Middle Ear

 Features

• The middle ear is also called the tympanic cavity, or tympanum.
• The middle ear is a narrow air filled space situated in the petrous part of the temporal bone between the external ear and the internal ear 

Shape and Size

• The middle ear is shaped like a cube. 
• Its lateral and medial walls are large, but the other walls are narrow, because the cube is compressed from side to side. 
• Its vertical and anteroposterior diameters are both about 15 mm. 
• When seen in coronal section the cavity of the middle ear is biconcave, as the medial and lateral walls are closest to each other in the centre. 
• The distances separating them are 6 mm near the roof, 2 mm in the centre, and 4 mm near the floor

Parts

• The cavity of the middle ear can be subdivided into the tympanic cavity proper which is opposite the tympanic membrane; and 
• the epitympanic recess which lies above the level of the tympanic membrane.

Communications

• The middle ear communicates anteriorly with the nasopharyrnx through the auditory tube, and 

• posteriorly with the mastoid antrum and mastoid air cells through the aditus to the mastoid antrum 

CONTENTS

The middle ear contains the following.

1. Three small bones or ossicles namely the malleus, the incus and the stapes. 

The upper half of the malleus, and the greater part of the incus lie in the epitympanic recess.

2. Ligaments of the ear ossicles.

3. Two muscles, the tensor tympani and the stapedius.

4. Vessels supplying and draining the middle ear.

5. Nerves: Chorda tympani and tympanic plexus.

6. Air.

• The mucous membrane lining the middle ear cavity invests all the contents and forms several vascular folds which project into the cavity. 
• This gives the cavity a honeycombed appearance.

BOUNDARIES

Roof or Tegmental wall

1. The roof separates the middle ear from the middle cranial fossa. 

• It is formed by a thin plate of bone called the tegmen tympani. 
• This plate is prolonged backwards as the roof of the canal for the tensor tympani.

2. In young children, the roof presents a gap at the unossified petrosquamous suture where the middle ear is in direct contact with the meninges. 

• In adults, the suture is ossified and transmits a vein from the middle ear to the superior petrosal sinus.

Floor or Jugular Wall

• The floor is formed by a thin plate of bone which separates the middle ear from the superior bulb of the internal jugular vein. 
• This plate is a part of the temporal bone.
• Near the medial wall, the floor presents the tympanic canaliculus which transmits the tympanic branch of the glossopharyngeal nerve to the medial wall of the middle ear.

Anterior or Carotid Wall

• The anterior wall is narrow due to the approximation of the medial and lateral walls, and because of descent of the roof.
• The uppermost part of the anterior wall bears the opening of the canal for the tensor tympani.
• The middle part has the opening of the auditory tube.
• The inferior part of the wall is formed by a thin plate of bone which forms the posterior wall of the carotid canal. 
• The plate separates the middle ear from the internal carotid artery. 
• This plate of bone is perforated by the superior and inferior sympathetic caroticotympanic nerves and the tympanic branch of the internal carotid artery 
• The bony septum between the canals for the tensor tympani and for the auditory tube is continued posteriorly on the medial wall as a curved lamina called the processes cochleariformis. 
• Its posterior end forms a pulley around which the tendon of the tensor tympani turns laterally to reach the upper part of the handle of the malleus.

Posterior or Mastoid walI

The posterior wall presents these features from above downwards.

1. Superiorly, there is an opening or aditus through which the epitympanic recess communicates with the mastoid or tympanic antrum.

2. The fossa incudis is a depression which lodges the short process of the incus.

3.  A conical projection, called the pyramid, lies near the junction of the posterior and medial walls. 

It has an opening at its apex for passage of the tendon of the stapedius muscle.

4. Lateral to pyramid and near the posterior edge of the tympanic membrane, is the posterior canaliculus for the chorda tympani through which the nerve enters the middle ear cavity

Lateral or Membranous Wall

1. The lateral wall separates the middle ear from the external acoustic meatus. 

It is formed:

a. Mainly by the tympanic membrane along with the tympanic ring and sulcus.

b. Partly by the squamous temporal bone, in the region of the epitympanic recess.

