Internal oblique line. Anatomy of the jaw bones
Mandibula, unpaired, forms the lower part of the facial. The bone is distinguished by a body and two processes called branches (running upward from the posterior end of the body).
The body, corpus, is formed from two halves connecting along the midline (mental symphysis, symphysis mentalis), which fuse into one bone in the first year of life. Each half is curved with a convex outward. Its height is greater than its thickness. On the body there is a lower edge - the base of the lower jaw, basis mandibulae, and an upper edge - the alveolar part, pars alveolaris.
On the outer surface of the body, in its middle sections, there is a small chin protuberance, protuberantia mentalis, outward from which the chin tubercle, tuberculum mentale, immediately protrudes. Above and outward from this tubercle lies the mental foramen, foramen mentale (the place of exit of the vessels and nerve). This hole corresponds to the position of the root of the second small molar. Posteriorly from the mental foramen, an oblique line, linea obliqua, goes upward, which passes into the anterior edge of the ramus of the lower jaw.
The development of the alveolar part depends on the teeth it contains.
This part is thinned and contains alveolar elevations, juga alveolaria. At the top it is limited by an arched free edge - the alveolar arch, arcus alveolaris. In the alveolar arch there are 16 (8 on each side) dental alveoli, alveoli dentales, separated from one another by interalveolar septa, septa interalveolaria.
On the inner surface of the body of the lower jaw, near the midline, there is a single or bifurcated mental spine, spina mentalis (the origin of the geniohyoid and genioglossus muscles). At its lower edge there is a depression - the digastric fossa, fossa digastrica, trace of attachment. On the lateral sections of the inner surface on each side and towards the branch of the lower jaw, the maxillary-hyoid line, linea mylohyoidea, runs obliquely (here the maxillary-hyoid muscle and the maxillary-pharyngeal part of the superior pharyngeal constrictor begin).
Above the maxillary-hyoid line, closer to the hyoid spine, there is a sublingual fossa, fovea sublingualis, a trace of the adjacent sublingual gland, and below and posterior to this line there is often a weakly defined submandibular fossa, fovea submandibularis, a trace of the adjacent submandibular gland.
The branch of the lower jaw, ramus mandibulae, is a wide bony plate that rises from the posterior end of the body of the lower jaw upward and obliquely backward, forming with the lower edge of the body mandibular angle, angulus mandibulae.
On the outer surface of the branch, in the area of the corner, there is a rough surface - the masticatory tuberosity, tuberositas masseterica, a trace of the attachment of the muscle of the same name. On the inner side, corresponding to the masticatory tuberosity, there is a smaller roughness - the pterygoid tuberosity, tuberositas pterygoidea, a trace of the attachment of the medial pterygoid muscle.
In the middle of the inner surface of the branch there is mandibular foramen, foramen mandibulae, limited from the inside and in front by a small bony protrusion - the tongue of the lower jaw, lingula mandibulae. This hole leads into the canal of the lower jaw, canalis mandibulae, in which vessels and nerves pass. The canal lies in the thickness of the spongy bone. On the anterior surface of the body of the lower jaw, it has an exit - the mental foramen, foramen mentale.
From the opening of the lower jaw, downward and forward, along the upper border of the pterygoid tuberosity, runs the mylohyoid groove, sulcus mylohyoideus (a trace of the occurrence of the vessels and nerves of the same name). Sometimes this groove or part of it is covered with a bone plate, turning into a canal. Slightly higher and anterior to the opening of the lower jaw is the mandibular ridge, torus mandibularis.
At the upper end of the ramus of the lower jaw there are two processes that are separated by the notch of the lower jaw, incisura mandibulae. The anterior coronoid process, processus coronoideus, often has roughness on its inner surface due to the attachment of the temporal muscle. The posterior condylar process, processus condylaris, ends with the head of the lower jaw, caput mandibulae. The latter has an ellipsoidal articular surface, which, together with the temporal bone of the skull, participates in the formation
The lower jaw is a movable bone of the facial skeleton, consisting of a body, a branch, and an angle.
The body consists of basal and alveolar parts.
The branch has two processes - the condylar, ending in the head of the lower jaw, and the coronoid.
The ratio of the height of the branch to the length of the body of the jaw in an adult is 6.5-7:10. The angle of the lower jaw is normally 120 degrees ± 5 (Tridents).
The shape of the dentition is parabolic.
The lower jaw is a horseshoe-shaped unpaired bone consisting of a body, two branches that end in two processes, a coronoid and an articular, and between the processes a semilunar notch.
