Structure of the Skin β The Bodyβs Protective Barrier
The skin is the largest organ of the human body. It covers the entire body and performs multiple essential functions including protection, sensation, temperature regulation, and immune defense. Structurally, the skin is composed of three main layers: epidermis, dermis, and hypodermis (subcutaneous tissue).
1. Epidermis β The Outer Protective Layer
The epidermis is the superficial, avascular layer made of stratified squamous keratinized epithelium. It provides a barrier against environmental hazards like pathogens, UV radiation, and dehydration.
Layers of the Epidermis (from deep to superficial):
Remember: βCome Letβs Get Sun Burnedβ
Stratum basale (basal layer)
Deepest layer; single row of stem cells
Contains melanocytes, keratinocytes, and Merkel cells
Site of cell division (mitosis)
Stratum spinosum
Several layers of keratinocytes
Contains Langerhans cells (immune function)
Cells connected by desmosomes
Stratum granulosum
3β5 cell layers with keratohyalin granules
Cells begin to die and become flat
Stratum lucidum
Thin, transparent layer
Present only in thick skin (palms, soles)
Stratum corneum
Outermost layer of dead keratinized cells
Continuously shed and replaced
Forms a protective barrier
Special Cells in the Epidermis
Keratinocytes: Main cells, produce keratin (waterproof protein)
Melanocytes: Produce melanin, which gives skin color and protects against UV radiation
Langerhans cells: Antigen-presenting immune cells
Merkel cells: Touch-sensitive mechanoreceptors
2. Dermis β The Strength and Support Layer
The dermis lies beneath the epidermis and is vascularized, thicker, and made of connective tissue, collagen, and elastic fibers. It supports the epidermis and houses sensory structures, blood vessels, and appendages like hair follicles and glands.
Pressure ulcers: Occur due to prolonged pressure affecting blood supply to dermis/hypodermis
Injections:
Intradermal: Into dermis (e.g., Mantoux test)
Subcutaneous: Into hypodermis (e.g., insulin, heparin)
Quick Recap Mnemonic:
βEvery Dog Has Super Skinβ β Epidermis β Dermis β Hypodermis β Sensation β Sweat
Anatomy of the Human Eye β The Organ of Sight
The human eye is a complex, spherical sensory organ that captures light and transforms it into electrical signals that are interpreted by the brain as vision. It is a precise optical system and a part of the nervous system, protected and supported by accessory structures.
Location and General Description
Situated in the bony orbit of the skull
Maintained in position by six extraocular muscles, fatty tissue, and connective support
Approximately 2.5 cm in diameter
Weight: ~7.5 grams
1. Fibrous Tunic (Outer Layer of the Eye)
The fibrous tunic is the outermost protective layer of the eyeball. It provides shape, support, and mechanical protection, and serves as an attachment site for extraocular muscles. This layer is avascular (contains no blood vessels) and is made up of dense connective tissue.
