Human Anatomy and Physiology: Chapter 7 - The Skeletal System

Organization of the Skeletal System and Bone Markings

• General Composition of the Skeletal System:

  • The adult human skeleton consists of approximately $206$ bones along with associated cartilage.

  • The skeletal system functions as an integrated complex of organs that collaborate to support, protect, and facilitate body locomotion.

  • The skeleton is anatomically divided into two major components: the axial skeleton and the appendicular skeleton.

• Axial Skeleton:

  • Forms the longitudinal axis of the body and encases internal cavities to protect essential underlying organs.

  • Skull: Composed of $22$ bones ($8$ cranial bones and $14$ facial bones); protects the brain and forms the facial framework.

  • Vertebral Column: Consists of $33$ bones; $24$ individual articulating vertebrae protect the spinal cord, while the remaining inferior vertebrae fuse into the sacrum and coccyx.

  • Thoracic Cage (Rib Cage): Composed of $12$ pairs of ribs, the sternum, and the thoracic region of the vertebral column; safeguards the lungs, heart, and major thoracic vessels.

• Appendicular Skeleton:

  • Composed of the bones of the limbs and the girdles that attach them to the axial skeleton; adapted primarily for motion, support, and muscle anchoring.

  • Pectoral Girdle: Formed by the clavicle and scapula; anchors the upper limb to the thoracic trunk.

  • Upper Limb: Divided into three regional compartments:

  • Arm (brachium): humerus.

  • Forearm (antebrachium): radius and ulna.

  • Wrist and Hand (manus): carpals, metacarpals, and phalanges.

  • Pelvic Girdle: Formed by two pelvic (coxal) bones and the sacrum, collectively called the pelvis; anchors the lower limbs to the trunk.

  • Lower Limb: Divided into three regional compartments:

  • Thigh: femur.

  • Leg (crus): tibia and fibula.

  • Ankle and Foot (pes): tarsals, metatarsals, and phalanges.

• Bone Markings and the Structure-Function Principle:

  • Bone surfaces display distinct morphological features adapted for specific physiological roles:

  • Depressions: Clefts of varying depth located where a bone meets another bone or a blood vessel; function as conduits or articular surfaces.

  • Facet: A shallow convex or concave articular surface (e.g., articular facet on a rib articulating with a vertebral transverse process).

  • Fossa (plural, fossae): An indentation into which another anatomical structure fits (e.g., olecranon fossa on the distal posterior humerus).

  • Fovea: A distinct shallow pit (e.g., fovea capitis on the head of the femur).

  • Groove (or sulcus): A long, narrow indentation along which a nerve or blood vessel travels (e.g., costal groove of a rib).

  • Openings: Penetrations that enclose and permit the passage of blood vessels and nerves, grant access to inner structures (such as middle and inner ear cavities), and protect neurovascular tissue from trauma.

  • Canal (or meatus): A tubular passage or tunnel through bone (e.g., external acoustic meatus of the temporal bone).

  • Fissure: A narrow slit within a bone or between adjacent bones (e.g., superior orbital fissure of the sphenoid bone).

  • Foramen (plural, foramina): A hole through a bone (e.g., supraorbital foramen of the frontal bone).

  • Projections: Outgrowths that serve as attachment points for muscles, tendons, and ligaments, or articulate with neighboring depressions to stabilize joints.

  • Condyle: A rounded articular prominence at the end of a bone (e.g., mandibular condyle of the mandible).

  • Crest: A prominent ridge or projection (e.g., iliac crest of the ilium).

  • Head: A rounded projection supported on the epiphysis of a bone (e.g., head of the humerus).

  • Tubercle and Tuberosity: A tubercle is a small, rounded projection; a tuberosity is a larger, rougher projection (e.g., deltoid tuberosity of the humerus).

  • Epicondyle: A projection situated proximal to a condyle (e.g., medial epicondyle of the humerus).

  • Process: Any prominent bony extension (e.g., coracoid process of the scapula).

  • Spine: A sharp, slender projection (e.g., spine of the scapula).

  • Protuberance: An outgrowth or swelling from a bone (e.g., external occipital protuberance of the occipital bone).

  • Trochanter: A massive, rough projection found exclusively on the femur (e.g., greater trochanter of the femur).

  • Line: A long, narrow, low ridge (e.g., linea aspera of the femur).

The Skull: Cranial and Facial Bones

• Organization of the Skull:

  • The skull contains $22$ bones grouped into $8$ cranial bones and $14$ facial bones.

