The Skeletal System: The Axial Skeleton

The Axial Skeleton and Homeostasis

• Protective Function: The bones of the axial skeleton maintain homeostasis by providing physical protection for vital organs:

  • Cranium: Encloses and protects the brain.

  • Vertebrae: Form a canal to surround and protect the spinal cord.

  • Ribs: Form a cage to protect the heart and lungs.

• Mineral Homeostasis: Bones serve as a reservoir for the storage and release of calcium ($Ca^{2+}$), which is a critical ion for various physiological processes.

Divisions of the Skeletal System

• Total Bone Count: The adult human skeleton consists of $206$ named bones.

• Axial Skeleton ($80$ Bones):

  • Skull: Consisting of cranial bones ($8$) and facial bones ($14$) for a total of $22$ bones.

  • Hyoid Bone: $1$ bone.

  • Auditory Ossicles: $6$ bones ($3$ in each ear).

  • Vertebral Column: $26$ bones.

  • Thorax: Consisting of the Sternum ($1$) and Ribs ($24$, or $12$ pairs).

• Appendicular Skeleton ($126$ Bones):

  • Pectoral Girdles: Clavicle ($2$) and Scapula ($2$).

  • Upper Limbs: Humerus ($2$), Ulna ($2$), Radius ($2$), Carpals ($16$), Metacarpals ($10$), and Phalanges ($28$).

  • Pelvic Girdle: Hip, pelvic, or coxal bones ($2$).

  • Lower Limbs: Femur ($2$), Patella ($2$), Fibula ($2$), Tibia ($2$), Tarsals ($14$), Metatarsals ($10$), and Phalanges ($28$).

Classification of Bones by Shape

• Long Bones: Characterized by having a greater length than width.

• Short Bones: Roughly cube-shaped.

• Flat Bones: Composed of thin layers of parallel plates.

• Irregular Bones: Possess complex shapes that do not fit other categories.

• Sesamoid Bones: Shaped like a sesame seed.

• Sutural Bones: Small, additional bone plates located specifically within the sutures (joints) of cranial bones.

Bone Surface Markings: Depressions and Openings

• General Purpose: These markings allow the passage of soft tissues (blood vessels, nerves, ligaments, tendons) or help form joints.

• Fissure: A narrow slit between adjacent parts of bones. Example: Superior orbital fissure of the sphenoid bone.

• Foramen: An opening or hole through which soft tissues pass. Example: Optic canal of the sphenoid bone.

• Fossa: A shallow depression or trench. Example: Coronoid fossa of the humerus or mandibular fossa.

• Sulcus: A furrow or groove along a bone surface that accommodates a soft tissue structure. Example: Intertubercular sulcus of the humerus.

• Meatus: A tube-like opening or passageway. Example: External acoustic meatus of the temporal bone.

Bone Surface Markings: Processes

• General Purpose: Projections or outgrowths that either form joints or serve as attachment points for connective tissues like ligaments and tendons.

• Processes that Form Joints:

  • Condyle: A large, round protuberance with a smooth articular surface at the end of a bone. Example: Lateral condyle of the femur.

  • Facet: A smooth, flat, slightly concave or convex articular surface. Example: Superior articular facet of a vertebra.

  • Head: A rounded articular projection supported on a constricted portion (the neck) of a bone. Example: Head of the femur.

• Processes that Form Attachment Points:

  • Crest: A prominent ridge or elongated projection. Example: Iliac crest of the hip bone.

  • Epicondyle: A roughened projection located above a condyle. Example: Medial epicondyle of the femur.

  • Line (Linea): A long, narrow ridge or border that is less prominent than a crest. Example: Linea aspera of the femur.

  • Spinous Process: A sharp, slender projection. Example: Spinous process of a vertebra.

  • Trochanter: A very large projection found only on the femur. Example: Greater trochanter of the femur.

  • Tubercle: A variably-sized rounded projection or knob. Example: Greater tubercle of the humerus.

  • Tuberosity: A variably-sized projection that has a rough, bumpy surface. Example: Ischial tuberosity of the hip bone.