It has two main parts:
A. Sclera β βThe White of the Eyeβ
Location & Structure
Forms the posterior 5/6 of the outer layer
Thick, opaque, white tissue
Composed of collagen and elastic fibers
Functions
Maintains the shape of the eyeball (important for proper vision)
Acts as a protective outer shell
Provides attachment for 6 extraocular muscles that move the eye
Helps resist internal pressure from fluids
Features
Covered externally by episclera (a thin vascular connective tissue)
Internally lined by suprachoroid lamina, in contact with the choroid
Pierced posteriorly by the optic nerve
Clinical Note
Scleritis: Inflammation of the sclera, often painful and associated with autoimmune disease
Blue sclera: Seen in conditions like osteogenesis imperfecta
B. Cornea β βThe Clear Window of the Eyeβ
Location & Structure
Forms the anterior 1/6 of the fibrous tunic
Transparent, dome-shaped structure
Composed of five microscopic layers:
Epithelium (outermost) β protective barrier
Bowmanβs layer
Stroma β makes up ~90% of corneal thickness
Descemetβs membrane
Endothelium (innermost) β pumps out excess fluid
Avascular but Rich in Nerve Endings
Nourished by diffusion from:
Aqueous humor
Tear film
Capillaries at the limbus (cornea-sclera junction)
Functions
Main refractive surface of the eye (bends light toward the retina)
Provides a smooth, clear surface for light to pass through
Protects the inner eye from dust, germs, and injury
Features
Highly sensitive to pain and touch
Transparent due to its nonvascularity and regular arrangement of collagen fibers
Clinical Note
Corneal abrasion: Scratch on the surface epithelium β painful, heals quickly
Keratitis: Inflammation of the cornea
Corneal ulcer: Infection leading to tissue damage
LASIK surgery: Reshapes the cornea to correct refractive errors
Junction Between Sclera and Cornea: The Limbus
Also called the corneoscleral junction
Important site for aqueous humor drainage
Contains the Canal of Schlemm, which plays a role in intraocular pressure regulation
Summary β Fibrous Tunic at a Glance
Part
Coverage
Function
Special Features
Sclera
Posterior 5/6
Shape, protection, muscle anchor
White, opaque, tough connective tissue
Cornea
Anterior 1/6
Refraction, barrier function
Transparent, highly innervated, no blood vessels
2. Vascular Tunic (Middle Layer / Uvea)
The vascular tunic, also known as the uvea, is the middle layer of the eyeball. It is rich in blood vessels and pigment, providing nutrients, oxygen, and light absorption for the inner layers of the eye.
This layer lies between the fibrous tunic (sclera and cornea) and the inner nervous tunic (retina), and it is composed of three continuous parts:
A. Choroid β The Nourishing Layer
Location
Lies between the sclera and retina
Extends from the optic disc at the back to the ciliary body in the front
Structure
Darkly pigmented, vascular membrane
Contains melanocytes, loose connective tissue, and many capillaries
Has four layers microscopically, including Bruchβs membrane (next to the retina)
Functions
Supplies oxygen and nutrients to the outer layers of the retina (esp. photoreceptors)
The melanin pigment absorbs stray light to prevent internal light reflection and glare
Assists in thermoregulation of the retina
Clinical Relevance:
Choroiditis: Inflammation, often part of uveitis
Choroidal neovascularization: Seen in age-related macular degeneration
B. Ciliary Body β The Focus Controller & Fluid Producer
Location
Extends from the choroid to the base of the iris
Forms a ring around the lens
Structure
Composed of:
Ciliary muscle: smooth muscle
Ciliary processes: finger-like projections
Suspensory ligaments (zonular fibers): connect to the lens
Functions
Accommodation:
Ciliary muscle contracts β lens becomes thicker β focuses on near objects
The color of the iris depends on the amount and type of melanin:
High melanin = brown eyes
Low melanin = blue/green eyes
Functions
Regulates the diameter of the pupil and thus the amount of light entering the eye
Protects retina from excessive light
Helps enhance contrast and visual sharpness
Clinical Relevance:
Iritis: Inflammation, often painful and light-sensitive
Anisocoria: Unequal pupil sizes β may indicate neurological problem
Mydriasis: Pupil dilation (e.g., from drugs or trauma)
Miosis: Pupil constriction (e.g., opioid effect)
Uvea Summary Table
Part
Location
Main Function
Choroid
Between sclera and retina
Nourishes outer retina; light absorption
Ciliary Body
Ring between choroid and iris
Accommodation; aqueous humor secretion
Iris
Front of eye, around pupil
Controls pupil size and light entry
Mnemonic to Remember Uvea Parts:
βCICβ β Choroid, Iris, Ciliary body Or βI See Clearlyβ β Iris, Sclera (connects), Ciliary body
3. Nervous Tunic (Inner Layer of the Eye) β The Retina
The nervous tunic, also known as the retina, is the innermost and most crucial layer of the eyeball, responsible for receiving visual images and transmitting them to the brain. It contains specialized photoreceptor cells that convert light energy into electrical impulses, which are carried by the optic nerve to the visual cortex.
