The bones which form the appendicular elements of the skullis group are the mandible (lower jaw), malleus, incus, stapes, hyoid, the styloid process of the temporal bone, and the internal pterygoid process of the sphenoid.
The mandible or lower jaw
The mandible (lower jaw or inferior maxilla) is in shape like a horseshoe; it consists of a horizontal portion or body, and two vertical portions or rami.
The body consists of a right and a left half, meeting in the middle line to form the symphysis. Each half presents two surfaces and two borders. The external surface is smooth and generally convex, and presents the following points of interest: - The symphysis ends inferiorly in a triangular surface which forms the chin. Near the symphysis is the incisive fossa, from which the levator menti arises; external to this is the mental foramen through which the mental nerve and artery issue. This foramen is in a line with the second bicuspid tooth. Extending backwards and upwards from the mental protuberance, so as to become continuous with the anterior border of the coronoid process, is the external oblique line; along its upper border the depressor labii inferioris and depressor anguli oris arise; the platysma is attached to its lower edge. The internal surface presents, at a point corresponding to the symphysis, two pairs of genial tubercles. The upper pair give origin to the genio-hyo-glossi, and the lower pair afford insertion to the genio-hyoid muscles. The tubercles occasionally form a single, median, irregularly shaped eminence. By the side of the genial tubercles there is a shallow smooth depression, the sublingual fossa; below this is the digastric fossa for the insertion of the anterior belly of the digastric muscle. Posterior to the genial tubercles, the internal oblique line (mylo-hyoid ridge) commences and extends backwards, becoming more and more prominent as it approaches the alveolar border. The mylo-hyoid muscle is inserted along the whole length of this ridge. At the posterior part the superior constrictor takes origin, and the pterygo- maxillary ligament is attached to its posterior extremity. Below the internal oblique line is the submaxillary fossa, which is in relation with the submaxillary gland.
The inferior border of the body of the mandible is smooth and rounded; near its junction with the ramus there is a groove for the facial artery. The superior border is composed of spongy bone, and is named the alveolus; it presents sockets for eight teeth. From the outer edge of the alveolus, as far forward as the first molar tooth, the buccinator muscle takes origin.
The ramus is quadrilateral in shape. It has two surfaces, four borders, and two processes. The external surface is for the insertion of the masseter muscle. The internal surface presents near its middle the mandibular (inferior dental) foramen which leads into the mandibular (inferior dental) canal which, traverses the body of the bone and emerges at the mental foramen. This canal presents a series of fine apertures above, through which filaments of the mandibular nerve and artery pass to the teeth. In its posterior two-thirds the canal is nearer the internal, in its outer third it is nearer the external surface of the mandible. The posterior orifice of the canal is surmounted by the mandibular spine to which the spheno-mandibular ligament is attached. Running obliquely downwards behind this spine is the mylo-hyoid groove, which lodges the mylo-hyoid nerve and artery. In the embryo, Meckel's cartilage also occupies the groove. The triangular rough space behind this groove is for the insertion of the internal pterygoid muscle.
The mandible. (outer view.)
The inferior border of the ramus is thick, rounded, and continuous with the lower border of the body of the bone. The posterior border is rounded; to its lower part the stylo-maxillary ligament is attached. This border is surmounted by the condyle, which is connected with the ramus by a somewhat constricted portion, the neck.
The condyle is oval in shape, with its long axis transverse to the upper border of the ramus, but oblique with regard to the median axis of the skull, so that the outer is more anterior than the inner angle, and presents the condyloid tubercle for the external lateral ligament of the temporo-mandibular articulation. The convex surface of the condyle is covered with cartilage and rests in the glenoid fossa; the neck is flattened in front and presents a pit, for the insertion of a portion of the external pterygoid muscle. The superior border of the ramus is known as the sigmoid notch; it is terminated anteriorly by the coronoid process. This is a pointed process with two borders and two surfaces; the inner surface presents a ridge, commencing at the tip, and becoming continuous with the inner edge of the alveolus. To this ridge, to the area of bone in front of it and the tip of the coronoid process, the temporal muscle is inserted; its outer surface affords attachment to the masseter and a few fibers of the temporal. The anterior border of the ramus is continuous with the external oblique line on the body of the bone.