The Skull

• Bone Count: Contains $22$ bones (excluding the $6$ auditory ossicles).

• Cranial Bones ($8$):

  • Frontal bone (comprising the frontal squama, supraorbital margin, and supraorbital foramen).

  • Parietal bones ($2$).

  • Temporal bones ($2$) (comprising the zygomatic process, mandibular fossa, external acoustic meatus, mastoid process, and styloid process).

  • Occipital bone (comprising the foramen magnum, occipital condyles, and external occipital protuberance).

  • Sphenoid bone (the "keystone" containing the sella turcica, hypophyseal fossa, and pterygoid processes).

  • Ethmoid bone (comprising the crista galli, cribriform plate, and nasal conchae).

• Facial Bones ($14$):

  • Nasal bones ($2$).

  • Maxillae ($2$) (comprising the alveolar process and infraorbital foramen).

  • Zygomatic bones ($2$).

  • Mandible ($1$) (the largest, strongest, and only movable skull bone, excluding auditory ossicles).

  • Lacrimal bones ($2$).

  • Palatine bones ($2$).

  • Inferior nasal conchae ($2$).

  • Vomer ($1$).

• Major Sutures: Immovable joints holding skull bones together, including the Coronal, Sagittal, Lambdoid, and Squamous sutures.

• Paranasal Sinuses: Mucous membrane-lined cavities in the frontal, maxillary, sphenoid, and ethmoid bones. Functions include acting as resonating chambers for the voice and increasing nasal mucosa surface area to moisten air.

• Fontanels: Areas of unossified mesenchyme in fetuses/infants that develop into dense connective tissue; they typically close via intramembranous ossification by $2$ years of age.

Principal Foramina of the Skull

• Carotid canal: Located in the petrous portion of the temporal bone; transmits the internal carotid artery and sympathetic nerves for the eyes.

• Hypoglossal canal: Superior to the base of occipital condyles; transmits the Hypoglossal ($XII$) nerve.

• Infraorbital foramen: Inferior to the orbit in the maxilla; transmits the infraorbital nerve and blood vessels.

• Jugular foramen: Between the temporal and occipital bones; transmits the internal jugular vein and cranial nerves $IX$ (glossopharyngeal), $X$ (vagus), and $XI$ (accessory).

• Foramen lacerum: Bounded by the sphenoid, temporal, and occipital bones; transmits a branch of the ascending pharyngeal artery.

• Foramen magnum: Located in the occipital bone; transmits the medulla oblongata, meninges, accessory ($XI$) nerve, and vertebral/spinal arteries.

• Mandibular foramen: Medial surface of the ramus of the mandible; transmits the inferior alveolar nerve and blood vessels.

• Mastoid foramen: Posterior border of the mastoid process; transmits an emissary vein to the transverse sinus.

• Mental foramen: Inferior to the second premolar in the mandible; transmits the mental nerve and vessels.

• Cribriform foramina: In the cribriform plate of the ethmoid bone; transmit the Olfactory ($I$) nerve.

• Optic canal: Between the wings of the sphenoid bone; transmits the Optic ($II$) nerve and ophthalmic vessels.

• Foramen ovale: In the greater wing of the sphenoid bone; transmits the mandibular division of the trigeminal ($V$) nerve.

• Foramen rotundum: Junction of parts of the sphenoid bone; transmits the maxillary division of the trigeminal ($V$) nerve.

• Stylomastoid foramen: Between the styloid and mastoid processes; transmits the Facial ($VII$) nerve.

• Supraorbital foramen: In the supraorbital margin of the frontal bone; transmits the supraorbital nerve and vessels.

Clinical Connections of the Skull

• Temporomandibular Joint (TMJ) Dysfunction: Symptoms include dull pain around the ear, tender jaw muscles, and clicking noises. Causes include misaligned teeth, grinding (bruxism), trauma, or arthritis. Treatment involves heat/ice, soft foods, and analgesics.

• Deviated Nasal Septum: Occurs when the septum does not run along the midline. Caused by trauma or developmental issues. Can lead to inflammation, congestion, and nosebleeds; surgery may be required for correction.