Location & General Features
Lies between the choroid (middle layer) and the vitreous body
It is attached firmly at two points:
Optic disc (where the optic nerve exits)
Ora serrata (anterior edge of the retina)
Thin, delicate, transparent neural tissue
Microscopic Layers of the Retina
The retina consists of 10 distinct layers, but they can be functionally grouped into three major layers:
1. Photoreceptor Layer
Contains two types of light-sensitive cells:
Rods β ~120 million, for dim light (night vision) and peripheral vision
Cones β ~6 million, for color vision, fine detail, and daylight (bright light)
Rods are concentrated in the periphery
Cones are concentrated in the fovea centralis
2. Bipolar Cell Layer
Transmits signals from photoreceptors to ganglion cells
Acts as a relay center in the retinal pathway
3. Ganglion Cell Layer
Axons from these cells bundle together to form the optic nerve (CN II)
These carry visual signals to the brainβs occipital lobe
Other Supporting Cells in Retina:
Horizontal cells: Integrate signals between photoreceptors
Amacrine cells: Modify signals between bipolar and ganglion cells
MΓΌller cells: Provide structural and metabolic support
Specialized Areas of the Retina
A. Macula Lutea
Small, yellowish central area of retina
Responsible for sharp central vision
Rich in cones
B. Fovea Centralis
Center of the macula
Contains only cones, no rods
Area of maximum visual acuity
C. Optic Disc (Blind Spot)
Point where optic nerve exits the eye
No photoreceptors present β insensitive to light
Appears pale on fundus examination
Retinal Pigmented Epithelium (RPE)
Lies just outside the photoreceptor layer
Absorbs stray light to enhance image clarity
Helps in nutrient transport, waste removal, and photoreceptor maintenance
Function of the Retina
Phototransduction: Converts light into electrical impulses
Signals pass through:
Photoreceptors (rods & cones)
Bipolar cells
Ganglion cells
Optic nerve
Brain (visual cortex in occipital lobe)
Clinical Relevance
Condition
Involvement
Retinal detachment
Separation of retina from choroid
Diabetic retinopathy
Damage to retinal blood vessels
Macular degeneration
Degeneration of the fovea
Retinitis pigmentosa
Genetic loss of photoreceptors
Glaucoma
Ganglion cell death due to pressure
Papilledema
Swelling of optic disc due to β ICP
Retina = inner sensory layer of the eye
Contains rods (night vision) and cones (color vision)
Key areas: macula, fovea, optic disc
Converts light into neural signals β sent to optic nerve β brain
Internal Structures of the Eye β The Optical Apparatus
The internal structures of the eye work collectively to refract (bend) and focus light rays onto the retina, facilitating clear and sharp vision.
1. Lens
A biconvex, transparent, avascular structure located behind the iris and in front of the vitreous humor
Held in place by zonular fibers (suspensory ligaments) attached to the ciliary body
Composed of a capsule, epithelial layer, and lens fibers (crystalline proteins)
Capable of changing shape to focus on near or distant objects (accommodation)
Accommodation Mechanism:
For near objects: ciliary muscles contract, tension on ligaments decreases β lens thickens
Anatomy of the Human Ear β The Organ of Hearing and Balance
The ear serves two essential functions:
Hearing (auditory function)
Balance (equilibrium or vestibular function)
The ear is anatomically divided into three main regions:
1. External Ear 2. Middle Ear 3. Inner Ear
External Ear β Structure and Function
The external ear is the outermost portion of the ear that is visible externally and functions to collect and direct sound waves into the ear canal toward the tympanic membrane (eardrum).