2. Near the tympanic notch, there are two small apertures.

a. The petrotympanic fissure lies in front of the upper

end of the bony rim. It lodges the anterior process of the malleus and transmits the tympanic branch of the maxillary artery.

b. The anterior canaliculus for the chordatympani nerve lies either in the fissure or just in front of it. 

The  nerve leaves the middle ear through this canaliculus to emerge at the base of the skull

Medial or Labyrinthine Wall

The medial wall separates the middle ear from the internal ear. It presents the following features.

1. The promontory is a rounded bulging produced by the first turn of the cochlea.

It is grooved by the tympanic plexus.

2. The fenestra vestibuli is an oval opening posterosuperior to the promontory. 

It leads into the vestibule of the internal ear and is closed by the foot-plate of the stapes.

3. The prominence of the facial canal runs backwards just above the fenestra vestibuli, to reach the lower margin of the aditus. 

The canal then descends behind the posterior wall to end at the stylomastoid foramen.

4. The fenestra cochleae is a round opening at the bottom of a depression posteroinferior to the promontory.

It opens into the scala tympani of the cochlea, and is closed by the secondary tympanic membrane.

5. The sinus tympani is a depression behind the promontory, opposite the ampulla of the posterior semicircular canal.

6. The processus cochleariformis.

7. Prominence of lateral semicircular canal above the facial canal.

Arterial Supply

The main arteries of the middle ear are as follows.

1. The anterior tympanic branch of the maxillary artery

which enters the middle ear through the petrotympanic fissure.

2. The posterior tympanic branch of the stylomastoid branch of the posterior auricular artery which enters through the stylomastoid foramen.

Venous Drainage

Veins from the middle ear drain into the superior petrosal sinus and the pterygoid plexus of the veins.

Lymphatic Drainage

Lymphatics pass to the preauricular and retropharyngeal lymph nodes.

Nerve Supply

The nerve supply is derived from the tympanic plexus which lies over the promontory. The plexus is formed by the following.

1. The tympanic branch of the glossopharyngeal nerve.

Its fibres are distributed to the mucous membrane of the middle ear, the auditory tube, the mastoid antrum and air cells. It also gives off the lesser petrosal nerve.

2. The superior and inferior caroticotympanic nerves

arise from the sympathetic plexus around the internal carotid artery. These fibres are vasomotor to the mucous membrane.

FUNCTIONS OF THE MIDDLE EAR

1. It transmits sound waves from the external ear to the internal ear through the chain of ear ossicles, and

thus transforms the air-borne vibrations from the tympanic membrane to liquid-borne vibrations in the

internal ear.

2. The intensity of the sound waves is increased ten times by the ossicles. It may be noted that the

frequency of sound does not change.

Watch lectures on Youtube:

Middle Ear | Situation | Dimensions | Parts | Boundaries | Contents

Tympanic Membrane

 TYMPANIC MEMBRANE 

This is a thin, translucent partition between the external acoustic meatus and the middle ear.
It is oval in shape,
measuring 9 x 10 mm. 

It is placed obliquely at an angle of 55 degrees with the floor of the meatus.
It faces downwards, forwards and laterally
The membrane has outer and inner surfaces.
The outer surface of the membrane is lined by thin skin. 

It is concave.
The inner surface provides attachment to the handle of the malleus which extends up to its centre. 

The inner surface is convex.
The point of maximum convexity lies at the tip of the handle of the malleus and is called the umbo. 

The membrane is thickened at its circumference which is fixed to the tympanic sulcus of the temporal bone on the tympanic plate.
Superiorly, the sulcus is deficient.
Here the membrane is attached to the tympanic notch.
From the ends of the notch, two bands, the anterior and posterior malleolar folds, are prolonged to the lateral process of the malleus.
While the greater part of the tympanic membrane is tightly stretched and is, therefore, called the pars tensa, 

the part between the two malleolar folds is loose and is called the pars flaccida.
The pars flaccida is crossed internally by the chorda tympani 

This part is more liable to rupture than the pars tensa. 