The Hyoid Bone

• Unique Property: It does not articulate with any other bone.

• Function: Supports the tongue and provides attachment sites for muscles of the neck and pharynx.

The Vertebral Column

• Structure: Composed of $26$ vertebrae in adults, divided into $5$ regions.

• Regions:

  • Cervical: $7$ vertebrae ($C1$ to $C7$).

  • Thoracic: $12$ vertebrae ($T1$ to $T12$).

  • Lumbar: $5$ vertebrae ($L1$ to $L5$).

  • Sacrum: $1$ bone (formed by the fusion of $5$ sacral vertebrae).

  • Coccyx: $1$ bone (formed by the fusion of $4$ coccygeal vertebrae).

• Normal Curves:

  • Fetal: A single anteriorly concave curve.

  • Adult: Four curves (Cervical, Thoracic, Lumbar, and Sacral). Primary curves (Thoracic and Sacral) retain the fetal orientation, while secondary curves (Cervical and Lumbar) develop later.

• Intervertebral Discs: Found from $C2$ to the sacrum.

  • Composition: Outer fibrocartilage ring (annulus fibrosus) and an inner soft, elastic nucleus pulposus.

  • Function: Absorb shock and facilitate movement by separating vertebrae.

Comparison of Vertebrae

• Cervical Vertebrae:

  • Size: Smallest.

  • Foramina: One vertebral and two transverse foramina.

  • Spinous Process: Slender, often bifid ($C2$-$C6$).

  • Unique features: The Atlas ($C1$) lacks a body and spinous process; the Axis ($C2$) features the dens (odontoid process) for rotation.

• Thoracic Vertebrae:

  • Size: Larger than cervical.

  • Foramina: One vertebral foramen.

  • Spinous Process: Long, fairly thick, projects inferiorly.

  • Unique features: Presence of costal facets for rib articulation.

• Lumbar Vertebrae:

  • Size: Largest and strongest.

  • Foramina: One vertebral foramen.

  • Spinous Process: Short, blunt, projects posteriorly.

  • Discs: Thickest intervertebral discs.

The Thorax

• Components: Sternum, ribs, and costal cartilages.

• Sternum Parts:

  • Manubrium: Superior segment; articulates with clavicles and first two ribs.

  • Body: Middle segment.

  • Xiphoid Process: Inferior segment.

• Ribs ($12$ pairs total):

  • True (Vertebrosternal) Ribs: Pairs $1$ through $7$; cartilage connects directly to the sternum.

  • False (Vertebrochondral) Ribs: Pairs $8$ through $12$; cartilage connects indirectly to the sternum ($8$-$10$).

  • Floating (Vertebral) Ribs: Pairs $11$ and $12$; do not connect to the sternum.

Disorders of the Skeletal System

• Herniated Disc: Protrusion of the nucleus pulposus through the annulus fibrosus, often due to trauma or aging.

• Abnormal Spinal Curvatures:

  • Scoliosis: Increased lateral (sideways) curvature.

  • Kyphosis: Increased thoracic curve (hunchback/bent forward).

  • Lordosis: Increased lumbar curve (swayback/bent backward).

• Spina Bifida: A congenital defect where the laminae of the vertebrae fail to develop normally. Ranges from spina bifida occulta (mild) to meningomyelocele (severe).

• Vertebral Fractures: Most commonly occur at the levels of $C1$, $C2$, $C4$-$C7$, and $T12$-$L2$. These can result in significant spinal cord or nerve damage.

Purpose of Axial Skeleton

• Provides main support for the body and protects vital organs.

Bones in Axial Skeleton

• Adult Human Skeleton Total: 206 named bones.

• Axial Skeleton: 80 bones, including:

  • Skull: 22 bones (8 cranial, 14 facial)

  • Hyoid Bone: 1 bone (supports the tongue and provides attachment for neck muscles)

  • Auditory Ossicles: 6 bones (3 in each ear, responsible for sound transmission)

  • Vertebral Column: 26 bones (composed of vertebrae)

  • Thorax: 1 sternum and 24 ribs (protects heart and lungs)

5 Different Shapes of Bones

• Long Bones: Bones that are longer than they are wide (e.g., femur).