It consists of three main components:
1. Auricle (Pinna)
Structure
A cartilaginous, funnel-shaped structure covered by skin
Made of elastic cartilage except for the earlobe (lobule), which is soft and fatty
Numerous grooves and ridges provide structural complexity
Parts of the Auricle:
Helix: the outer rim
Antihelix: a curved prominence parallel to the helix
Tragus: small projection in front of the external auditory canal
Antitragus: opposite the tragus, separated by intertragic notch
Concha: deep cavity leading to the ear canal
Lobule (earlobe): lower, soft portion with rich blood supply
Function
Acts like a satellite dish, collecting sound waves and funneling them toward the ear canal
Its unique shape helps to localize sound direction
Extends from the concha of auricle to the tympanic membrane
Composed of:
Outer 1/3: cartilaginous portion (lined with skin, hair follicles, and ceruminous glands)
Inner 2/3: bony portion (within the temporal bone)
Ceruminous Glands:
Modified sweat glands that produce cerumen (earwax)
Cerumen + hair trap dust, debris, and microbes
Function
Conducts sound vibrations to the eardrum
Protects deeper structures by:
Maintaining an acidic environment (prevents microbial growth)
Self-cleaning via migration of epithelial cells
3. Tympanic Membrane (Eardrum)
Though technically a boundary between the external and middle ear, it is often included in the external ear for anatomical teaching.
Structure
Thin, cone-shaped semi-transparent membrane
Made up of three layers:
Outer layer β continuous with skin of the canal
Middle layer β fibrous connective tissue
Inner layer β continuous with mucosa of middle ear
It is anchored to the bony canal by the tympanic ring
Function
Vibrates in response to incoming sound waves
Transmits mechanical energy to malleus (ear ossicle) in the middle ear
Clinical Relevance
Condition
Description
Otitis externa
Infection of the external ear canal (swimmerβs ear); redness, itching, pain
Impacted cerumen
Excessive earwax can block sound and cause hearing loss
Tympanic membrane perforation
Tear in the eardrum due to trauma, infection, or pressure changes (barotrauma)
Congenital malformations
Abnormal development of auricle or canal β may affect hearing
Innervation (Nerve Supply)
Auriculotemporal nerve (branch of CN V3 β mandibular nerve)
Great auricular nerve (from cervical plexus)
Auricular branch of the vagus nerve (Arnoldβs nerve) β may cause cough reflex when cleaning ears!
Blood Supply
Posterior auricular artery and superficial temporal artery
Veins drain into the external jugular vein
Summary
Part
Structure
Function
Auricle (Pinna)
Cartilage + skin
Collects sound
Auditory Canal
Cartilage + bony tube
Channels sound; produces cerumen
Tympanic Membrane
Thin trilaminar membrane
Vibrates to transmit sound to ossicles
Middle Ear (Tympanic Cavity) β Structure and Function
The middle ear is an air-filled cavity located within the petrous part of the temporal bone, situated between the external ear and the inner ear. It functions as a mechanical amplifier, transmitting sound vibrations from the eardrum to the inner ear.
Boundaries of the Middle Ear
The middle ear cavity is shaped like a box and has six walls:
Wall
Location
Key Features
Lateral wall
Outer boundary
Formed by the tympanic membrane (eardrum)
Medial wall
Inner boundary
Contains oval window, round window, and promontory (overlying cochlea)
Anterior wall
Faces carotid artery
Opening for the Eustachian (auditory) tube
Posterior wall
Towards mastoid
Leads to mastoid air cells (via aditus)
Roof
Tegmen tympani (bony)
Separates middle ear from cranial cavity
Floor
Thin bone above jugular bulb
Separates ear from internal jugular vein
Main Components of the Middle Ear
Tympanic Membrane (Eardrum)
Though part of the external ear anatomically, functionally it belongs to the middle ear.
It vibrates when sound waves hit it and transmits the energy to the ossicles.