  • All skull bones are united by immovable joints called sutures, with the sole exception of the freely movable mandible at the temporomandibular joint (TMJ).

• The Cranial Bones ($8$ Total):

  • Frontal Bone ($1$):

  • Forms the forehead (frontal squama) and the superior margins of the orbits.

  • Supraorbital Margins: Sharp ridges forming the superior border of the orbit.

  • Supraorbital Foramen: A small perforation located along the supraorbital margin transmitting nerves and vessels to the forehead.

  • Glabella: The smooth region situated directly between the orbits and superior to the nasion.

  • Internally houses the paired frontal sinuses.

  • Parietal Bones ($2$):

  • Paired bones forming the superior roof and superolateral walls of the cranial vault.

  • Meet along the sagittal suture at the midline.

  • Articulate with the frontal bone anteriorly at the coronal suture.

  • Articulate with the temporal bones inferolaterally at the squamous sutures.

  • Articulate with the occipital bone posteriorly at the lambdoid suture.

  • Occipital Bone ($1$):

  • Forms the posterior wall and floor of the cranial cavity (posterior cranial fossa).

  • Foramen Magnum: The large aperture through which the brainstem continues inferiorly as the spinal cord.

  • Occipital Condyles: Smooth oval projections lateral to the foramen magnum that articulate with the superior articular facets of the first cervical vertebra ($C_1$, atlas).

  • Nuchal Lines: Transverse ridges consisting of the superior nuchal line and inferior nuchal line that serve as neck muscle attachment sites.

  • External Occipital Protuberance: A prominent midline bump on the posterior surface that continues inferiorly as the external occipital crest.

  • Temporal Bones ($2$):

  • Form the inferolateral walls of the cranium and part of the middle cranial fossa; each consists of four distinct morphological regions:

    • Squamous Region: A flat plate featuring the anteriorly projecting zygomatic process (which forms the posterior half of the zygomatic arch) and the mandibular fossa (which receives the condyle of the mandible).

    • Tympanic Region: Surrounds the external acoustic meatus (ear canal) and projects an inferior, needle-like styloid process for muscle and ligament attachment.

    • Mastoid Region: Located posterior to the tympanic region; contains the mastoid process filled with tiny, air-filled mastoid air cells.

    • Petrous Region: A dense, wedge-shaped internal ridge forming part of the middle and posterior cranial fossae. Contains the internal acoustic meatus (transmitting cranial nerves from the inner ear), the jugular foramen (passage for the internal jugular vein and cranial nerves), the carotid canal (passage for the internal carotid artery), and the foramen lacerum (slit-like canal covered by cartilage in life, transmitting small vessels and nerves).

  • Sphenoid Bone ($1$):

  • Known as the "keystone" of the cranium because it articulates with every other cranial bone; resembles a bat or stingray.

  • Central Body: Hollow middle region containing the paired sphenoidal sinuses.

  • Sella Turcica: A saddle-shaped depression on the superior surface of the body ("Turk's saddle") housing the hypophysis (pituitary gland).

  • Optic Canals (Optic Foramina): Openings situated anterior to the sella turcica transmitting the optic nerves ($\text{CN II}$) and ophthalmic arteries.

  • Greater Wings: Lateral expanses forming parts of the middle cranial fossa, the temple, and the posterior orbit. Feature three serial openings: foramen rotundum, foramen ovale, and foramen spinosum.

  • Lesser Wings: Slender, horn-like superior extensions; separated from the greater wings by the elongated superior orbital fissure.

  • Pterygoid Processes: Paired inferior projections providing anchoring surfaces for the pterygoid muscles of mastication.

  • Ethmoid Bone ($1$):

  • Positioned deep in the anterior cranium between the orbits, posterior to the nasal bones.

  • Cribriform Plate: Forms the roof of the nasal cavity; perforated by tiny cribriform (olfactory) foramina that transmit olfactory nerve fibers ($\text{CN I}$).

  • Crista Galli: A superior midline ridge ("rooster's comb") extending above the cribriform plate to anchor the falx cerebri meningeal dura mater.

  • Perpendicular Plate: A thin vertical lamina projecting inferiorly into the nasal cavity, forming the superior portion of the bony nasal septum.

  • Lateral Masses: Flank the perpendicular plate; contain thin-walled ethmoidal sinuses (air cells). Their lateral sides form the thin medial orbital plates, and their medial aspects form the superior nasal conchae and middle nasal conchae.