The mandible is very vascular, and receives a large supply from the mandibular branch of the internal maxillary artery. This constitutes its main supply. It receives twigs also from the facial artery.
The mandible. (Inner view.)
It gives attachment to the following muscles:
- Depressor labii inferioris.
- Depressor anguli inferioris.
- Levator menti.
- Superior constrictor of pharynx.
- Orbicularis oris.
- Internal pterygoid.
- External pterygoid.
- Platysma myoides.
The mandible has six points of ossification for each lateral half. All these, with the exception of one, are deposited in membrane. The nuclei are deposited very early (between the sixth and eighth week), and fuse so rapidly that observations on the development of this bone are unusually difficult.
Its six centers are mainly named according to their position.
This is deposited in the distal end of Meckel's (mandibular) cartilage, and gives rise to that portion of the bone between the symphysis and the mental foramen.
This forms the lower border and outer plate, and supports the teeth, hence its name.
This gives rise to the process of that name.
This forms the condyle and adjacent portion of the neck of the bone.
This gives rise to the angle of the bone.
This center appears three weeks later than the portions already mentioned. It forms the inner plate of the mandible from near the symphysis to the mandibular foramen. The mandibular spine represents the posterior extremity of the splenial. Its line of junction with the dentary is indicated in the adult bone by the mylo-hyoid groove.
At birth the mandible is represented by two nearly horizontal troughs of bone lodging unerupted teeth. Each half is joined at the symphysis by fibrous tissue. The upper edge of the symphysis and the condyles are nearly on a level. The mandibular nerve lies in a shallow groove between the dentary and splenial plates.
During the first year the two halves ankylose, union taking place from below upwards, but the ankylosis is not complete until the second year. After the first dentition, the ramus forms with the body of the mandible an angle of about 140°, and the mental foramen is situated midway between the upper and lower borders of the bone opposite the second milk-molar. In the adult, the angle formed by the ramus and body is nearly a right angle, and the mental foramen is opposite the second bicuspid, so that its relative position remains unaltered after the first dentition. In old age, after the fall of the teeth, the alveolar margin is absorbed, the angle formed by the ramus and body becomes obtuse, and the mental foramen approaches to the alveolar margin. In a young and vigorous adult the mandible is, with the exception of the petrosal, the densest bone in the skeleton, and resists decay longest; in old age it becomes exceedingly porous, and often so soft that it may be broken easily.
The hyoid, the styloid process, and the ear bones
The hyoid or lingual bone consists of a body and four processes. The body (basi-hyal) forms the central portion of the bone; it is somewhat oblong in shape. Its anterior aspect is convex and divided by a longitudinal ridge into a superior and an inferior portion. Frequently it presents a median vertical ridge, and at the point where the horizontal and vertical ridges intersect, a tubercle, sometimes measuring four millimeters in length, is formed. The whole of the anterior surface is crowded with the origin and insertions of muscles. The posterior surface is deeply concave, and frequently presents several small depressions near the middle line which lodge accessory thyroid bodies.
The inferior border is free, the superior gives attachment to the thyro-hyoid membrane. Between this membrane and the concavity of the hyoid a large bursa is occasionally found. The lateral borders are in relation with the greater cornua, but remain separated from them until late in life.
The greater cornua (thyro-hyals) project backwards and upwards. Their upper and lower borders and anterior surfaces are occupied with muscles. Each corner terminates posteriorly in a rounded tubercle, to which the thyro-hyoid ligament is attached.
The lesser cornua (cerato-hyals) are small conical pieces of bone occupying the upper part of the suture between the body and the greater cornua. Their tips are continuous with the stylo-hyoid ligaments.
Muscles attached to the hyoid bone
- Middle constrictor.
- Omo-hyoid (when present).
- and the thyro-hyoid membrane.