• Short Bones: Roughly cube-shaped bones (e.g., carpals).

• Flat Bones: Bones composed of thin layers of parallel plates (e.g., skull bones).

• Irregular Bones: Complex shaped bones that do not fit into other categories (e.g., vertebrae).

• Sesamoid Bones: Bones shaped like sesame seeds, found in tendons (e.g., patella).

Bones of the Skull

• Total: Contains 22 bones (excluding the 6 auditory ossicles).

  • Cranial Bones: Frontal, parietal (2), temporal (2), occipital, sphenoid, ethmoid.

  • Facial Bones: Nasal (2), maxillae (2), zygomatic (2), mandible (1), lacrimal (2), palatine (2), inferior nasal conchae (2), vomer (1).

Bones of the Vertebral Column

• Composed of 26 vertebrae divided into 5 regions:

  • Cervical (7): Bones of the neck.

  • Thoracic (12): Bones of the upper back, each articulating with ribs.

  • Lumbar (5): Bones of the lower back, supporting most body weight.

  • Sacrum (1): Formed from the fusion of 5 sacral vertebrae.

  • Coccyx (1): Formed from the fusion of 4 coccygeal vertebrae.

Unique Features of Vertebrae

• Cervical: Smallest, bifid (split) spinous processes; Atlas (C1) lacks a body and spinous process; Axis (C2) has a dens for head rotation.

• Thoracic: Larger bodies than cervical; presence of costal facets for rib articulation.

• Lumbar: Thickest and largest vertebrae with robust spinous processes.

Bones of the Rib Cage

• Consists of 12 pairs of ribs:

  • True Ribs (1-7): Directly connect to the sternum via their own costal cartilage.

  • False Ribs (8-12): Do not attach directly to the sternum (8-10 connect indirectly via the cartilage of rib 7).

  • Floating Ribs (11-12): Do not attach to the sternum at all.

Parts of the Sternum

• Manubrium: The upper part of the sternum.

• Body: The middle portion of the sternum.

• Xiphoid Process: The small, cartilaginous lower end of the sternum.

Depressions and Openings

• Major types include:

  • Fissures: Narrow slits between adjacent bones.

  • Foramina: Holes allowing passage for nerves and vessels.

  • Fossae: Shallow depressions for muscle attachment or joint articulation.

  • Sulci: Grooves along a bone surface for soft tissue accommodation.

  • Meatuses: Tube-like passages through a bone.

• Function: Allow passage of soft tissues or help form joints.

Processes

• Major types include:

  • Condyles: Enlarged ends that form joints (e.g., femoral condyles).

  • Facets: Smooth, flat surfaces for articulation (e.g., vertebral facets).

  • Heads: Rounded projections at the end of a bone (e.g., femoral head).

  • Crests: Prominent ridges for muscle attachment (e.g., iliac crest).

  • Epicondyles: Rough projections above condyles for ligament attachment (e.g., medial epicondyle of the femur).

  • Lines (Linea): Narrow ridges less prominent than crests (e.g., linea aspera).

  • Spinous Processes: Sharp projections for muscle attachment (e.g., vertebrae).

  • Trochanters: Large projections on the femur (e.g., greater and lesser trochanters).

  • Tubercles: Round projections (e.g., greater tubercle of the humerus).

  • Tuberosities: Variously sized rough projections (e.g., ischial tuberosity).

• Function: Projections for joint formation or attachment for ligaments and tendons.

Dysfunctions of the Axial Skeleton

• Common problems include:

  • Herniated Discs: Protrusion of the nucleus pulposus.

  • Scoliosis: Abnormal lateral curvature of the spine.

  • Kyphosis: Increased thoracic curve (hunchback appearance).

  • Lordosis: Increased lumbar curve (swayback appearance).

  • Spina Bifida: A congenital defect where the laminae of the vertebrae fail to develop normally.