• The Facial Bones ($14$ Total):

  • Nasal Bones ($2$): Small rectangular paired bones forming the bridge of the nose; articulate inferiorly with flexible hyaline nasal cartilages.

  • Lacrimal Bones ($2$): Delicate, fingernail-sized bones located in the medial orbital wall; feature the lacrimal fossa through which lacrimal fluid (tears) drains into the nasal cavity.

  • Zygomatic Bones ($2$): Form the prominent cheekbones, the lateral margins and floor of the orbits, and join the zygomatic process of the temporal bone to complete the zygomatic arch.

  • Palatine Bones ($2$): L-shaped bones positioned in the posterior nasal cavity; their horizontal plates unite to form the posterior third of the hard palate, while their perpendicular plates form portions of the lateral nasal walls and orbit apex.

  • Maxillary Bones / Maxillae ($2$): Keystone bones of the facial skeleton that fuse at the intermaxillary suture to form the upper jaw:

  • Alveolar Processes: Inferior arches containing dental alveoli (deep tooth sockets).

  • Palatine Processes: Horizontal medial projections forming the anterior two-thirds of the hard palate.

  • Maxillary Sinuses: Large paranasal sinuses within the bodies of the maxillae.

  • Anterior Nasal Spine: A sharp anterior projection at the base of the nasal septum.

  • Infraorbital Foramen: Located immediately below the orbit; conducts sensory nerves and vessels to the facial skin.

  • Inferior Nasal Conchae ($2$): Independent, curved scroll-like bones projecting medially from the lateral walls of the nasal cavity, situated inferior to the middle nasal conchae of the ethmoid.

  • Vomer ($1$): An unpaired, plow-shaped bone that forms the inferior and posterior parts of the bony nasal septum.

  • Mandible ($1$): The inferior jawbone; the only movable bone of the adult skull:

  • Mandibular Body: Horizontal, U-shaped curved portion forming the chin.

  • Mandibular Rami: Paired vertical arms ascending from the body at the mandibular angles.

  • Mandibular Condyle (Condylar Process): Posterior projection articulating with the mandibular fossa of the temporal bone.

  • Coronoid Process: Anterior projection serving as an attachment point for the temporalis muscle.

  • Mandibular Notch: U-shaped depression separating the condylar and coronoid processes.

  • Mental Foramen: Lateral opening on the body transmitting sensory nerves from the chin and lower lip.

  • Mandibular Foramen: Medial opening on the ramus granting neurovascular access to the lower teeth.

Cavities of the Skull and Associated Structures

• Cranial Cavity Architecture:

  • Divided into a superior cranial vault (calvarium) and an inferior cranial base.

  • The base contains three stepped fossae that cradle brain lobes:

  • Anterior Cranial Fossa: Formed by the frontal bone, ethmoid bone, and lesser wings of the sphenoid; supports the frontal lobes.

  • Middle Cranial Fossa: Formed by the sphenoid body, greater wings, and petrous temporal bones; cradles the temporal lobes.

  • Posterior Cranial Fossa: Deepest fossa, formed by the occipital and petrous temporal bones; encloses the cerebellum, pons, and medulla oblongata.

• The Orbits:

  • Bony sockets housing the eyeballs, lacrimal glands, extrinsic ocular muscles, adipose tissue, and cranial nerves ($\text{CN II}$, $\text{CN III}$, $\text{CN IV}$, $\text{CN V}_1$, $\text{CN VI}$).

  • Constructed from seven individual bones:

  • Superior Wall (Roof): Frontal bone and lesser wing of the sphenoid.

  • Lateral Wall: Zygomatic bone and greater wing of the sphenoid.

  • Inferior Wall (Floor): Maxilla, zygomatic bone, and palatine bone.

  • Medial Wall: Ethmoid bone (orbital plate), lacrimal bone, frontal bone, and maxilla.

• The Nasal Cavity:

  • First anatomical compartment of the respiratory tract, subdivided into right and left nasal fossae by the nasal septum.

  • Roof: Cribriform plate of the ethmoid bone.

  • Floor (Hard Palate): Palatine processes of the maxillae and horizontal plates of the palatine bones.

  • Lateral Walls: Ethmoid bone (superior and middle conchae), perpendicular plates of the palatine bones, inferior nasal conchae, maxillae, and lacrimal bones.

