The Ear

• The ear contains the vestibulocochlear organ and consists of three main parts: external, middle, and internal.
• It has two functions, balance and hearing.

• The tympanic membrane (eardrum) separates the external ear from the middle ear.
• The auditory tube joins the middle ear or tympanic cavity to the nasopharynx.

Ligaments of the Joint

• The fibrous capsule is thickened laterally to form the lateral (temporomandibular) ligament. It reinforces the lateral part of this capsule.
• The base of this triangular ligament is attached to the zygomatic process of the temporal bone and the articular tubercle.
• Its apex is fixed to the lateral side of the neck of the mandible.

• Two other ligaments connect the mandible to the cranium but neither provides much strength.

• The stylomandibular ligament is a thickened band of deep cervical fascia.
• It runs from the styloid process of the temporal bone to the angle of the mandible and separates the parotid and submandibular salivary glands.

• The sphenomandibular ligament is a long membranous band that lies medial to the joint.
• This ligament runs from the spine of the sphenoid bone to the lingula on the medial aspect of the mandible.

• U-shaped bone
• Body
• Greater horn
• Lesser horn

• Suspended by ligaments from the styloid process

Digastric Muscle

• Origin:
  • Anterior Belly: Digastric fossa of the mandible.
  • Posterior Belly: Mastoid notch of the temporal bone.
• Insertion: Intermediate tendon attached to the body of the hyoid bone.
• Nerve Supply:
  • Anterior Belly: Nerve to mylohyoid (branch of the trigeminal nerve, CN V3).
  • Posterior Belly: Facial nerve (CN VII).
• Arterial Supply:
  • Anterior Belly: Branch of the submental artery.
  • Posterior Belly: Muscular branch of the posterior auricular artery and occipital artery.
• Action: Raises the hyoid bone and base of the tongue, steadies the hyoid bone, and opens the mouth by lowering the mandible.

The External Ear

• The auricle (L. auris, ear) is the visible, shell-like part of the external ear.
• It consists of a single elastic cartilage that is covered on both surfaces with thin, hairy skin.
• The external ear contains hairs, sweat glands, and sebaceous glands.
• The cartilage is irregularly ridged and hollowed, which gives the auricle its shell-like form.
• It also shapes the orifice of the external acoustic meatus.

The Ear Lobule

• The ear lobule (earlobe) consists of fibrous tissue, fat and blood vessels that are covered with skin.
• The arteries are derived mainly from the posterior auricular artery and the superficial temporal artery.
• The skin of the auricle is supplied by the great auricular and auriculotemporal nerves.

• The great auricular nerve supplies the superior surface and the lateral surface inferior to the external acoustic meatus with nerve fibres from C2.
• The auriculotemporal nerve supplies the skin of the auricle superior to the external acoustic meatus.

The External Acoustic Meatus

• This passage extends from the concha (L. shell) of the auricle to the tympanic membrane (L. tympanum, tambourine). It is about 2.5 cm long in adults.
• The lateral 1/3 of the S-shaped canal is cartilaginous, whereas its medial 2/3 is bony.

• The lateral third of the meatus is lined with the skin of the auricle and contains hair follicles, sebaceous glands, and ceruminous glands.
• The latter glands produce cerumen (L. cera, wax).

• The medial two-thirds of the meatus is lined with very thin skin that is continuous with the external layer of the tympanic membrane.

• The lateral end of the meatus is the widest part. It has the diameter about that of a pencil.
• The meatus becomes narrow at its medial end, about 4 mm from the tympanic membrane.
• The constricted bony part is called the isthmus.

• Innervation of the external acoustic meatus is derived from three cranial nerves:

1. The auricular branch of the auriculotemporal nerve (derived from the mandibular, CN V³).
2. The facial nerve (CN VII) by the branches from the tympanic plexus.
3. The auricular branch of the vagus nerve (CN X).

The Tympanic Membrane

• This is a thin, semi-transparent, oval membrane at the medial end of the external acoustic meatus.
• It forms a partition between the external and middle ears.
• The tympanic membrane is a thin fibrous membrane, that is covered with very thin skin externally and mucous membrane internally.

• The tympanic membrane shows a concavity toward the meatus with a central depression, the umbo, which is formed by the end of the handle of the malleus.
• From the umbo, a bright area referred to as the cone of light, radiates anteroinferiorly.

• The external surface of the tympanic membrane is supplied by the auriculotemporal nerve.
• Some innervation is supplied by a small auricular branch of the vagus nerve (CN X); this nerve may also contain some glossopharyngeal and facial nerve fibres.

Tongue 
Appears at 4th week.
Musculature derived from mesoderm of occipital somites.  Precursor muscles cells migrate to region of tongue and are innervated by general sensory efferent fibers of CN XII.
Mucosa derived from anterior endoderm lining arches 1-4; accordingly, innervation depends on arch derivation:
              Mucosa of anterior 2/3 of tongue comes from the first arch -> CN V
              Mucosa of posterior 1/3 of tongue comes from third and forth arch -> CN IX, X
Special taste of anterior 2/3 of tongue comes from CN VII.
Special taste of posterior 1/3 of tongue comes from CN X.
Tongue freed from floor of mouth by extensive degeneration of underlying tissue.  Midline frenulum continues to anchor tongue to floor of mouth.

