NEET MDS Lessons
Anatomy
-> Most of the facial skeleton is formed by nine bones: four paired (nasal, zygomatic, maxilla, and palatine) and one unpaired (mandible).
-> The calvaria of the new-born infant is large compared with the relatively small fascial skeleton.
-> This results from the small size of the jaws and the almost complete absence of the maxillary and other paranasal sinuses in the new-born skull.
-> These sinuses form large spaces in the adult facial skeleton. As the teeth and sinuses develop during infancy and childhood, the facial bones enlarge.
-> The growth of the maxillae between the ages of 6 and 12 years accounts for the vertical elongation of the child’s face.
The Nasal Bones
-> These bones may be felt easily because they form the bridge of the nose.
-> The right and left nasal bones articulate with each other at the internasal suture.
-> They also articulate with the frontal bones, the maxillae, and the ethmoid bones.
-> The mobility of the anteroinferior portion of the nose, supported only by cartilages, serves as a partial protection against injure (e.g., a punch in the nose). However, a hard blow to the anterosuperior bony portion of the nose may fracture the nasal bones (broken nose).
-> Often the bones are displaced sideways and/or posteriorly.
The Maxillae
-> The skeleton of the face between the mouth and the eyes is formed by the two maxillae.
-> They surround the anterior nasal apertures and are united in the medial plane at the intermaxillary suture to form the maxilla (upper jaw).
-> This suture is also visible in the hard palate, where the palatine processes of the maxillae unite.
-> Each adult maxilla consists of: a hollow body that contains a large maxillary sinus; a zygomatic process that articulates with its mate on the other side to form most of the hard palate; and alveolar processes that form sockets for the maxillary (upper) teeth.
-> The maxillae also articulate with the vomer, lacrimal, sphenoid, and palatine bones.
-> The body of the maxilla has a nasal surface that contributes to the lateral wall of the nasal cavity; an orbital surface that forms most of the floor of the orbit; an infratemporal surface that forms the anterior wall of the infratemporal fossa; and an anterior surface that faces partly anteriorly and partly anterolaterally and is covered buy facial muscles.
-> The relatively large infraorbital foramen, which faces inferomedially, is located about 1 cm inferior to the infraorbital margin; it transmits the infraorbital nerve and vessels.
-> The incisive fossa is a shallow concavity overlying the roots of the incisor teeth, just a shallow concavity overlying the roots of the incisor teeth, just inferior to the nasal cavity. This fossa is the injection site for anaesthesia of the maxillary incisor teeth.
-> If infected maxillary teeth are removed, the bone of the alveolar processes of the maxillae begins to be reabsorbed. As a result, the maxilla becomes smaller and the shape of the face changes.
-> Owing to absorption of the alveolar processes, there is a marked reduction in the height of the lower face, which produces deep creases in the facial skin that pass posteriorly from the corners of the mouth.
The Mandible
-> This is a U-shaped bone and forms the skeleton of the lower jaw and the inferior part of the face. It is the largest and strongest facial bone.
-> The mandibular (lower) teeth project superiorly from their sockets in the alveolar processes.
-> The mandible (L. mandere, to masticate) consists of two parts: a horizontal part called the body, and two vertical oblong parts, called rami.
-> Each ramus ascends almost vertically from the posterior aspect of the body.
-> The superior part of the ramus has two processes: a posterior condylar process with a head or condyle and a neck, and a sharp anterior coronoid process.
-> The condylar process is separated from the coronoid process by the mandibular notch, which forms the concave superior border of the mandible.
-> Viewed from the superior aspect, the mandible is horseshoe-shaped, whereas each half is L-shaped when viewed laterally.
-> The rami and body meet posteriorly at the angle of the mandible.
-> Inferior to the second premolar tooth on each side of the mandible is a mental foramen (L. mentum, chin) for transmission of the mental vessels and the mental nerve.
-> In the anatomical position, the rami of the mandible are almost vertical, except in infants and in edentulous (toothless) adults.
-> On the internal aspect of the ramus, there is a large mandibular foramen.
-> It is the oblong entrance to the mandibular canal that transmits the inferior alveolar vessels and nerve to the roots of the mandibular teeth.
-> Branches of these vessels and the mental nerve emerge from the mandibular canal at the mental foramen.