  • Nasal Septum: Formed superiorly by the perpendicular plate of the ethmoid, inferiorly/posteriorly by the vomer, and anteriorly by hyaline septal cartilage.

• Paranasal Sinuses:

  • Air-filled, ciliated mucous membrane-lined cavities located inside four bones: frontal, ethmoid, sphenoid, and maxillary.

  • Communicate directly with the nasal cavity via small ostia.

  • Functions:

  • Warm, humidify, and filter inhaled air.

  • Significantly reduce the overall mass/weight of the skull.

  • Act as resonating chambers to enhance voice acoustic qualities.

• Oral Cavity:

  • Houses the tongue, teeth, and openings of the salivary glands; forms the initial portion of the digestive tract.

  • Roof is formed by the bony hard palate.

  • Anterior and lateral boundaries are formed by the maxillae, mandible, and teeth.

  • Bony posterior wall and floor are absent, lined instead by soft muscular and connective tissues.

• The Hyoid Bone:

  • A C-shaped bone located in the anterior neck between the mandible and larynx.

  • Does not articulate directly with any other bone in the human body.

  • Suspended by the stylohyoid ligaments from the styloid processes of the temporal bones and connected to the thyroid cartilage of the larynx.

  • Consists of an anterior central body, paired lesser horns, and paired greater horns.

  • Serves as an attachment platform for tongue, suprahyoid, and infrahyoid muscles involved in swallowing and speech production.

Development, Age, and Forensic Anatomy of the Skull

• The Fetal Skull and Fontanels:

  • Fetal cranial bones are not rigidly fused, possessing fibrous connective tissue membranes termed fontanels ("soft spots").

  • Allow the fetal cranium to compress and deform slightly during passage through the maternal birth canal.

  • Accommodate rapid postnatal brain expansion; typically close completely by $18\text{–}24$ months of age.

  • Major Fontanels:

  • Anterior Fontanel: Located at the junction of the sagittal, coronal, and frontal (metopic) sutures; the largest fontanel, closing around $18\text{–}24$ months.

  • Posterior Fontanel: Located at the junction of the sagittal and lambdoid sutures; typically closes around $2\text{–}3$ months.

  • Sphenoid Fontanels (paired): Located at the pterion region where the sphenoid, parietal, temporal, and frontal bones meet.

  • Mastoid Fontanels (paired): Located at the junction of the squamous and lambdoid sutures near the mastoid region.

  • The frontal bone is divided in the fetus by the frontal (metopic) suture, which normally fuses during childhood into a single bone.

• Forensic Skull Anatomy and Sexual Dimorphism:

  • Forensic identification of human skeletal remains relies on primary morphological distinctions between male and female skulls:

  • Forehead Contour: Males exhibit a sloping forehead (receding frontal squama); females exhibit a vertical, straight forehead.

  • Supraorbital Margins and Ridges: Males possess thick, prominent, rounded supraorbital margins with pronounced superciliary arches; females have sharp, thin supraorbital margins.

  • Mandibular Angle: The angle of the mandibular ramus to the body in males is sharp and approaches approximately $90^\circ$; in females, the mandibular angle is more obtuse, exceeding $90^\circ$.

  • Mastoid Process: Male mastoid processes are substantially larger and more robust; female mastoid processes are smaller and pointed.

  • Cranial size, suture closure stages, and dental eruption profiles provide markers for age estimation, while discrete facial indices provide information regarding ethnic ancestry.

The Vertebral Column

• Overview and Vertebral Classes:

  • The vertebral column (spine) typically consists of $33$ vertebrae categorized into five distinct anatomical regions:

  • Cervical vertebrae: $7$ ($C_1\text{–}C_7$) located in the neck.

  • Thoracic vertebrae: $12$ ($T_1\text{–}T_{12}$) articulating with ribs.

  • Lumbar vertebrae: $5$ ($L_1\text{–}L_5$) situated in the lower back.

  • Sacral vertebrae: $5$ fused vertebrae forming the sacrum.

  • Coccygeal vertebrae: $3\text{–}5$ (typically $4$) fused vertebrae forming the coccyx.

  • Intervertebral foramina are lateral openings formed between adjacent vertebrae that provide exit routes for spinal nerve roots.

• Spinal Curvatures:

  • Primary Curvatures (Thoracic and Sacral): Present during fetal development; retain a continuous anteriorly concave (posteriorly convex) C-curve.