Auditory Ossicles (Ear Bones)
The three smallest bones in the human body, which form a chain from the tympanic membrane to the inner ear:
a) Malleus (βhammerβ)
Handle attached to tympanic membrane
Head articulates with the incus
b) Incus (βanvilβ)
Intermediate bone; articulates with both malleus and stapes
c) Stapes (βstirrupβ)
Footplate fits into the oval window (fenestra vestibuli)
Transmits vibrations to the fluid-filled cochlea of the inner ear
Function of Ossicles
Amplify sound vibrations ~20x from air (low impedance) to fluid (high impedance)
Transform sound waves into mechanical energy for the cochlea
Eustachian Tube (Pharyngotympanic Tube)
Connects middle ear to the nasopharynx
Lined with mucous membrane and opens during swallowing or yawning
Equalizes pressure on both sides of the eardrum, preventing rupture
Clinical note:
In children, the Eustachian tube is shorter and more horizontal, increasing the risk of otitis media
Middle Ear Muscles
These tiny muscles regulate sound transmission:
a) Tensor Tympani Muscle
Originates from the auditory tube and inserts on the malleus
Contracts in response to loud sounds β tightens tympanic membrane β reduces vibration
b) Stapedius Muscle
Smallest skeletal muscle in the body
Attaches to stapes
Dampens excessive vibrations (especially from oneβs own voice)
Innervated by Facial Nerve (CN VII)
Clinical note:
Loss of stapedius reflex (e.g., in Bellβs palsy) may cause hyperacusis (sensitivity to sound)
Windows of the Middle Ear
a) Oval Window
Covered by the footplate of stapes
Transmits vibrations to the inner ear (cochlea)
b) Round Window
Covered by a membrane
Acts as a pressure release valve, allowing fluid displacement within cochlea
Nerve Relations in the Middle Ear
Facial Nerve (CN VII):
Passes through the facial canal, giving rise to the nerve to stapedius and chorda tympani (taste from anterior 2/3 tongue)
Tympanic Plexus:
Formed by Glossopharyngeal Nerve (CN IX) β provides sensory innervation to middle ear mucosa
Clinical Relevance
Condition
Description
Otitis media
Infection of middle ear (common in children)
Barotrauma
Pressure difference injury (e.g., during flights or diving)
Cholesteatoma
Abnormal skin growth in middle ear; destructive if untreated
Otosclerosis
Stapes fixation due to bone overgrowth β conductive deafness
Mastoiditis
Infection spreading to mastoid air cells
Summary Table
Structure
Function
Tympanic membrane
Receives sound and vibrates
Ossicles (M-I-S)
Amplify and transmit vibrations to the cochlea
Eustachian tube
Equalizes pressure between middle ear and pharynx
Stapedius & Tensor Tympani
Protect inner ear from loud sounds
Oval & round windows
Interface between middle and inner ear
Inner Ear (Labyrinth) β Structure and Function
The inner ear is a complex, fluid-filled structure housed in the petrous part of the temporal bone. It serves two main functions:
Hearing via the cochlea
Balance (equilibrium) via the vestibular system (vestibule and semicircular canals)
Structural Overview
The inner ear has two main parts:
Bony labyrinth β a system of hollow, rigid, bone-encased canals
Membranous labyrinth β a delicate, fluid-filled membrane suspended within the bony labyrinth
Both contain fluids:
Perilymph: fills the space between the bony and membranous labyrinths (like CSF)
Endolymph: fills the membranous labyrinth (rich in potassium, like intracellular fluid)
Three Major Divisions of the Inner Ear
1. Cochlea β Hearing Apparatus
Structure:
Snail-shaped spiral canal (~2.5 turns)
Bony cochlea contains three parallel fluid-filled chambers:
Scala vestibuli (perilymph)
Scala media (cochlear duct) β contains endolymph and houses the Organ of Corti
Scala tympani (perilymph)
Organ of Corti:
Located on the basilar membrane inside the cochlear duct
Contains hair cells (mechanoreceptors) with stereocilia
Hair cells synapse with the cochlear nerve fibers (CN VIII)
When sound-induced vibrations move the basilar membrane, hair cells bend, triggering nerve impulses
Function:
Converts mechanical vibrations into electrical nerve signals β sent to the auditory cortex via the cochlear branch of the vestibulocochlear nerve (CN VIII)
2. Vestibule β Static Balance & Linear Acceleration
Structure:
Central part of the bony labyrinth
Contains two sac-like structures of the membranous labyrinth:
Utricle
Saccule
Each contains a macula: a patch of hair cells covered by a gelatinous membrane with embedded otoliths (calcium crystals).