The membrane is held tense by the inward pull of the tensor tympani muscle which is inserted into the upper end of the handle of the malleus. 

Structure 

The tympanic membrane is composed of the
following three layers: 

1. The outer cuticular layer of skin.
2. The middle fibrous layer made up of superficial radiating fibres and deep circular fibres.
The circular fibres are minimal at the centre and maximal at the periphery.
The fibrous layer is replaced by loose areolar tissue in the pars flaccida.
3. The inner mucous layer is lined by a low ciliated columnar epithelium. 

Blood Supply 

1. The outer surface is supplied by the deep auricular branch of the maxillary artery.
2. The inner surface is supplied by the anterior
tympanic branch of the maxillary artery and by the posterior tympanic branch of the stylomastoid branch of the posterior auricular artery.

Venous Drainage 

Veins from the outer surface drain into the external jugular vein.
Those from the inner surface drain into the transverse sinus and into the venous plexus around the auditory tube. 

Lymphatic Drainage 

Lymphatics pass to the preauricular and retropharyngeal lymph nodes. 

Nerve Supply 

I. Outer surface: The anteroinferior part is supplied by the auriculotemporal nerve, and the posterosuperior part by the auricular branch of the vagus nerve with a communicating branch from facial nerve.
2 Inner surface: This is supplied by the tympanic branch of the glossopharyngeal nerve through the tympanic plexus 

Clinical Anatomy 

For examination of the meatus and tympanic membrane, the auricle should be drawn upwards, backwards and slightly laterally.
However, in infants, the auricle is drawn downwards and backwards because the canal is only cartilaginous and the outer surface of the tympanic membrane is directed mainly downwards 

Boils and other infections of the external auditory meatus cause little swelling but are extremely painful, due to the fixity of the skin to the 

underlying bone and cartilage. 

Ear should be dried after head bath or swimming.
Irritation of the auricular branch of the vagus in the external ear by ear wax or syringing may reflexly produce persistent cough called eat cough, vomiting or even death due to sudden cardiac inhibition.
On the other hand, mild stimulation of this nerve may reflexly produce increased appetite.
Accumulation of wax in the external acoustic meatus is often a source of excessive itching,although fungal infection and foreign bodies should be excluded.
Troublesome impaction of large foreign bodies like seeds, grains, insects is common.
Syringing is done to remove these
Involvement of the ear in herpes zoster of the geniculate ganglion depends on the connection between the auricular branch of the vagus and the facial nerve within the petrous temporal bone. 

Small pieces of skin from the lobule of the pinna are commonly used for demonstration of lepra bacilli to confirm the diagnosis of leprosy.
Pinna is used as grafting material.
Hair on pinna in male represents y-linked inheritance. 

Infection of elastic cartilage may cause perichondritis.
Bleeding within the auricle occurs between the perichondrium and auricular cartilage.
If left untreated fibrosis occurs as haematoma compromises blood supply to cartilage. Fibrosis leads to "cauliflower ear". It is usually seen in wrestlers.
Tympanic membrane is divided into an upper smaller sector, the pars flaccida bounded by
anterior and posterior malleolar folds and a larger sector, the pars tensa. 

Behind pars flaccida lies the chorda tympani, so disease in pars flaccida should be treated carefully 

When the tympanic membrane is illuminated for examination, the concavity of the membrane produces a 'cone of light’ over the anteroinferior quadrant which is the farthest or deepest quadrant with its apex at the umbo. 

Through the membrane, one can see the underlying handle of the malleus and the long process of the incus.
The membrane is sometimes incised to drain pus present in the middle ear.
The procedure is called myringotomy. 

The incision for myringotomy is usually made in the posteroinferior quadrant of the membrane where the bulge is most prominent.
In giving an incision, it has to be remembered that the chorda tympani nerve runs downwards and forwards across the inner surface of the membrane, lateral to the long process of the incus, but medial to the neck of the malleus. 