The lower edge of the body and the posterior edge of the branch form an angle of 110-130°
Inner surface:
1. In the area of the central incisors there are mental spines;
2. Next to them is the digastric fossa, the place of attachment of the muscle of the same name;
3. Laterally (from the fossa) the bone ridge is the internal oblique line (mylohyoid);
4. In the area of the angle on the inner side there is the pterygoid tuberosity, the place of attachment of the muscle of the same name;
5. On the inner surface of the lower jaw branch there is a hole, the exit point of the neurovascular bundle.
Outer surface:
1. Mental protuberance, mental foramina in the area of the second premolars;
2. The external oblique line goes superiorly and posteriorly, merging with the internal oblique line to form a space behind the retromolar;
3. In the area of the corner there is a masticatory tuberosity.
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(mandibula), unpaired, horseshoe-shaped (Fig. 1). This is the only movable bone of the skull. It consists of two symmetrical halves, completely fused by the end of the 1st year of life. Each half has a body and a branch. In old age, a bony protrusion forms at the junction of both halves.
IN body (corpus mandibulae) differentiate base of the lower jaw (basis mandibulae) And alveolar part (pars alveolaris). The body of the jaw is curved, its outer surface is convex and its inner surface is concave. At the base of the body, the surfaces transform into one another. The right and left halves of the body converge at individually different angles, forming the basal arch.
The height of the jaw body is greatest in the area of the incisors, the smallest is at the level of the 8th tooth. The thickness of the jaw body is greatest in the region of the molars, and the smallest in the region of the premolars. The cross-sectional shape of the jaw body is not the same in different areas, which is determined by the number and position of the roots of the teeth. In the area of the front teeth it approaches triangular with the base facing downwards. In areas of the body corresponding to large molars, it is close to a triangle with the base facing upward.
Rice. 1.
a — topography of the lower jaw;
b — side view: 1 — coronoid process; 2 - notch of the lower jaw; 3 - pterygoid fossa; 4 - head of the lower jaw; 5 - condylar process; 6 - neck of the lower jaw; 7 - chewing tuberosity; 8 - angle of the lower jaw; 9 - base of the lower jaw; 10 - mental tubercle; 11 - chin protuberance; 12 - chin hole; 13 - alveolar part; 14 - oblique line; 15 - branch of the lower jaw;
c — view from the inner surface: 1 — condylar process; 2 - coronoid process; 3 - tongue of the lower jaw; 4 - opening of the lower jaw; 5 - maxillary-hyoid line; 6 - mental spine; 7 - sublingual fossa; 8 - mylohyoid groove; 9 - mandibular ridge; 10 - pterygoid tuberosity; 11—submandibular fossa; 12—digastric fossa; 13 - angle of the lower jaw; 14 - neck of the lower jaw;
d — top view: 1 — alveolar arch; 2 - retromolar fossa; 3 - temporal crest; 4 - coronoid process; 5 - tongue of the lower jaw; 6 - pterygoid fossa; 7 - head of the lower jaw; 8 - oblique line; 9 - mandibular pocket; 10—base of the lower jaw; 11 - mental tubercle; 12—chin protuberance; 13 - dental alveoli; 14 - interalveolar septa; 15 - chin hole; 16 — interroot septa; 17 - neck of the lower jaw; 18 - condylar process;
d — position of the opening of the lower jaw; e - the magnitude of the angle of the lower jaw
In the middle outer surface the body of the jaw is located chin protuberance (protuberantia mentalis), which is a characteristic feature of modern man and causes the formation of a chin. The angle of the chin to the horizontal plane in modern humans ranges from 46 to 85°. In the apes, Pithecanthropus, Heidelberg man and Neanderthal, the mental protrusion is absent, the angle of the chin in the first three is obtuse, and in the Neanderthal it is straight. From 1 to 4 participate in the formation of the human chin protuberance. chin bones (ossicula mentales), which arise at the time of birth and later fuse with the jaw. On both sides of the mental protuberance, closer to the base of the jaw, there are mental tubercles (tubercula mentalia).
Outward from each tubercle is located mental foramen (foramen mentale)- outlet of the mandibular canal. The vessels and nerve of the same name exit through the mental foramina. Most often, this hole is located at the level of the 5th tooth, but it can move anteriorly to the 4th tooth, and posteriorly to the space between the 5th and 6th teeth. The dimensions of the mental foramen range from 1.5 to 5 mm; it is oval or round, sometimes double. The mental foramen is removed from the base of the jaw by 10-19 mm. On the jaws of newborns this hole is located closer to the base, and on the toothless jaws of adults with an atrophied alveolar part - closer to the upper edge of the jaw.