Thyroid Gland

Develops as in growth of mucosal epithelium located in the midline of the tongue (at foramen cecum).  It descends along front of pharyngeal gut, but remains connected to tongue by thyrooglossal duct, which is obliterated later in development.  Thyroid gland descends to a point just caudal to laryngeal cartilages. 

Facial structures (general)

a) medial nasal prominence forms midline of nose, philtrum and primary palate
b) lateral nasal prominence forms alae of nose
c) maxillary prominence forms cheek region and lateral lip
d) clefts can form at inter-prominence fusion lines

Nose

At the time of anterior neural tube closure, mesenchyme around forebrain, frontonasal prominence (FNP), has smooth rounded extended contour.  Nasal placodes (thickening of surface ectoderm to become peripheral neural tissue) develop on frontolateral aspects of FNP.  Mesenchyme swells around nasal placode producing a medial and lateral nasal prominence (nasomedial and nasolateral processes).  These nasal prominences form the nose.

Mouth 

Stomadeum (primitive oral cavity) forms between frontonasal prominence and first pharyngeal arch.  The first pharyngeal arch forms the dorsal maxillary prominence and ventral mandibular prominence.  The maxillary prominence will merge with medial nasal prominences, pushing them closer to cause fusion.  Fused medial nasal prominences will form midline of nose and midline of upper lip (philtrum) and primary palate (first 4 teeth).

Nasolacrimal structures

Maxillary and lateral nasal prominences are separated by deep furrow, the nasolacrimal groove.  Ectoderm in floor of groove forms epithelial cord, which detaches from overlying ectoderm.  The epithelial cord canalizes to form the nasolacrimal duct.  The upper end of the duct widens to form the lacrimal sac.  After detachment of the cord, the maxillary and lateral nasal prominences merge with each other, resulting in the formation of a nasolacrimal duct that runs from the medial corner of the eye to the inferior meatus of the nasal cavity.  
The maxillary prominences enlarge to form the cheeks and maxillae.
The lateral nasal prominences form the alae of the nose.

Secondary (hard) palate

Main part of definitive palate formed by two palatine shelves derived from intraoral bilateral extensions of the maxillary prominences.  These appear at the 6th week.  They are directed obliquely downward on each side of the tongue; they move down when mandible gets bigger.  
At the seventh week, they ascend to attain a horizontal position, then fuse to form the secondary palate.  At the time the palatine shelves fuse, the nasal septum (an outgrowth of median tissue of the frontonasal prominence) grows down and joins the cephalic aspect of the newly formed palate
Anteriorly, shelves fuse with triangular primary palate.  The incisive foramen marks the midline between the primary and secondary palate.

External Ear

The auricle is derived from 6 auricular hillocks (mesenchymal proliferations) along the dorsal aspect of arches 1 (top of ear) and 2 (bottom of ear).  These fuse to form the definitive auricle.  At the mandible grows, the ear is pushed upward and backward from its initial horizontal position on the neck.
The EAM is derived from the 1st pharyngeal arch.  
The eardrum (tympanic membrane) is composed of 3 layers of cells: 1) ectodermal epithelial lining of bottom of EAM; 2) endodermal epithelium lining of tympanic cavity; 3) intermediate layer of connective tissue.
The eardrum is composed of multiple cell layers because it represents the first pharyngeal membrane, and thus lies at the junction of the first pharyngeal pouch and cleft.

Middle Ear

The middle ear consists of an auditory tube (from the 1st pharyngeal pouch, along with tympanic cavity) and the ossicles (from pharyngeal arches 1 and 2 cartilage).  
The first arch cartilage forms the malleus and incus.  The tensor tympani (muscle of the malleus) is derived from the fourth somitomere (associated with the first arch) and is therefore innervated by CN V.
The second arch cartilage forms the stapes.  The stapedius (muscles of the stapes) is derived from the sixth somitomere (associated with the second arch) and is therefore innervated by CN VII.
The ossicles are initially embedded in mesenchyme, but in the 8th month, the mesenchyme degenerates and an endodermal epithelial lining of the tympanic cavity envelops the ossicles and connects them to the wall of the cavity in a mesentery-like fashion.

Inner Ear

The inner ear is derived thickening of surface ectoderm on both sides of the hindbrain (otic placodes).  The placodes invaginate to form otic vesicles (otocytes).  The vesicles then divide into ventral and dorsal components.
The ventral component forms the saccule and cochlear duct.
The dorsal component forms the utricle and semicircular canals and endolymphatic duct.

Cochlear Duct

Derived from an outgrowth of the saccule during the 6th week.  The outgrowth penetrates the surrounding mesenchyme in a spiral fashion.  The surrounding mesenchyme forms the cartilage and undergoes vacuolization.
The scala vestibule and scale tympani form and surround the cochlear duct.  They are filled with periplymp to receive mechanical vibrations of ossicles. The mechanical stimuli activates sensory (ciliary) cells in the cochlear duct.  

Semicircular canals

The utricle is initially three flattened outpocketings, which lose the central core.  From this three semicircular canals are forms, each at 90 degree angles from one another.  Sensory cells arise in the ampulla at one end of each canal, in the utricle and saccule.