Structure of the Nasal Septum

• This part bony, part cartilaginous septum divides the chamber of the nose into two narrow nasal cavities.
• The bony part of the septum is usually located in the median plane until age 7; thereafter, it often deviates to one side, usually the right.

• The nasal septum has three main components: (1) the perpendicular plate of the ethmoid bone; (2) the vomer, and (3) the septal cartilage.

• The perpendicular plate, which forms the superior part of the septum, is very thin and descends from the cribiform plate of the ethmoid bone.
• The vomer, which forms the posteroinferior part of the septum, is a thin, flat bone. It articulates with the sphenoid, maxilla and palatine bones.

•  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

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. 

• The six muscles rotate the eyeball in the orbit around three axes (sagittal, horizontal and vertical).
• The action of the muscles can be deduced by their site of insertion on the eyeball.

The Palate

• The palate forms the arched roof of the mouth and the floor of the nasal cavities.
• The palate consists of two regions: the anterior 2/3 or bony part, called the hard palate, and the mobile posterior 1/3 or fibromuscular part, known as the soft palate.

The Hard Palate

• The anterior bony part of the palate is formed by the palatine process of the maxillae and the horizontal plates of the palatine bones.

• Anteriorly and laterally, the hard palate is bounded by the alveolar processes and the gingivae.
• Posteriorly, the hard palate is continuous with the soft palate.

• The incisive foramen is the mouth of the incisive canal.
• This foramen is located posterior to the maxillary central incisor teeth.
• This foramen is the common opening for the right and left incisive canals.
• The incisive canal and foramen transmit the nasopalatine nerve and the terminal branches of the sphenopalatine artery.

• Medial to the third molar tooth, the greater palatine foramen pierces the lateral border of the bony palate.
• The greater palatine vessels and nerve emerge from this foramen and run anteriorly into two grooves on the palate.

• The lesser palatine foramen transmits the lesser palatine nerve and vessels.
• This runs to the soft palate and adjacent structures.

The Oral Cavity

• The oral cavity (mouth) consists of two parts: the vestibule and the mouth proper.

• The vestibule is the slit-like spaced between the cheeks and the lips and the teeth and gingivae.
• It is the entrance of the digestive tract and is also used for breathing.
• The vestibule communicates with the exterior through the orifice of the mouth.

• The oral cavity is bounded:
• Externally: by the cheeks and lips.
• Roof of oral cavity: formed by the palate.
• Posteriorly: the oral cavity communicates with the oropharynx.