Function:
Detects linear acceleration and gravity (e.g., head tilting, moving forward/backward)
Sends signals to the brain via the vestibular nerve (CN VIII)
Positioned at right angles to detect movement in all 3 dimensions
Each canal has an enlarged area called the ampulla, which contains the crista ampullaris
Crista Ampullaris:
Contains hair cells with stereocilia embedded in a gelatinous structure called the cupula
When the head rotates, endolymph lags behind, bending the cupula β stimulates hair cells β sends impulse via vestibular nerve
Function:
Detects rotational (angular) movement of the head
Nerve Supply
The Vestibulocochlear Nerve (CN VIII) has two branches:
Cochlear nerve β carries hearing signals from cochlea
Vestibular nerve β carries balance information from semicircular canals, utricle, and saccule
These enter the brainstem at the pons-medulla junction and relay to the auditory cortex or cerebellum.
Clinical Relevance
Condition
Description
Sensorineural hearing loss
Damage to cochlea or CN VIII
Meniereβs disease
Excess endolymph β vertigo, tinnitus, hearing loss
Labyrinthitis
Inflammation of inner ear β dizziness, nausea
Vestibular neuritis
Viral infection of vestibular nerve β sudden vertigo
Benign Paroxysmal Positional Vertigo (BPPV)
Otoliths displaced into semicircular canals β brief vertigo
Summary Table
Part
Structure
Function
Cochlea
Organ of Corti with hair cells
Hearing
Vestibule
Utricle + Saccule with macula
Linear acceleration and gravity
Semicircular Canals
3 canals with crista ampullaris
Rotational balance
CN VIII
Cochlear + Vestibular branches
Transmits sound and balance signals
Anatomy of the Nose β The Organ of Smell and Airway Gateway
The nose is a vital component of the respiratory system and the olfactory (smell) system. It serves as the entry point for air, filters and humidifies it, and houses sensory receptors for the sense of smell.
Main Divisions of the Nose
The nose is anatomically divided into two parts:
1. External Nose 2. Internal Nose (Nasal Cavity)
1. External Nose
Structure:
Pyramidal shape, projecting from the face
Made of bones, cartilage, muscles, and skin
Skeletal Framework:
A. Bony Part:
Nasal bones (bridge of nose)
Frontal process of maxilla
Nasal part of frontal bone
B. Cartilaginous Part:
Lateral nasal cartilages
Septal cartilage (midline divider)
Alar cartilages (surround nostrils)
Features:
Nostrils (nares): external openings leading to nasal cavity
Columella: soft tissue between nostrils
Alae: lateral, wing-like flaps of the nostrils
Function:
Allows inhalation/exhalation of air
Provides initial filtering, warming, and humidification
Involved in facial expression and aesthetics
2. Internal Nose (Nasal Cavity)
A large, air-filled space behind the external nose, divided into right and left halves by the nasal septum.
