NEET MDS Lessons
Anatomy
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:
- The auricular branch of the auriculotemporal nerve (derived from the mandibular, CN V3).
- The facial nerve (CN VII) by the branches from the tympanic plexus.
- 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.
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.
The Masseter Muscle
- This is a quadrangular muscle that covers the lateral aspect of the ramus and the coronoid process of the mandible.
- Origin: inferior border and medial surface of zygomatic arch.
- Insertion: lateral surface of ramus of mandible and its coronoid process.
- Innervation: mandibular nerve via masseteric nerve that enters its deep surface.
- It elevates and protrudes the mandible, closes the jaws and the deep fibres retrude it.
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.
Movements of the Temporomandibular Joint
- The two movements that occur at this joint are anterior gliding and a hinge-like rotation.
- When the mandible is depressed during opening of the mouth, the head of the mandible and articular disc move anteriorly on the articular surface until the head lies inferior to the articular tubercle.
- As this anterior gliding occurs, the head of the mandible rotates on the inferior surface of the articular disc.
- This permits simple chewing or grinding movements over a small range.
- Movements that are seen in this joint are: depression, elevation, protrusion, retraction and grinding
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| Motor Innervation | All muscles by hypoglossal nerve (CN XII) except palatoglossus muscle (by the pharyngeal plexus) | ||
| General Sensory Innervation |
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| Special Sensory Innervation |
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The Articular Capsule
- The capsule of this joint is loose.
- The thin fibrous capsule is attached to the margins of the articular area on the temporal bone and around the neck of the mandible.