The Tongue

• The tongue (L. lingua; G. glossa) is a highly mobile muscular organ that can vary greatly in shape.
• It consists of three parts, a root, body, and tip.

• The tongue is concerned with mastication, taste, deglutition (swallowing), articulation (speech), and oral cleansing.
• Its main functions are squeezing food into the pharynx when swallowing, and forming words during speech.

Gross Features of the Tongue

• The dorsum of the tongue is divided by a V-shaped sulcus terminalis into anterior oral (presulcal) and posterior pharyngeal (postsulcal) parts.
• The apex of the V is posterior and the two limbs diverge anteriorly.
• The oral part forms about 2/3 of the tongue and the pharyngeal part forms about 1/3.

Oral Part of the Tongue

• This part is freely movable, but it is loosely attached to the floor of the mouth by the lingual frenulum.

• On each side of the frenulum is a deep lingual vein, visible as a blue line.
• It begins at the tip of the tongue and runs posteriorly.
• All the veins on one side of the tongue unite at the posterior border of the hyoglossus muscle to form the lingual vein, which joins the facial vein or the internal jugular vein.

• On the dorsum of the oral part of the tongue is a median groove.
• This groove represents the site of fusion of the distal tongue buds during embryonic development.

The Lingual Papillae and Taste Buds

• The filiform papillae (L. filum, thread) are numerous, rough, and thread-like.
• They are arranged in rows parallel to the sulcus terminalis.

• The fungiform papillae are small and mushroom-shaped.
• They usually appear are pink or red spots.

• The vallate (circumvallate) papillae are surrounded by a deep, circular trench (trough), the walls of which are studded with taste buds.

• The foliate papillae are small lateral folds of lingual mucosa that are poorly formed in humans.

• The vallate, foliate and most of the fungiform papillae contain taste receptors, which are located in the taste buds.

The Pharyngeal Part of the Tongue

• This part lies posterior to the sulcus terminalis and palatoglossal arches.
• Its mucous membrane has no papillae.

• The underlying nodules of lymphoid tissue give this part of the tongue a cobblestone appearance.
• The lymphoid nodules (lingual follicles) are collectively known as the lingual tonsil.

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.

Intrinsic muscles

    all innervated by recurrent laryngeal nerve except cricothyroid: external laryngeal nerve
    adductors of vocal folds: bring folds together at midline

    Transverse and oblique arytenoids: pull arytenoids together
    Lateral cricoartenoids: spin and slide arytenoids up
    only one abductor of vocal folds
    
    Posterior cricoarytenoids—down and up
    adjustors of shape and tension of vocal folds

    Cricothyroid muscle
    
o    superficial to lateral cricoarytenoid
o    tenses vocal folds by tilting thyroid cartilage forward and sliding forward

    Thyroartenoid and vocalis muscles
    
o    vocalis: sometimes treated as medial most fibers of thyroartenoid muscle
o    different fiber directions
    
    lateral: adduct
    medial: change shape of folds
    control voice by bring bringing together different parts of folds

o    as move from epithelium to vocalis muscle, fold becomes stiffer
o    near connections, vocal folds are stiffer
o    vocal fold: complex, multilayered vibrator

Endochondral ossification

• A cartilage model exists
• Through intramembraneous ossification in the perichondrium a collar of bone forms around the middle part of the cartilage model
• The perichondrium change to a periostium
• The bone collar cuts off the nutrient and oxygen supply to the chondrocytes in the cartilage model
• The chondrocytes then increase in size and resorb the surrounding cartilage matrix until only thin vertical septae of matrix are left over
• These thin plates then calcify after which the chondrocytes die
• The osteoclasts make holes in the bone collar through which blood vessels can now enter the cavities left behind by the chondrocytes
• With the blood vessels osteoprogenitor cells enter the tissue
• They position themselves on the calcified cartilage septae, change into osteoblasts and start to deposit bone to form trabeculae
• In the mean time the periosteum is depositing bone on the outside of the bone collar making it thicker and thicker
• The trabeculae,consisting of a core of calcified cartilage with bone deposited on top of it, are eventually resorbed by osteoclasts to form the marrow cavity
• The area where this happens is the primary ossification centre and lies in what is called the diaphysis (shaft)
• This process spreads in two directions towards the two ends of the bone the epiphysis
• In the two ends (heads) of the bone a similar process takes place
• A secondary ossification centre develops from where ossification spreads radially
• Here no bone collar forms
• The outer layer of the original cartilage remains behind to form the articulating cartilage
• Between the primary and the secondary ossification centers two epiphyseal cartilage plates remain
• This is where the bone grows in length

• From the epiphyseal cartilage plate towards the diaphysis a number of zones can be identified:

 Resting zone of cartilage

 Hyaline cartilage

 Proliferation zone

 Chondrocytes divide to form columns of cells that mature.