If the nerve is injured taste from most of anterior two thirds of tongue is not perceived. Also salivation from submandibular and sublingual glands gets affected. 

Watch lectures on YouTube

Tympanic Membrane | Situation | Parts | Recess | Layers | Development | Deep Relations |Blood Supply

External Acoustic meatus

 Features 

The external auditory meatus conducts sound waves from the concha to the tympanic membrane.
The canal is S-shaped.
Its outer part is directed medially, forwards and upwards. The middle part is directed medially, backwards and upwards. 

The inner part is directed medially, forwards and downwards.
The meatus can be straightened for examination by pulling the auricle upwards, backwards and slightly laterally. 

The meatus or canal is about 24mm long,
of which the medial two-thirds or 16 mm is bony, and the lateral one-third or 8 mm is cartilaginous.
Due to the obliquity of the tympanic membrane, the anterior wall and floor are longer than the posterior wall and roof
The canal is oval in section.
The greatest diameter is vertical at the lateral end, and anteroposterior at the medial end.
The bony part is narrower than the cartilaginous part. The narrowest point, the isthmus, lies about 5 mm from the tympanic membrane. 

The bony part is formed by the tympanic plate of the temporal bone which is C-shaped in cross-section.
The posterosuperior part of the plate is deficient. 

Here the wall of the meatus is formed by a part of the squamous temporal bone.
The meatus is lined by thin skin, firmly adherent to the periosteum. 

The cartilaginous part is also C-shaped in section;
the gap of the 'C' is filled with fibrous tissue. The lining skin is adherent to the perichondrium, contains hairs, sebaceous glands, and ceruminous or wax glands. Ceruminous glands are modified sweat glands. 

Blood Supply 

The outer part of the canal is supplied by the superficial temporal and posterior auricular arteries, and
the inner part, by the deep auricular branch of the maxillary artery. 

Lymphatics 

The lymphatics pass to preauricular, postauricular and superficial cervical lymph nodes. 

Nerve Supply 

The skin lining the anterior half of the meatus is supplied by the auriculotemporal nerve, the posterior half, by the auricular branch of the vagus. 

Watch lectures on YouTube:

External Ear - Auricle | External Acoustic Meatus|Features |Blood Supply | Nerve Supply| Development

External Ear

 The external ear consists of:

. The auricle or pinna.
. The external acoustic meatus. 

AURICLE/PINNA 

The auricle is the part seen on the surface, The greater part of it is made up of a single crumpled plate of elastic cartilage which is lined on both sides by skin. 

However, the lowest part of the auricle is soft and consists only of fibrofatty tissue covered by skin
This part is called the lobule. 

The rest of the auricle is divided into four parts These are helix, antihelix, concha, tragus, scaphoid fossa 

the large depression called the concha; it leads into the external acoustic meatus.
In relation to the auricle, there are a number of muscles. These are all vestigial in man. 

In lower animals, the intrinsic muscles alter the shape of the auricle, while the extrinsic muscles move the auricle as a whole. 

Nerve Supply 

The upper two-thirds of the lateral surface of the auricle are supplied by the auriculotemporal nerve;
the lower one-third by the great auricular nerve 

The upper two-thirds of the medial surface are supplied by the lesser occipital nerve; 

the lower one-third by the great auricular nerve.
The root of the auricle is supplied by the auricular branch of the vagus 

The auricular muscles are supplied through branches of the facial nerve. 

BIood Supply 

The blood supply of the auricle is derived from the posterior auricular and superficial temporal arteries

The lymphatics drain into the preauricular, and postauricular lymph nodes. 

Watch lectures on YouTube:

External Ear - Auricle | External Acoustic Meatus|Features |Blood Supply | Nerve Supply| Development

Tongue

 The tongue is a muscular organ situated in the floor of the mouth. It is associated with the functions of 

(i) taste,

(ii) speech,

(iii) chewing, and  

(iv) deglutition. 

PARTS 

The tongue has: 

1. A root,
   
2. A tip, and
   
3. A body, which has:
   

• A curved upper surface or dorsum
  
• An inferior surface.
  The dorsum is divided into oral and pharyngeal parts by a V-shaped, the sulcus terminalis.
  The inferior surface is confined to the oral part only. 