  • Vertebral Fractures: Commonly occur at specific vertebrae levels with potential spinal cord damage.

Facial Bones

• Total: 14 bones including nasal (2), maxillae (2), zygomatic (2), mandible (1), lacrimal (2), palatine (2), inferior nasal conchae (2), and vomer (1).

Why is the Mandible Special?

• The mandible is the largest, strongest, and the only movable bone of the skull (excluding auditory ossicles).

Nasal Septum Components

• Composed of vomer (bone), perpendicular plate of the ethmoid (bone), and cartilage.

Major Foramina of the Skull

• Key foramina include:

  • Carotid Canal: Transmits the internal carotid artery.

  • Hypoglossal Canal: Transmits the hypoglossal nerve (CN XII).

  • Jugular Foramen: Transmits the internal jugular vein and cranial nerves IX, X, and XI.

  • Foramen Magnum: Connects cranial cavity to vertebral canal; transmits the medulla oblongata.

Four Paranasal Sinuses

• Sinuses Include:

  • Frontal Sinus: In the frontal bone, above the eyes.

  • Maxillary Sinus: Largest, located in the maxilla.

  • Sphenoid Sinus: In the sphenoid bone, behind the eyes.

  • Ethmoid Sinus: Located between the nose and eyes.

• Function: Act as resonating chambers for voice, increasing nasal mucosa surface area.

Fetal Skull Fontanels

• Areas of unossified mesenchyme (soft spots) that allow for brain growth; close by age 2.

Hyoid Bone

• A unique bone that does not articulate with others; supports the tongue and provides attachment for neck muscles.

Parts of the Vertebral Column

• Consists of cervical, thoracic, lumbar regions, sacrum, and coccyx.

Curvatures of the Vertebral Column

• Curvatures: Anteriorly concave fetal curvature; adults possess cervical, thoracic, lumbar, and sacral curves.

Intervertebral Discs

• Fibrocartilaginous discs providing cushioning between vertebrae, composed of an outer annulus fibrosus and inner nucleus pulposus.

Unique Vertebrae

• C1 (Atlas):

  • Unique Feature: Lacks a body and spinous process.

  • Function: Allows nodding of the head (flexion and extension).

• C2 (Axis):

  • Unique Feature: Contains the dens (odontoid process).

  • Function: Permits rotation of the head by acting as a pivot.

Differences Between Vertebrae Types

• Cervical Vertebrae:

  • Size: Smallest vertebrae.

  • Foramina: One vertebral foramen and two transverse foramina.

  • Spinous Process: Slender and often bifid (split) from C2 to C6.

• Thoracic Vertebrae:

  • Size: Larger than cervical vertebrae.

  • Foramina: One vertebral foramen.

  • Spinous Process: Long, fairly thick, projecting inferiorly.

  • Unique Features: Presence of costal facets for rib articulation.

• Lumbar Vertebrae:

  • Size: Largest and strongest.

  • Foramina: One vertebral foramen.

  • Spinous Process: Short and blunt, projects posteriorly.

  • Discs: Thickest intervertebral discs, supporting most body weight.

Sacrum

• Structure: Formed by the fusion of five sacral vertebrae.

• Function: Connects the spine to the hip bones, providing strength and stability.

Coccyx

• Structure: Formed from the fusion of four coccygeal vertebrae.

• Function: Forms the tailbone and serves as an attachment site for ligaments and muscles.

Herniated Disc

• Definition: Protrusion of the nucleus pulposus through the annulus fibrosus, often due to trauma or aging.

• Symptoms: Pain, numbness, or weakness in the affected area or down the limbs.

Disorders of the Vertebral Column

• Scoliosis: Abnormal lateral curvature of the spine.

• Kyphosis: Increased thoracic curve (hunchback appearance).

• Lordosis: Increased lumbar curve (swayback appearance).

• Spina Bifida: A congenital defect where the laminae of the vertebrae fail to develop normally.

• Vertebral Fractures: Commonly occur at specific vertebrae levels with potential for spinal cord damage.