-> Running inferiorly and slightly anteriorly on the internal surface of the mandible from the mandibular foramen is a small mylohyoid groove (sulcus), which indicates the course taken by the mylohyoid nerve and vessels.
-> These structures arise from the inferior alveolar nerve and vessels, just before they enter the mandibular foramen.
-> The internal surface of the mandible is divided into two areas by the mylohyoid line, which commences posterior to the third molar tooth. -> Just superior to the anterior end of the mylohyoid line are two small, sharp mental spines (genial tubercles), which serve as attachments for the genioglssus muscles.
The Zygomatic Bones
-> The prominences of the cheeks (L. mala), the anterolateral rims and much of the infraorbital margins of the orbits, are formed by the zygomatic bones (malar bones, cheekbones).
-> They articulate with the frontal, maxilla, sphenoid, and temporal bones.
-> The frontal process of the zygomatic bone passes superiorly, where it forms the lateral border of the orbit (eye socket) and articulates with the frontal bone at the lateral edge of the supraorbital margin.
-> The zygomatic bones articulate medially with the greater wings of the sphenoid bone. The site of their articulation may be observed on the lateral wall of the orbit.
-> On the anterolateral aspect of the zygomatic bone near the infraorbital margin is a small zygomaticofacial foramen for the nerve and vessels of the same name.
-> The posterior surface of the zygomatic bone near the base of its frontal process is pierced by a small zygomaticotemporal foramen for the nerve of the same name.
-> The zygomaticofacial and zygomaticotemporal nerves, leaving the orbit through the previously named foramina, enter the zygomatic bone through small zygomaticoorbital foramina that pierces it orbital surface.
-> The temporal process of the zygomatic bone unites with the zygomatic process of the temporal bone to form the zygomatic arch.
-> This arch can be easily palpated on the side of the head, posterior to the zygomatic prominence (malar eminence) at the inferior boundary of the temporal fossa (temple).
-> The zygomatic arches form one of the useful landmarks for determining the location of the pterion. These arches are especially prominent in emaciated persons.
-> A horizontal plane passing medially from the zygomatic arch separates the temporal fossa superiorly from the infratemporal fossa inferiorly.
Other Bones
There are several other, very important bones in the skull, including the palatine bone, ethmoid bone, vomer, inferior concha and the ossicles of the ear (malleus, incus and stapes). These, however, are covered to greater detail where they are relevant in the head (e.g., ethmoid bone with the orbit and nasal cavity).
Muscles Moving the Auditory Ossicles
The Tensor Tympani Muscle
- This muscle is about 2 cm long.
- Origin: superior surface of the cartilaginous part of the auditory tube, the greater wing of the sphenoid bone, and the petrous part of the temporal bone.
- Insertion: handle of the malleus.
- Innervation: mandibular nerve (CN V3) through the nerve to medial pterygoid.
- The tensor tympani muscle pulls the handle of the malleus medially, tensing the tympanic membrane, and reducing the amplitude of its oscillations.
- This tends to prevent damage to the internal ear when one is exposed to load sounds.
The Stapedius Muscle
- This tiny muscle is in the pyramidal eminence or the pyramid.
- Origin: pyramidal eminence on the posterior wall of the tympanic cavity. Its tendon enters the tympanic cavity by traversing a pinpoint foramen in the apex of the pyramid.
- Insertion: neck of the stapes.
- Innervation: nerve to the stapedius muscle, which arises from the facial nerve (CN VII).
- The stapedius muscle pulls the stapes posteriorly and tilts its base in the fenestra vestibuli or oval window, thereby tightening the anular ligament and reducing the oscillatory range.
- It also prevents excessive movement of the stapes.
Innervation of the Pharynx
- The motor and most of the sensory supply of the pharynx is derived from the pharyngeal plexus of nerves on the surface of the pharynx.
- The plexus is formed by pharyngeal branches of the vagus (CN X) and glossopharyngeal (CN IX) nerves, and by sympathetic branches for the superior cervical ganglion.
- The motor fibres in the pharyngeal plexus are derived from the cranial root of accessory nerve (CN XI), and are carried by the vagus nerve to all muscles of the pharynx and soft palate.
- The exceptions are stylopharyngeus (supplied by CN IX) and the tensor veli palatini (supplied by CN V3).
The Sublingual Glands
- These are the smallest of the three paired salivary glands and the most deeply situated.