Boundaries of the Nasal Cavity:
Wall
Structures
Roof
Nasal bones, frontal bone, cribriform plate of ethmoid
Lingual frenulum: mucosal fold that anchors tongue to floor of the mouth
Sublingual caruncles: openings of submandibular ducts (saliva)
Tongue Papillae (Taste and Texture)
The dorsum of the tongue (top surface) contains four types of papillae:
Type
Shape & Features
Function
Filiform
Thin, cone-shaped (most numerous)
Provide friction; no taste buds
Fungiform
Mushroom-shaped; scattered across tip/sides
Contain taste buds
Circumvallate
Large, round; arranged in a V-shape near sulcus terminalis
Contain many taste buds
Foliate
Leaf-like folds on lateral edges
Contain taste buds (especially in children)
Muscles of the Tongue
The tongue is made entirely of skeletal muscle. It has:
Intrinsic Muscles (within the tongue)
Change the shape of the tongue (e.g., curling, flattening, narrowing)
Includes: superior longitudinal, inferior longitudinal, transverse, vertical
Extrinsic Muscles (attach tongue to bone)
Move the tongue in different directions
Include:
Genioglossus β protrudes tongue
Hyoglossus β depresses tongue
Styloglossus β elevates and retracts tongue
Palatoglossus β elevates posterior tongue (also part of soft palate)
Nerve Supply of the Tongue
Motor Supply:
All tongue muscles are supplied by the Hypoglossal Nerve (CN XII)
Except Palatoglossus, supplied by Vagus Nerve (CN X)
Sensory Supply:
Region
General Sensation
Taste Sensation
Anterior 2/3 (oral part)
Lingual nerve (branch of CN V3)
Chorda tympani (branch of CN VII)
Posterior 1/3 (pharyngeal part)
Glossopharyngeal nerve (CN IX)
Glossopharyngeal nerve (CN IX)
Base of tongue (epiglottis)
Vagus nerve (CN X)
Vagus nerve (CN X)
Blood Supply
Lingual artery (branch of external carotid artery)
Venous drainage via lingual vein β internal jugular vein
Lymphatic Drainage
Tip of tongue β submental nodes
Lateral anterior tongue β submandibular nodes
Posterior tongue β deep cervical nodes
Important: Cancers of the tongue often metastasize to submandibular or deep cervical lymph nodes
Clinical Relevance
Condition
Description
Glossitis
Inflammation of the tongue (e.g., due to B12 deficiency)
Tongue tie (ankyloglossia)
Shortened frenulum restricts movement (speech/breastfeeding issues)
Leukoplakia
White patches, possibly precancerous
Oral cancers
Common at lateral border of tongue; tobacco is a major risk
Hypoglossal nerve palsy
Tongue deviates toward affected side on protrusion
Summary Table
Feature
Details
Regions
Tip, body (oral part), root (pharyngeal part)
Surface
Dorsal (with papillae), ventral (smooth)
Muscles
Intrinsic (shape), Extrinsic (movement)
Taste buds
On fungiform, foliate, circumvallate papillae
Motor nerve
CN XII (except Palatoglossus β CN X)
Taste nerves
CN VII, IX, X
Blood supply
Lingual artery and vein
The Sensory Organs β Applications & Implications in Nursing Practice
Introduction
The five special sensory organs β eyes, ears, nose, tongue, and skin β are essential for perceiving the external environment. They provide sensory input that helps the body maintain homeostasis, avoid danger, communicate, and enjoy life.
Nurses must understand the structure and function of each sense organ to provide holistic care, recognize early signs of disorders, and ensure patient comfort, safety, and education.