An obliquely located ridge runs along the lateral half of the outer surface of the body of the lower jaw - oblique line (linea obliqua), the anterior end of which corresponds to the level of the 5-6th tooth, and the posterior end, without sharp boundaries, passes to the anterior edge of the lower jaw branch.
On inner surface of the body of the jaw, near the midline, there is a bone spine, sometimes double, - mental spine (spina mentalis). This place is the beginning of the geniohyoid and genioglossus muscles. Below and lateral to the mental spine is determined digastric fossa (fossa digastrica), in which the digastric muscle begins. Above the digastric fossa there is a flat depression - sublingual fossa (fovea sublingualis)- trace from the adjacent sublingual salivary gland. Further posteriorly visible mylohyoid line (linea mylohyoidea), on which the muscle of the same name and the superior pharyngeal constrictor begin. The mylohyoid line begins below the hyoid fossa and ends on the inner surface of the jaw branch. In some cases it is barely noticeable, in others it is represented by a strongly pronounced bone ridge. Under the maxillary-hyoid line at the level of the 5-7th tooth there is submandibular fossa (fovea submandibularis)- a trace from the submandibular salivary gland located in this place. Below and parallel to the maxillary-hyoid line there is a groove of the same name, to which the vessels and nerve are adjacent. The groove begins on the inner surface of the jaw branch near the opening of the lower jaw and ends under the posterior part of the mylohyoid line. Sometimes for some distance it turns into a canal.
So, the lower jaw consists of a body, corpus mandibulae formed by two horizontal branches and paired vertical branches , rami mandibulae connecting to the body at an obtuse angle. The body of the lower jaw bears a row of lower teeth.
The junction of the body and branches of the mandible forms the angle of the mandible , angulus mandibulae, to which the masticatory muscle is attached externally, causing the appearance of the tuberosity of the same name, tuberositas masseterica. On the inner surface of the angle there is a pterygoid tuberosity , tuberositas pterigoidea, to which the internal pterygoid muscle is attached, m. pterigoideus medialis. In newborns and elderly people, this angle is approximately 140-150 degrees; in adults, the angle of the lower jaw is close to straight. This is directly related to chewing activity.
Rice. Anatomy of the lower jaw (according to H. Milne, 1998): 1 – body of the lower jaw; 2 – mental tubercle; 3 – mental spine; 4 – mental foramen; 5 – alveolar part; 6 – branch of the lower jaw; 7 – angle of the lower jaw; 8 – condylar process; 9 – neck of the lower jaw; 10 – pterygoid fossa; 11 – coronoid process; 12 – notch of the lower jaw; 13 – opening of the lower jaw; 14 – tongue of the lower jaw.
The structure and relief of the body of the lower jaw are determined by the presence of teeth and its participation in the formation of the mouth (M.G. Prives et al., 1974).
The outer surface of the body of the lower jaw is convex, protruding forward by the chin protuberance, protuberantia mentalis. The mental protrusion is divided by the mental symphysis, symphysis mandibulae (mentalis), on the sides of which there are two mental tubercles, tubercula mentali. Above them and slightly lateral to the symphysis (at the level of the space between the 1st and 2nd small molars) are the mental fossae, where the mental foramina are located, foramen mentale, representing the exits of the mandibular canals, canalis mandibulae. The third branches of the trigeminal nerve pass through them. External oblique line, linea oblique, goes from the mental protuberance to the upper edge of the vertical branch. Alveolar arch , arcus alveolaris, runs along the upper edge of the body of the lower jaw and carries dental cells, alveoli dentales. In old age, the alveolar part often atrophies and the whole body becomes thin and low.
The inner surface of the body of the lower jaw is concave with a pronounced hyoid oblique line, linea mylohyoidea, running from front to back from the upper mental protuberances to the upper edge of the vertical branch. Above this line there is a hypoglossal fossa, fossa sublingualis where the sublingual gland is located. Below the line is the submandibular fossa, fossa submaxillaris, - location of the submandibular gland.
In the area of the symphysis, two mental spines protrude on the inner surface, spina mentales, - places of tendon attachment mm. genioglossi. The tendinous method of attachment of the tongue muscles contributed to the development of articulate speech. The mental spines are the attachment sites for the genioglossus, mm. genioglossi, and geniohyoid muscles, mm. geniohyoidei.
On either side of spina mentalis, closer to the lower edge of the lower jaw there are places of attachment of the digastric muscle, fossae digastricae.
vertical branches, rami mandibulae, - flat bones with two protrusions: the condylar process, processus condylaris, and the coronoid process, processus coronoideus, separated by the mandibular notch, incisura mandibula.