- They are almond-shaped and lie in the floor of the mouth between the mandible and the genioglossus muscle.
- The paired glands unite to form a horseshoe-shaped glandular mass around the lingual frenulum.
- Numerous small ducts (10 to 12) open into the floor of the mouth.
- Sometimes one of the ducts opens into the submandibular duct.
- The nerves the accompany the submandibular and sublingual glands are derived from the lingual and chorda tympani nerves and from the sympathetic nerves.
- The parasympathetic secretomotor fibres are from the submandibular ganglion.
- Provides a rigid support system
- Protects delicate structures (e. g., the protection provided by the bones of the vertebral column to the spinal cord)
- Bones supply calcium to the blood; are involved In the formation of blood cells (hemopoiesis)
- Bones serve as the basis of attachment of muscles; form levers in the joint areas, aIlowing movement
Walls of the Tympanic Cavity or Middle Ear
- This cavity is shaped like a narrow six-sided box that has convex medial and lateral walls.
- It has the shape of the biconcave lens in cross-section (like a red blood cell).
The Roof or Tegmental Wall
- This is formed by a thin plate of bone, called the tegmen tympani (L. tegmen, roof).
- It separates the tympanic cavity from the dura on the floor of middle cranial fossa.
- The tegmen tympani also covers the aditus ad antrum.
The Floor or Jugular Wall
- This wall is thicker than the roof.
- It separates the tympanic cavity from the superior bulb of the internal jugular vein. The internal jugular vein and the internal carotid artery diverge at the floor of the tympanic cavity.
- The tympanic nerve, a branch of the glossopharyngeal nerve (CN IX), passes through an aperture in the floor of the tympanic cavity and its branches form the tympanic plexus.
The Lateral or Membranous Wall
- This is formed almost entirely by the tympanic membrane.
- Superiorly it is formed by the lateral bony wall of the epitympanic recess.
- The handle of the malleus is incorporated in the tympanic membrane, and its head extends into the epitympanic recess.
The Medial or Labyrinthine Wall
- This separates the middle ear from the membranous labyrinth (semicircular ducts and cochlear duct) encased in the bony labyrinth.
- The medial wall of the tympanic cavity exhibits several important features.
- Centrally, opposite the tympanic membrane, there is a rounded promontory (L. eminence) formed by the first turn of the cochlea.
- The tympanic plexus of nerves, lying on the promontory, is formed by fibres of the facial and glossopharyngeal nerves.
- The medial wall of the tympanic cavity also has two small apertures or windows.
- The fenestra vestibuli (oval window) is closed by the base of the stapes, which is bound to its margins by an annular ligament.
- Through this window, vibrations of the stapes are transmitted to the perilymph window within the bony labyrinth of the inner ear.
- The fenestra cochleae (round window) is inferior to the fenestra vestibuli.
- This is closed by a second tympanic membrane.
The Posterior or Mastoid Wall
- This wall has several openings in it.
- In its superior part is the aditus ad antrum (mastoid antrum), which leads posteriorly from the epitympanic recess to the mastoid cells.
- Inferiorly is a pinpoint aperture on the apex of a tiny, hollow projection of bone, called the pyramidal eminence (pyramid).
- This eminence contains the stapedius muscle.
- Its aperture transmits the tendon of the stapedius, which enters the tympanic cavity and inserts into the stapes.
- Lateral to the pyramid, there is an aperture through which the chorda tympani nerve, a branch of the facial nerve (CN VII), enters the tympanic cavity.
The Anterior Wall or Carotid Wall
- This wall is a narrow as the medial and lateral walls converge anteriorly.
- There are two openings in the anterior wall.
- The superior opening communicates with a canal occupied by the tensor tympani muscle.
- Its tendon inserts into the handle of the malleus and keeps the tympanic membrane tense.
- Inferiorly, the tympanic cavity communicates with the nasopharynx through the auditory tube.
The Superior Roof of the Orbit
- The superior wall or roof of the orbit is formed almost completely by the orbital plate of the frontal bone.
- Posteriorly, the superior wall is formed by the lesser wing of the sphenoid bone.
- The roof of the orbit is thin, translucent, and gently arched. This plate of bone separates the orbital cavity and the anterior cranial fossa.
- The optic canal is located in the posterior part of the roof.