Five Main Sensory Organs and Their Clinical Applications
1. Eyes (Organ of Vision)
β€ Functions:
Detects light, color, shapes
Enables perception of distance and motion
β€ Nursing Implications:
Assessment: Visual acuity (Snellen chart), pupil reaction, eye movement
Care of blind/partially sighted patients: Orientation, mobility support
Medication administration: Eye drops/ointments β use sterile technique
Structure of Bone β Gross and Microscopic Anatomy
I. Gross Structure of Bone (Macroscopic Anatomy)
Bones are organs made of osseous tissue, blood vessels, bone marrow, nerves, and connective tissue. Letβs take the example of a long bone (e.g., femur) to understand the structure:
1. Diaphysis (Shaft)
The long, cylindrical middle portion of the bone
Composed of thick compact bone
Encloses the medullary (marrow) cavity, which contains:
Yellow marrow (fat storage in adults)
Red marrow in children
2. Epiphyses (Ends of Bone)
Expanded ends of the bone (proximal and distal)
Composed mostly of spongy (cancellous) bone
Contains red marrow (site of blood cell production)
Covered by articular (hyaline) cartilage β smooth, reduces friction at joints
3. Metaphysis
Region between diaphysis and epiphysis
In growing children, contains the epiphyseal plate (growth plate)
In adults, becomes the epiphyseal line
4. Periosteum
Tough, fibrous membrane covering the outer surface of bones (except joints)
Has two layers:
Outer fibrous layer: dense connective tissue
Inner osteogenic layer: contains osteoblasts and osteoclasts
Rich in nerves and blood vessels
Function: Bone growth, repair, and serves as an attachment point for muscles, tendons, and ligaments
5. Endosteum
Thin membrane lining the internal surfaces (medullary cavity, Haversian canals)
Contains osteoblasts and osteoclasts
6. Medullary Cavity (Marrow Cavity)
Central cavity in diaphysis
Contains:
Red marrow in children (hematopoietic)
Yellow marrow in adults (fat storage)
II. Microscopic Structure of Bone (Histology)
Bone tissue is of two types:
Compact bone (dense) Spongy bone (cancellous)
A. Compact Bone
Highly organized, strong, and forms the outer layer of bones.
Perpendicular canals connecting osteons; carry vessels and nerves
B. Spongy Bone (Cancellous Bone)
Found in epiphyses, flat bones, vertebrae, etc.
Made of trabeculae (lattice of thin bone plates)
Spaces between trabeculae are filled with red bone marrow
Lighter and less dense than compact bone
No osteons β nutrients diffuse through canaliculi from marrow spaces
Types of Bone Cells
Cell
Function
Osteoblasts
Build new bone matrix (bone-forming cells)
Osteocytes
Mature bone cells that maintain the matrix
Osteoclasts
Break down bone matrix (bone-resorbing cells)
Osteoprogenitor cells
Stem cells that differentiate into osteoblasts
Bone Matrix Composition
Component
Function
Organic matrix (35%)
Collagen fibers (strength + flexibility)
Inorganic salts (65%)
Calcium phosphate crystals (bone hardness)
Clinical Correlation
Condition
Involvement
Fractures
Break in continuity of compact/spongy bone
Osteoporosis
Loss of bone density, especially in trabecular bone
Osteomalacia/Rickets
Poor mineralization due to vitamin D deficiency
Bone cancer
May originate in marrow (leukemia) or bone matrix
Summary Table
Feature
Details
Gross Parts
Diaphysis, epiphysis, metaphysis, periosteum
Compact bone
Dense outer layer; contains osteons
Spongy bone
Lattice-like; contains red marrow
Bone cells
Osteoblasts, osteocytes, osteoclasts
Marrow types
Red (hematopoiesis), Yellow (fat storage)
Classification of Bones: Types, Structure & Clinical Relevance
The human skeleton is composed of 206 bones, and each bone is classified based on its shape, internal structure, and function. Understanding these types helps in identifying bone behavior during growth, movement, trauma, or disease.
Overview: Why Do We Classify Bones by Shape?
To understand mechanical function
To determine locations of hematopoiesis
To identify fracture risks and clinical relevance
To aid in radiographic interpretation
The 5 Main Types of Bones
1. Long Bones
Characteristics:
Longer than they are wide
Have a central shaft (diaphysis) and two expanded ends (epiphyses)
Mostly compact bone in the shaft and spongy bone at the ends