  • Secondary Curvatures (Cervical and Lumbar): Develop postnatally as infants achieve motor milestones; are anteriorly convex (posteriorly concave):

  • Cervical Curvature: Develops when an infant begins to lift and support its head ($3\text{–}4$ months).

  • Lumbar Curvature: Develops when a toddler begins standing and walking, shifting the center of gravity over the sacral base for bipedal balance.

  • Abnormal Curvatures:

  • Scoliosis: Pathological lateral curvature of the vertebral column, often with rotational deformity.

  • Lordosis ("Swayback"): Exaggerated lumbar (and sometimes cervical) curvature; seen during pregnancy or abdominal obesity.

  • Kyphosis ("Hunchback"): Exaggerated thoracic curvature; frequently associated with osteoporosis and vertebral wedge compression fractures.

• General Structure of a Vertebra:

  • Body (Centrum): The anterior, thick, cylindrical disc-shaped mass; primary weight-bearing component.

  • Vertebral Foramen: Large central opening bounded by the body and the vertebral arch; the aligned foramina of all articulated vertebrae form the vertebral canal enclosing the spinal cord.

  • Vertebral Arch: Formed by two lateral pedicles and two posterior laminae:

  • Pedicles: Short, thick bony pillars projecting posteriorly from the body; possess superior and inferior vertebral notches that form intervertebral foramina when stacked.

  • Laminae: Broad, flat plates fusing along the posterior midline to enclose the vertebral foramen.

  • Bony Processes ($7$ Total):

  • Spinous Process ($1$): Median posterior projection extending from the junction of the laminae; site for muscle and ligament attachment.

  • Transverse Processes ($2$): Extend laterally from the junctions of the pedicles and laminae.

  • Superior and Inferior Articular Processes ($4$): Paired projections with hyaline cartilage-covered articular facets that form synovial facet joints between successive vertebrae.

• Intervertebral Discs:

  • Consists of $23$ fibrocartilaginous pads positioned between adjacent vertebral bodies from $C_2$ down to the sacrum; none exists between the occiput and $C_1$, nor between $C_1$ and $C_2$.

  • Functions to absorb compressive shocks, facilitate spine flexibility, and anchor adjacent vertebrae together.

  • Nucleus Pulposus: Central gelatinous core with high water content; acts as a resilient hydraulic shock absorber.

  • Anulus Fibrosus: Outer ring of concentric fibrocartilage lamellae and collagen fibers; confines the nucleus pulposus and resists rotational stresses.

• Pathology of Intervertebral Discs and Vertebral Bodies:

  • Herniated Disc ("Slipped Disc"):

  • Structural compromise or tearing of the anulus fibrosus (predominantly posterolateral, where the posterior longitudinal ligament is thinnest), allowing the nucleus pulposus to protrude.

  • Compresses adjacent spinal nerve roots traversing the intervertebral foramen, generating radiating pain, paresthesias, sensory loss, or muscle weakness.

  • Treatments: Anti-inflammatory medications, physical therapy, epidural corticosteroid injections, and surgical decompression (discectomy).

  • Vertebral Compression Fractures:

  • Structural failure reducing the height of the vertebral body by greater than $20\%$.

  • Wedge Fractures: Collapse isolated to the anterior aspect of the body; common in elderly females with osteoporosis or thoracic kyphosis; can be induced by minor mechanical stresses (coughing, sneezing, or rolling in bed).

  • Burst Fractures: High-energy axial compression causing comminuted collapse of the entire vertebral body; bone fragments carry severe risk of spinal cord laceration.

Regional Vertebrae, the Sacrum, and the Coccyx

• Regional Distinctions Among Vertebrae:

  • Cervical Vertebrae ($C_1\text{–}C_7$):

  • Smallest vertebral bodies; oval in shape.

  • Possess a triangular vertebral foramen.

  • Transverse processes feature unique transverse foramina that transmit the vertebral arteries and veins.

  • Spinous processes are typically bifid (fork-shaped) in vertebrae $C_2\text{–}C_6$.

  • Vertebra $C_7$ is termed the vertebra prominens, possessing a long, non-bifid, easily palpable spinous process.

  • Atlas ($C_1$): Lacks a vertebral body and spinous process; consists of an anterior arch, posterior arch, and lateral masses bearing concave superior articular facets that articulate with the occipital condyles (enables nodding the head "yes").