Pectoral Girdle

• Composed of the clavicles (collarbones) and scapulae (shoulder blades).

• Functions to connect the upper limb to the trunk and support arm movements.

Pelvic Girdle

• Composed of the hip bones (coxal bones).

• Functions to support the weight of the body during sitting and standing, protect pelvic organs, and form the birth canal in females.

Parts of the Clavicle

• Sternal End: Connects to the sternum.

• Acromial End: Connects to the acromion of the scapula.

• Conoid Tubercle: Serves as an attachment point for ligaments.

Articulations Between Bones of the Appendicular Skeleton

• Ball-and-socket joints (e.g., shoulder and hip joints).

• Hinge joints (e.g., elbow and knee joints).

• Pivot joints (e.g., between radius and ulna).

Parts of the Scapula

• Spine: Prominent ridge on the posterior side.

• Acromion: An extension of the spine that forms a joint with the clavicle.

• Glenoid cavity: Socket for the head of the humerus.

Parts of the Humerus

• Head: Round proximal end that fits into the glenoid cavity.

• Greater and Lesser Tubercle: Provide attachment sites for rotator cuff muscles.

• Shaft: Long central part of the bone.

• Condyles: Include the capitulum and trochlea for articulation with the forearm.

Parts of the Radius and Ulna

• Radius: Thinner bone on the lateral side of the forearm; includes the radial head and styloid process.

• Ulna: Thicker bone on the medial side; includes the olecranon (elbow) and styloid process.

Carpal Bones, Metacarpals, and Phalanges

• Carpal Bones: 8 bones in the wrist; arranged in two rows: proximal (scaphoid, lunate, triquetrum, pisiform) and distal (trapezium, trapezoid, capitate, hamate).

• Metacarpals: 5 bones forming the palm of the hand.

• Phalanges: 14 bones in the fingers; 3 in each finger (proximal, middle, distal) and 2 in the thumb (proximal, distal).

Parts of the Hip

• Ilium: Largest part; includes the iliac crest.

• Ischium: Lower part; forms the sit bone.

• Pubis: Anterior part that meets at the pubic symphysis.

Parts of the Femur

• Head: Round proximal end articulating with the acetabulum of the hip.

• Neck: Narrow area just below the head.

• Greater and Lesser Trochanters: Projections for muscle attachment.

• Condyles: Medial and lateral condyles at the distal end for knee joint articulation.

Parts of the Tibia and Fibula

• Tibia: Larger weight-bearing bone; includes the tibial tuberosity, medial malleolus, and lateral malleolus (the ankle).

• Fibula: Thinner bone; mainly provides support and stability.

Tarsal Bones, Metatarsals, and Phalanges

• Tarsal Bones: 7 bones in the ankle (e.g., talus, calcaneus).

• Metatarsals: 5 bones forming the arch of the foot.

• Phalanges: 14 bones in the toes; similar to fingers with each toe having three (proximal, middle, distal) except for the big toe.

Arches of the Foot

• Medial Longitudinal Arch: Highest arch running from heel to toe.

• Lateral Longitudinal Arch: Lower arch, also from heel to toe.

• Transverse Arch: Crosswise arch at the proximal part of the metatarsals.

Skeletal Tissue in Fetal Development

• Initially composed mostly of hyaline cartilage which later ossifies into bone during growth.

Development of Bones of the Face

• Includes the mandible, zygomatic bones, nasal bones, and maxilla; primarily formed from intramembranous ossification.

Development of Bones of the Skull

• Formed by a combination of intramembranous and endochondral ossification leading to the cranial vault and base development.

Development of Bones of the Limbs

• Limb bones develop through endochondral ossification from cartilage models, shaping into long bones.

False (Greater) Pelvis vs. True (Lesser) Pelvis

• False Pelvis: Superior part of the pelvis; supports the intestines.

• True Pelvis: Inferior part; defines the birth canal.

Differences Between the Male and Female Pelves

• Male pelvis: Narrower, heavier, and deeper for bipedal locomotion.

• Female pelvis: Wider, lighter, and broader for childbirth.