Hypertrophic cartilage zone

 Chondrocytes become larger, accumulate glycogen, resorb the surrounding matrix so that only thin septae of cartilage remain 

Calcification and degeneration zone

The thin septae of cartilage become calcified.

The calsified septae cut off the nutrient supply to the chondrocytes so subsequently they die.

Ossification zone.

Osteoclasts make openings in the bone collar through which blood vessels then invade the spaces left vacant by the chondrocytes that died.

Osteoprogenitor cells come in with the blood and position themselves on the calcified cartilage

septae, change into osteoblasts and start to deposit bone.

 When osteoblasts become trapped in bone they change to osteocytes.

Growth and remodeling of bone

Long bones become longer because of growth at the epiphyseal plates

They become wider because of bone formed by the periosteum

The marrow cavity becomes bigger because of resorbtion by the osteoclasts

Fracture repair

When bone is fractured a blood clot forms

 Macrophages then remove the clot, remaining osteocytes and damaged bone matrix

The periosteum and endosteum produce osteoprogenitor cells that form a cellular tissue in the fracture area

 Intramembranous and endochondral ossification then take place in this area forming trabeculae.

Trabeculae connect the two ends of the broken bone to form a callus

Remodelling then takes place to restore the bone as it was

Joints

The capsule of a joint seals off the articular cavity,  

The capsule has two layers

 fibrous (outer)

synovial (inner)

The synovial layer is lined by squamous or cuboidal epithelial cells,  Under this layer is a layer of loose or dense CT, The lining cells consists of two types:

- A cells

- B cells

They secrete the synovial fluid

They are different stages of the same cell, They are also phagocytic., The articular cartilage has fibres that run perpendicular to the bone and then turn to run parallel to the surface

Eye 

At week 4, two depressions are evident on each of the forebrain hemispheres.  As the anterior neural fold closes, the optic pits elongate to form the optic vesicles.  The optic vesicles remain connected to the forebrain by optic stalks. 
The invagination of the optic vesicles forms a bilayered optic cup.  The bilayered cup becomes the dual layered retina (neural and pigmented layer)
Surface ectoderm forms the lens placode, which invaginates with the optic cup.
The optic stalk is deficient ventrally to contain choroids fissure to allow blood vessels into the eye (hyaloid artery).  The artery feeds the growing lens, but will its distal portion will eventually degenerate such that the adult lens receives no hyaloid vasculature.
At the 7th week, the choroids fissure closes and walls fuse as the retinal nerve get bigger.
The anterior rim of the optic vesicles forms the retina and iris.  The iris is an outgrowth of the distal edge of the retina.
Optic vesicles induces/maintains the development of the lens vesicle, which forms the definitive lens.  Following separation of the lens vesicle from the surface ectoderm, the cornea develops in the anterior 1/5th of the eye.
The lens and retina are surrounded by mesenchyme which forms a tough connective tissue, the sclera, that is continuous with the dura mater around the optic nerve.  
Iridopupillary membrane forms to separate the anterior and posterior chambers of the eye.  The membrane breaks down to allow for the pupil
Mesenchyme surrounding the forming eye forms musculature (ciliary muscles and pupillary muscles – from somitomeres 1 and 2; innervated by CN III), supportive connective tissue elements and vasculature.

Eyelids

Formed by an outgrowth of ectoderm that is fused at its midline in the 2nd trimester, but later reopen.

Nerves of the Palate

• The sensory nerves of the palate, which are branches of the pterygopalatine ganglion, are the greater and lesser palatine nerves.
• They accompany the arteries through the greater and lesser palatine foramina, respectively.

• The greater palatine nerve supplies the gingivae, mucous membrane, and glands of the hard palate.

• The lesser palatine nerve supplies the soft palate.

• Another branch of the pterygopalatine ganglion, the nasopalatine nerve, emerges from the incisive foramen and supplies the mucous membrane of the anterior part of the hard palate.