The hyoid receives twigs from the arteries supplying the muscles attached to it, in addition to direct supply from the superior thyroid and lingual arteries.
At the third month the hyoid consists of hyaline cartilage; it is directly continuous with the styloid process. In the fourth month a nucleus appears on each side of the middle line; they become quickly confluent to form the body of the bone. In the fifth month each greater cornu has a conspicuous nucleus. The centers for the lesser cornua are delayed until the second year.
The greater cornua remain separate from the body until after middle life. The lesser cornua rarely ankylose with the body of the bone. As a rule, they are small and inconspicuous; occasionally they are very long, and are sometimes continuous with the styloid process of their respective sides.
The styloid process
The styloid process is a thin cylindrical spike of bone wedged in between the tympanic plate and the petrosal immediately anterior to the stylo-mastoid foramen. It consists of two parts: a tympano-hyal segment which in the adult is hidden behind the tympanic plate, and a free projecting portion of variable length. As a rule it varies from five to fifty millimeters. When short it is hidden by the vaginal process, but it may reach to the hyoid bone. Its base forms the anterior boundary of the stylo-mastoid foramen. The free portion gives origin to the following muscles: The stylo-pharyngeus arises from the base posteriorly; the stylo-hyoid from the outer aspect near the middle; and the stylo-glossus from the front near the tip. The extremity of the process is continuous with the stylo-hyoid ligament. A band of fibrous tissue - the stylo-mandibular ligament- passes from the process below the origin of the stylo-glossus to the angle of the mandible.
Muscles attached to the styloid process
The morphology and development of this process are described on page 98.
This is the most external of the auditory ossicles, and comes in relation with the tympanic membrane. Its upper portion, or head, is lodged in the attic of the tympanum. It is of rounded shape, and presents posteriorly an elliptical depression for articulation with the incus. Below the head is a constricted portion or neck. From beneath the neck three processes diverge. The largest is the handle or manubrium, which is slightly twisted and flattened. It forms an obtuse angle with the head of the bone, and lies between the tympani membrane and the mucous membrane covering its inner surface.
The tensor tympani tendon is inserted into the manubrium near its junction with the neck on the inner side. The slender process (gracilis or Folian) is a long, slender, delicate spiculum of bone (rarely seen of full length except in the fetus), projecting nearly at right angles to the anterior aspect of the neck, and extending obliquely downwards. It lies in the Glaserian fissure, and in the adult usually becomes converted into connective tissue, except a small basal stump. The short process is a conical projection from the outer aspect of the base of the manubrium. Its apex is connected to the upper part of the tympanic membrane, and its base receives the external ligament of the malleus. The malleus also gives attachment to the suspensory ligament, and to the long anterior ligament of the malleus which was formerly described as the laxator tympani muscle.
This bone is situated between the malleus externally, and the stapes internally. It presents for examination a body and two processes. The body is deeply excavated anteriorly for the reception of the head of the malleus.
The short process projects backwards, and is connected by means of ligamentous fibers to the posterior wall of the tympanum, near the entrance to the mastoid antrum. The long^ process is slender, and directed downwards and inwards; it lies parallel with the handle of the malleus. On the inner aspect of the distal extremity of this process is the orbicular tubercle, connected with the process by a narrow neck. Its free surface articulates with the head of the stapes. The orbicular tubercle is separate in early life.
The stapes is the innermost ossicle. It has a head directed horizontally outwards, capped at its outer extremity by a disc resembling the head of the radius. The cup-shaped depression receives the orbicular tubercle of the incus. The base occupies the fenestra ovalis, and like this opening the inferior border is straight, and the superior curved. The base is connected with the head by means of two crura, and a narrow piece of bone called the neck. Of the two crura, the anterior is the shorter and straighter. The crura with the base form a stirrup- shaped arch, of which the inner margin presents a groove for the reception of the membrane which is stretched across the hollow of the stapes. In the early embryo this hollow is traversed by the stapedial artery. The neck is very short, and receives on its posterior border the tendon of the stapedius muscle.