On the inner surface there is a mandibular foramen, foramen mandibulae, leading into the mandibular canal. The inner edge of the hole protrudes in the form of a tongue of the lower jaw lingual mandibulae, to which the sphenomandibular ligament is attached, lig. sphenomandibulare. To the pterygoid tuberosity, tuberositas pterygoidea, the internal pterygoid muscle is attached. At the junction of the body and vertical branches, gonion, there is an attachment of the stylomandibular ligament, lig.stylomandibulare.
At the top, as already mentioned, the branch ends in two processes: condylar and coronoid. The coronoid process was formed under the influence of the traction of the temporalis muscle. On the inner surface of the branch towards the coronoid process, the crest of the buccal muscle rises from the level of the last molars , Christa Buccinatoria. The condylar process has a head, caput mandibulae, and neck, collum mandibulae. In front of the neck there is a fossa to which the external pterygoid muscle is attached , m. pterigoideus lateralis.
Outer surface of the lower jaw It is distinguished by the following anatomical features: the chin protuberance (protuberantia mentalis) is located in the symphysis region - at the fusion of the two halves of the lower jaw. Fusion occurs, as stated above, in the first year of the child’s extrauterine life. Subsequently, this part of the chin fuses with the mental bones (ossicula mentalia I-4 bones according to Meckel). These bones also take part in the formation of the chin protrusion.
chin protuberance on the side it is limited by the mental foramen (foramen mentale), which serves as the exit point for the mental nerves and vessels and is located between the first and second premolars. An external oblique line stretches upward and posteriorly from the opening, located on the border between the body of the lower jaw and the alveolar process. On the outer surface of the angle of the lower jaw there is a roughness formed as a result of the traction of the masticatory muscle attached in this place, the so-called masticatory tuberosity (tuberositas masseterica). The external oblique line, like the internal one, serves to strengthen the lower molars and protect them from loosening in the bucco-lingual direction during transversal chewing movements (A. Ya. Katz).
Between the articular head and coronoid process there is a mandibular notch formed as a result of phylogenetic development (incisura mandibulae). Some authors consider one of the reasons for its formation to be the traction of the muscles attached here. The external pterygoid muscle pulls the articular head inward and slightly upward, and the horizontal bundles of the temporal muscle pull the coronoid process posteriorly and upward. This direction of muscle traction caused the formation of a semilunar notch as a result of species development.
Interesting in a nutshell dwell on the phylogeny of the mental protuberance (protuberantia mentalis). Different authors explain the formation of the chin in different ways.
Some attribute the emergence chin action of the pterygoid muscles. The external and internal pterygoid muscles, acting on both sides in opposite directions, create an area of dangerous section in the area of the mental protuberance and stimulate the bone tissue in the mental region to grow and thicken, which protects the lower jaw from fracture. This theory is one-sided.
Others explain chin formation the emergence of articulate speech and rich facial expressions, distinguishing modern man from his ancestors. Various emotional experiences, reflected on the face and requiring continuous and specialized mobility of the facial muscles, cause increased functional irritation of the bone tissue and, as a result, the formation of a chin protrusion. This idea is confirmed by the fact that all modern people have a pronounced chin, while primitive people, who stood at a low level of the phylogenetic ladder, had no chin.
Still others explain chin formation reduction of the alveolar process due to the reverse development of the lower dentition, the basal arch of the mandible therefore protrudes.
In our opinion, chin development is not determined by one reason, but by many factors depending on the relationship between form and function and the ability of a living organism to adapt to environmental conditions. These are the main features that distinguish the relief of the lower jaw as a place of attachment of the masticatory muscles. Under the influence of increased functional activity of the lower jaw, not only the relief, but also the internal structure of this bone changes. It is known that the beams of spongy substance and their direction are always in a natural connection with the development of traction and pressure. Pressure and traction in any bone cause special compression and rupture curves. These lines of thrust and pressure are called trajectories.
Trajectories detected also when studying the architecture of the lower jaw. Walkhoff, while studying the functional structure of the lower jaw, examined the structure of the bone using x-rays and found that the trajectories go from the point of load through the area of application of the force of the masticatory muscles and are directed to the articular heads. It distinguishes 8 directions of trajectories.
A. Ya. Katz also studied spongy substances of the lower jaw. He made cuts in the jaw in three mutually perpendicular planes. Research by A. Ya. Katz showed that the direction of the trabeculae of the spongy substance reflects the functional activity of the lower jaw. The spongy substance of the retromolar region and branches is characterized by a lamellar structure.