  • Axis ($C_2$): Features the dens (odontoid process) projecting superiorly from its body, anchored against the anterior arch of the atlas by the transverse ligament; acts as a pivot around which the atlas rotates (enables shaking the head "no").

  • Thoracic Vertebrae ($T_1\text{–}T_{12}$):

  • Larger, heart-shaped vertebral bodies.

  • Circular vertebral foramina.

  • Long, slender spinous processes that project sharply inferiorly (resembling a giraffe profile in lateral view).

  • Bear articular facets for ribs: superior costal facets and inferior costal facets on the lateral body margin articulate with the heads of ribs; transverse costal facets on the transverse processes articulate with rib tubercles.

  • Lumbar Vertebrae ($L_1\text{–}L_5$):

  • Largest and heaviest vertebrae, adapted for axial weight-bearing; bodies are kidney-shaped.

  • Vertebral foramina are flattened and triangular.

  • Short, thick, blunt, rectangular spinous processes pointing directly posteriorly (resembling a moose profile in lateral view).

  • Possess short, thick pedicles and laminae without costal or transverse foramina.

• The Sacrum and Coccyx:

  • Sacrum ($5$ fused vertebrae):

  • Forms the posterior, posterosuperior wall of the pelvic cavity.

  • Base: Broad superior surface articulating with $L_5$ at the superior articular processes.

  • Sacral Promontory: Anterior midline lip of the sacral base that projects into the pelvic inlet.

  • Alae: Smooth, broad lateral wings expanding from the sacral base.

  • Sacral Canal: Tubular continuation of the vertebral canal transmitting spinal nerve roots (cauda equina).

  • Median Sacral Crest: Posterior midline ridge formed by the fused spinous processes.

  • Lateral Sacral Crests: Longitudinal ridges representing remnants of the fused transverse processes.

  • Sacral Foramina: Four pairs of anterior and posterior apertures for the exit of sacral spinal nerves.

  • Sacral Hiatus: Inferior termination of the sacral canal near the apex where laminae fail to fuse.

  • Auricular Surfaces: Lateral ear-shaped articular surfaces forming the sacroiliac (SI) joints with the ilium.

  • Coccyx ($3\text{–}5$, typically $4$ fused vertebrae):

  • Terminal portion of the vertebral column; articulates superiorly with the sacral apex.

  • Features transverse ridges demarcating fusion lines and paired superior coccygeal cornua.

The Thoracic Cage

• Components and Functions:

  • Formed by the sternum anteriorly, $12$ pairs of ribs and costal cartilages anterolaterally, and $12$ thoracic vertebrae posteriorly.

  • Encloses and protects thoracic viscera (heart, lungs, great vessels), supports the pectoral girdles, and changes volume during pulmonary ventilation.

• The Sternum (Breastbone):

  • A flat, dagger-shaped bone positioned along the anterior midline of the thorax; consists of three segments:

  • Manubrium: Superior segment; features the superior suprasternal (jugular) notch and paired lateral clavicular notches (articulating with the clavicles). The first costal cartilages attach directly below the clavicular notches.

  • Sternal Angle (Angle of Louis): The transverse cartilaginous ridge where the manubrium and sternal body meet; landmark indicating the level of the second rib attachment and the $T_4\text{–}T_5$ vertebral interspace.

  • Body: Long, central plate; features articular notches along its lateral edges for the costal cartilages of ribs $3\text{–}7$.

  • Xiphoid Process: Slender, cartilaginous inferior projection that ossifies around age $40$; serves as an attachment point for rectus abdominis and the linea alba.

  • Clinical Relevance (Cardiopulmonary Resuscitation - CPR): Hands must be placed over the lower half of the sternal body, between the nipples. Direct compressions over the fragile xiphoid process can fracture it and cause laceration of underlying organs such as the liver or diaphragm.

• The Ribs and Costal Cartilages:

  • Classification of Ribs ($12$ Pairs):

  • True Ribs (Vertebrosternal, Ribs $1\text{–}7$): Attach directly to the sternum via individual strips of hyaline costal cartilage.

  • False Ribs (Ribs $8\text{–}12$): Do not possess a direct attachment to the sternum:

    • Vertebrochondral Ribs (Ribs $8\text{–}10$): Their costal cartilages fuse with the cartilage of rib $7$ immediately superior to them, contributing to the costal margin.

    • Floating Ribs (Vertebral, Ribs $11\text{–}12$): Completely lack an anterior costal attachment; embedded solely in the lateral abdominal musculature.