The root is attached to the styloid process and soft palate above, and to mandible and the hyoid bone below.
Because of these attachments, we are not able to swallow the tongue itself. 

In between the mandible and hyoid bones, it is related to the geniohyoid and mylohyoid muscles.
The tip of the tongue forms the anterior free end which, at rest, lies behind the upper incisor teeth. 

The dorsum of the tongue is convex in all directions.
It is divided into
1. An oral part or anterior two-thirds.
2. A pharyngeal part or posterior one-third, 

by a faint V-shaped groove, the sulcus terminalis. The two limbs of the 'V' meet at a median pit, named the foramen caecum.
They run laterally and forwards up to the palatoglossal arches. 

The foramen caecum represents the site from which the thyroid diverticulum grows down in the embryo. The oral and pharyngeal parts of the tongue differ in their development, topography, structure, and function 

3. Small posterior most part 

The oral or papillary part of the tongue is placed on the floor of the mouth.
Its margins are free and in contact with the gums and teeth. 

Just in front of the palatoglossal arch, each margin shows 4 to 5 vertical folds, named the foliate papillae. The superior surface of the oral part shows a median furrow and is covered with papillae which make it rough.
The inferior surface is covered with a smooth mucous membrane, which shows a median fold called the frenulum linguae. 

On either side of the frenulum, there is a prominence produced by the deep lingual veins. 

More laterally there is a fold called the plica fimbriata that is directed forwards and medially towards the tip of the tongue 

The pharyngeal or lymphoid part of the tongue lies behind the palatoglossal arches and the sulcus terminalis.
Its posterior surface, sometimes called the base of the tongue, forms the anterior wall of the oropharynx. 

The mucous membrane has no papillae,
but has many lymphoid follicles that collectively constitute the lingual tonsil. 

Mucous glands are also present. 

The posterior most part of the tongue is connected to the epiglottis by three-folds of mucous membrane. 

These are the median glossoepiglottic fold and the right and left lateral glossoepiglottic folds. 

On either side of the median fold, there is a depression called the vallecula
The lateral folds separate the vallecula from the piriform fossa. 

PAPILLAE OF THE TONGUE 

• These are projections of mucous membrane or corium which give the anterior two-thirds of the tongue its characteristic roughness.
• These are of the following three types. 

1. Vallate or circumvallate papillae;
• They are large in size 1-2 mm in diameter and
• are 8-12 in number.
• They are situated immediately in front of the sulcus terminalis. 

• Each papilla is a cylindrical projection surrounded by a circular
  sulcus.
  
• The walls of the papilla have taste buds.
  

2. The fungiform papillae are numerous near the tip and margins of the tongue, but some of them are also scattered over the dorsum. 

• These are smaller than the vallate papillae but larger than the
  filiform papillae.
  
• Each papilla consists of a narrow pedicle and a large rounded
  head.
  
• They are distinguished by their bright red colour.
  

3. The filiform papillae or conical papillae cover the presulcal area of the dorsum of the tongue, and 

• give it a characteristic velvety appearance.
  
• They are the smallest and most numerous of the lingual
  papillae.
  
• Each is pointed and covered with keratin; the apex is often split
  into filamentous processes.
  

4. Few foliate papillae are also present. 

MUSCLES OF THE TONGUE 

• A middle fibrous septum divides the tongue into right and left halves.
• Each half contains four intrinsic and four extrinsic muscles. lntrinsic Muscles 

1. Superior longitudinal
   
2. Inferior longitudinal
   
3. Transverse
   
4. Vertical.
   

• The intrinsic muscles occupy the upper part of the tongue, 

and are attached to the submucous fibrous layer and to the median fibrous septum. 

• They alter the shape of the tongue.
  
• The superior longitudinal muscle lies beneath the mucous membrane.
  
• The inferior longitudinal muscle is a narrow band lying close to the inferior surface of the tongue between the genioglossus and the hyoglossus.
  
