioglossus and the hyoglossus muscles. Posteriorly some of its fibres extend to the hyoid bone.

M. Transversus Linguæ.-The transversus linguæ (O.T. transverse fibres) arises from the median raphe, and radiates outwards to the dorsum and sides of the tongue, intermingling with the extrinsic muscles and the fibres of the vertical muscle. It occupies the substance of the tongue between the superior and inferior longitudinal muscles.

M. Verticalis Linguæ.-The verticalis linguæ (O.T. vertical fibres) arises from the dorsal surface of the tongue, and sweeps downwards and laterally to its sides, intermingled with the fibres of the preceding muscle and the insertions of the extrinsic muscles. The transverse and vertical muscles form a very considerable part of the total muscular substance of the organ.

Nerve-Supply. All these muscles except the glossopalatinus are supplied by the hypoglossal nerve. The glossopalatinus is supplied by the accessory nerve through the pharyngeal plexus.

Actions.-The genioglossus and the hyoglossus are both elevators of the hyoid bone besides having actions in relation to the tongue. The tongue is protruded by the action of the posterior fibres of the genioglossus, retracted by the anterior fibres aided by the styloglossus. The styloglossus and glossopalatinus are elevators, while the genioglossus and hyoglossus are depressors of the tongue.

Actions of the Infra-hyoid and Supra-hyoid Muscles, and the Muscles of the Tongue. These muscles have a complexity of action, owing to their numerous attachments to more or less movable points. The movements for which they are responsible in whole or part are: (1) movements of the hyoid bone in mastication and deglutition, (2) movements of the thyreoid cartilage, (3) movements of the tongue, (4) movements of the head, (5) movements of the shoulder, and (6) respiration.

(1) Movements of the Hyoid Bone. The hyoid bone is elevated or depressed, and moved forwards or backwards along with the mandible and tongue, in speech, mastication, and swallowing.

(2) Movements of the Thyreoid Cartilage. The thyreoid cartilage is raised and lowered during speech and deglutition.

(3) Movements of the Tongue. The chief movements of the tongue in speech and deglutition are elevation and depression, protrusion and retraction, and lateral movements.

(4) Movements of the Head.-The sterno-mastoid muscles, acting together, flex the head on the vertebral column, assisted by the supra-hyoid and infra-hyoid muscles. The sterno-mastoid muscle of one side, acting alone, bends the head to the same side, and simultaneously rotates it to the opposite side, as seen in torticollis (wryneck).

(5) Movements of the Shoulder Girdle. The omo-hyoid and sterno-mastoid muscles have already been included among the elevators of the shoulder girdle.

(6) Respiration. The muscles in the front of the neck are auxiliary muscles in extraordinary or difficult inspiration. The masseter and temporal muscles fix the mandible; the hyoid bone is raised and fixed by the supra-hyoid muscles; and the sternum is raised by the sterno-mastoid and infra-hyoid muscles.

The Muscles of the Pharynx.

The muscular envelope of the pharynx is composed of two strata. The external or circular layer consists of the three fan-shaped constrictor muscles; the internal or longitudinal layer consists of the fibres of the stylopharyngeus and pharyngopalatinus muscles.

M. Constrictor Pharyngis Superior. The superior constrictor muscle arises successively from the inferior half of the posterior border of the medial lamina of the pterygoid process (pterygopharyngeus), from the pterygomandibular raphe (buccopharyngeus), from the mylo-hyoid line of the mandible (mylopharyngeus) (Fig. 410, p. 461), and from the mucous membrane of the floor of the mouth (glossopharyngeus).

The muscular fibres radiate backwards, and are inserted, for the most part, into a raphe extending down the posterior wall of the pharynx in the median plane. The highest fibres are attached to the pharyngeal tubercle of the occipital bone (Fig. 396, p. 444), and the lowest fibres are overlapped by the middle constrictor. A crescentic interval occurs above the muscle, below the base of the skull, in which the auditory tube and the levator and tensor veli palatini muscles appear. Its lower border is separated from the middle constrictor by the stylopharyngeus

muscle.

M. Constrictor Pharyngis Medius. The middle constrictor muscle arises from the stylo-hyoid ligament and from both cornua of the hyoid bone (chondropharyngeus, ceratopharyngeus).

From its origin the muscular fibres radiate backwards, to be inserted into the median raphe on the posterior aspect of the pharynx.

The superior fibres overlap the inferior part of the superior constrictor; the inferior fibres are concealed from view by the inferior constrictor muscle. In the interval between the middle and inferior constrictors are found the superior laryngeal artery and internal laryngeal nerve.

M. Constrictor Pharyngis Inferior.-The inferior constrictor muscle arises from the oblique line of the thyreoid cartilage (thyreopharyngeus), and from the side of the cricoid cartilage (cricopharyngeus).

Its fibres radiate backwards, to be inserted into the median raphe on the posterior wall of the pharynx, the superior fibres overlapping the inferior part of the middle constrictor, the inferior fibres blending with the muscular fibres of the œsophagus. Below the inferior border of the muscle the inferior laryngeal artery and nerve enter into relation with the larynx.

Nerve-Supply. The constrictors of the pharynx receive their nerve-supply through the pharyngeal plexus from the accessory nerve. The inferior constrictor is supplied also by the external laryngeal and recurrent branches of the vagus nerve.

The deeper longitudinal stratum of muscles in the pharyngeal wall is composed of the insertions of the stylopharyngeus and pharyngopalatinus muscles.

M. Stylopharyngeus. The stylopharyngeus arises from the root of Pterygo-mandithe styloid process on its medial side, and passes downwards between the external and internal carotid arteries. It enters the wall of the pharynx in the interval between the superior and middle constrictor muscles.

Spreading out beneath the middle constrictor muscle, it is inserted into the superior and posterior borders of the thyreoid cartilage and into the wall of the pharynx itself, becoming continuous posteriorly with the palatopharyngeus. In the neck the glossopharyngeal nerve winds round it on its way to the tongue.

Nerve-Supply.-Glossopharyngeal

nerve.

FIG. 412.-POSTERIOR VIEW OF THE PHARYNX AND
CONSTRICTOR MUSCLES.

M. Pharyngopalatinus. The pharyngopalatinus (O.T. palatopharyngeus) occupies the soft palate and the pharyngeal wall. In the substance of the soft palate it consists of two layers, a postero-superior layer, thin, and continuous across the median plane with the corresponding layer on the opposite side, and an antero-inferior layer, which is thicker, and is attached to the posterior border of the hard palate. The levator veli palatini and the musculus uvulæ are enclosed between the two layers, which unite at the posterior edge of the palate, receiving at the same time additional fibres arising from the auditory tube (salpingopharyngeus). The muscle descends to the pharynx in the

pharyngo-palatine arch.

Its fibres spread out in the form of a thin sheet in the wall of the pharynx, in continuity anteriorly with the stylopharyngeus, and are inserted into the posterior border of the thyreoid cartilage, and behind that into the aponeurosis of the pharynx, reaching down as far as the inferior border of the inferior constrictor. The muscle is placed beneath the middle and inferior constrictors, and the fibres