• The transverse muscle extends from the median septum to the margins.
  
• The vertical muscle is found at the borders of the anterior part of the tongue
  

Extrinsic Muscles 

1. Genioglossus
   
2. Hyoglossus
   
3. Styloglossus
   
4. Palatoglossus
   The extrinsic muscles connect the tongue to the mandible via genioglossus;
   to the hyoid bone through hyoglossus;
   to the styloid process via styloglossus, and
   to the palate via palatoglossus
   

Palatoglossus 

Origin - Oral surface of palatine aponeurosis
Insertion-
Descends in the palatoglossal arch to the side of tongue at the junction of oral and pharyngeal parts 

Nerve supply - the cranial root of the accessory nerve through the pharyngeal plexus.
Action - Pulls up the root of tongue, approximates the palatoglossal arches and thus closes the oropharyngeal isthmus 

Hyoglossus 

Origin - Whole length of greater cornua and lateral part of hyoid bone
Insertion - Side of tongue between styloglossus and inferior longitudinal muscle of tongue 

Nerve Supply - Hypoglossal Nerve 

Action - Depresses tongue, makes dorsum convex, retracts the protruded tongue 

Styloglossus 

Origin - Tip and part of anterior surface of styloid process
Insertion - lnto the side of tongue
Nerve Supply - Hypoglossal Nerve 

Action - Pulls tongue upwards and back wards, i.e. retracts the tongue 

Genioglossus 

fan shaped bulky muscle
Origin - Upper genial tubercle of mandible 

Insertion- 

Upper fibres into the tip of tongue 

Middle fibres into the dorsum 

Lower fibres into the hyoid bone 

Nerve Supply - Hypoglossal Nerve 

Action - 

upper fibres - Retracts the tongue
middle fibres - Depresses the tongue
lower fibres - Pulls the posterior part of tongue forwards and protrude the tongue forwards.
It is a life-saving muscle ( Safety muscle of the tongue ) 

Intrinsic muscles Actions

Superior longitudinal - Shortens the tongue makes its dorsum concave 

Inferior longitudinal   - Shortens the tongue makes its dorsum convex 

Transverse - Makes the tongue narrow and elongated

Vertical - Makes tongue broad and flattened. 

Extrinsic Muscles Actions.

Genioglossus - Protrudes the tongue

Hyoglossus  - Depresses the tongue

Styloglossus -  Retracts the tongue 

Palatoglossus - Elevates the tongue 

Arterial Supply of Tongue 

It is derived from the tortuous lingual artery a branch of the external carotid artery.
The root of the tongue is also supplied by the tonsillar artery a branch of facial artery, and ascending pharyngeal branch of external carotid 

Venous Drainage 

The arrangement of the vena comitantes/veins of the tongue is variable.
Two venae comitantes accompany the lingual artery, and one vena comitant accompanies the hypoglossal nerve. 

The deep lingual vein is the largest and principal vein of the tongue.
It is visible on the inferior surface of the tongue. It runs backwards and crosses the genioglossus and the hyoglossus below the hypoglossal nerve.
These veins unite at the posterior border of the hyoglossus to form the lingual vein which ends in the internal jugular vein. 

Lymphatic Drainage 

1. The tip of the tongue drains bilaterally to the submental nodes.
2 .The right and left halves of the remaining part of the anterior two-thirds of the tongue drain unilaterally to the submandibular nodes. 

A few central lymphatics drain bilaterally to the deep cervical nodes.
3. The posteriormost part and posterior one third of the tongue drain bilaterally into the upper deep cervical lymph nodes including jugulodigastric nodes. 

4. The whole lymph finally drains to the jugulo- omohyoid nodes. These are known as the lymph nodes of the tongue. 

Nerve Supply 

Motor Supply 

All the intrinsic and extrinsic muscles, except the palatoglossus, are supplied by the hypoglossal nerve. 

The palatoglossus is supplied by the cranial root of the accessory nerve through the pharyngeal plexus. 

Sensory Supply 

The lingual nerve is the nerve of general sensation and the chorda tympani is the nerve of taste for the anterior two-thirds of the tongue except vallate papillae 

The glossopharyngeal nerve is the nerve for both general sensation and taste for the posterior one- third of the tongue including the circumvallate papillae. 

The posteriormost part of the tongue is supplied by the vagus nerve through the internal laryngeal branch 

Clinical Anatomy:

Carcinoma of the tongue is quite common. The affected side of the tongue is removed surgically.
All the deep cervical lymph nodes are also removed, i.e. block dissection of neck because recurrence of malignant disease occurs in lymph nodes.
Carcinoma of the posterior one-third of the tongue is more dangerous due to bilateral lymphatic spread.
Sorbitrate is taken sublingually for immediate relief from angina pectoris. It is absorbed fast because of rich blood supply of the tongue and bypassing of portal circulation.
Genioglossus is called the 'safety muscle of the tongue, because if it is paralysed, the tongue will fall back on the oropharynx and block the air passage.
During anaesthesia, the tongue is pulled forwards to clear the air passage.
Genioglossus is the only muscle of the tongue which protrudes it forwards.
It is used for testing the integrity of hypoglossal nerve.
If hypoglossal nerve of right side is paralysed, the tongue on protrusion will deviate to the right side.
Normal Ieft genioglossus will pull the base to left side and apex will get pushed to right side. 

Histology :

The bulk of the tongue is made up of striated muscles.
2. The mucous membrane consists of a layer of connective tissue (corium), lined by stratified squamous
epithelium. 

On the oral part of the dorsum, it is thin, forms papillae, and is adherent to the muscles.
On the pharyngeal part of the dorsum, it is very rich in 

On the inferior surface, it is thin and smooth.
Numerous glands, both mucous and serous lie deep to the mucous membrane. 

3. Taste buds are most numerous on the sides of the circumvallate papillae, and on the walls of the surrounding sulci. 

Taste buds are numerous over the foliate papillae and over the posterior one-third of the tongue; and sparsely distributed on the fungiform papillae, the soft palate, the epiglottis and the pharynx. 

There are no taste buds on the middorsal region of the oral part of the tongue. 

Development :

Epithelium 

I. Anterior two-thirds: From two lingual swellings, which arise from the first branchial arch.
Therefore, it is supplied by lingual nerve (post trematic) of 1st arch and chorda tympani (pre trematic) of 2nd arch. 

2. Posterior one-third: From cranial large part of the hypobranchial eminence, 

Therefore, it is supplied nerve. i.e. from the third arch. by the glossopharyngeal
3. Posteriormost part - from the fourth arch. This is supplied by the vagus nerve. 

Muscles
The muscles develop from the occipital myotomes which are supplied by the hypoglossal nerve.

Connective Tissue

The connective tissue develops from the local mesenchyme. 

Taste Pathway 

1. The taste from anterior two-thirds of tongue except from vallate papillae is carried by chorda tympani branch of facial till the geniculate ganglion.
The central processes go to the tractus solitarius in the medulla. 

2. Taste from posterior one-third of tongue including the circumvallate papillae is carried by cranial nerve IX till the inferior ganglion.
The central processes also reach the tractus solitarius 3. Taste from posteriormost part of tongue and epiglottis travels through vagusnetve till the inferior ganglion of vagus. 

These central processes also reach tractus solitarius.
4. After a relay in tractus solitarius, the solitario-thalamic tract is formed which becomes a part of trigeminal lemniscus and 

reaches postero-ventromedial nucleus of thalamus of the opposite side. Another relay here takes them to lowest part of postcentral gyrus, which is the area for taste. 

Clinical Anatomy 

Injury to any part of the pathway causes abnormality in appreciation of taste. 

Referred pain is felt in the ear in diseases of posterior part of the tongue, as ninth and tenth nerves are common supply to both the regions. 

Watch lectures on YouTube:

The Tongue | Parts| Features| Papillae| Muscles |Nerve & Blood supply|Lymphatic Drainage|Development