  • Structure of a Typical Rib:

  • Head: Posterior extremity bearing superior and inferior articular facets that articulate with the costal facets on the bodies of thoracic vertebrae.

  • Neck: Constricted zone directly lateral to the head.

  • Tubercle: Posterior projection lateral to the neck possessing a smooth articular facet that articulates with the transverse costal facet of the matching vertebra.

  • Angle: The point where the shaft bends sharply anterolaterally.

  • Shaft (Body): Flat, curved ribbon of bone; its inferior internal border displays a costal groove housing the intercostal vein, artery, and nerve.

The Pectoral Girdle and Upper Limb

• The Pectoral Girdle:

  • Consists of the clavicle and scapula; anchors each upper limb to the axial skeleton.

  • Possesses only one direct bony articulation with the axial skeleton: the sternoclavicular joint.

  • Clavicle (Collarbone):

  • Slender, S-shaped strut bone that holds the scapula and upper limb laterally away from the thorax.

  • Sternal End: Medial, bulbous, blunt end articulating with the manubrium at the sternoclavicular joint.

  • Acromial End: Lateral, flattened end articulating with the acromion of the scapula at the acromioclavicular (AC) joint.

  • Conoid Tubercle: Roughened projection on the inferolateral surface providing attachment for the coracoclavicular ligament.

  • Scapula (Shoulder Blade):

  • Flat, triangular bone positioned over posterior ribs $2\text{–}7$.

  • Features three borders: superior, medial (vertebral), and lateral (axillary); and three angles: superior, inferior, and lateral.

  • Spine: Prominent ridge on the posterior surface separating the superior supraspinous fossa from the inferior infraspinous fossa.

  • Acromion: Flattened, lateral expanded terminal process of the spine; articulates with the clavicle and caps the shoulder joint.

  • Coracoid Process: Hook-like anterior projection extending beneath the clavicle; anchors biceps brachii and pectoral muscles.

  • Glenoid Cavity: Shallow lateral socket that articulates with the head of the humerus to form the glenohumeral (shoulder) joint.

  • Subscapular Fossa: Smooth, broad anterior depression seating the subscapularis muscle.

• The Humerus (Arm / Brachium):

  • Longest and largest bone of the upper limb.

  • Proximal Epiphysis: Hemispherical head articulates with the glenoid cavity; bounded by an anatomical neck. Features a lateral greater tubercle and a medial lesser tubercle, separated by the intertubercular sulcus (bicipital groove) through which the tendon of the long head of the biceps brachii passes.

  • Surgical Neck: Tapering region distal to the tubercles; frequent site of fractures.

  • Diaphysis: Features a lateral roughened deltoid tuberosity and an oblique posterior radial groove conveying the radial nerve.

  • Distal Epiphysis: Medial epicondyle and lateral epicondyle serve as origin points for forearm muscles. Articular condyles include:

  • Capitulum: Lateral, rounded button articulating with the head of the radius.

  • Trochlea: Medial, spool-shaped pulley articulating with the trochlear notch of the ulna.

  • Radial Fossa: Shallow depression anterior to the capitulum receiving the radial head during full elbow flexion.

  • Coronoid Fossa: Anterior depression proximal to the trochlea receiving the coronoid process of the ulna.

  • Olecranon Fossa: Deep posterior depression receiving the olecranon process of the ulna upon full extension.

• The Forearm (Antebrachium: Radius and Ulna):

  • Connected along their interosseous borders by a dense fibrous sheet termed the interosseous membrane; articulate proximally and distally at proximal and distal radioulnar joints.

  • Radius (Lateral):

  • Proximal head is disc-shaped; articulates with the humeral capitulum and the radial notch of the ulna.

  • Followed by a constricted radial neck and a medial radial tuberosity (insertion site for the biceps brachii).

  • The expanded distal epiphysis features a medial ulnar notch (articulating with the ulnar head) and a lateral pointed radial styloid process.

  • Ulna (Medial):

  • Proximal end features the deep U-shaped trochlear notch, bounded posteriorly by the prominent olecranon ("point of the elbow") and anteriorly by the coronoid process.

  • The radial notch of the ulna is a lateral articular facet receiving the circumference of the radial head.

  • Distal end terminates in a small ulnar head and a posteromedial ulnar styloid process.

• The Wrist, Hand, and Digits:

  • Carpals ($8$ Short Bones): Arranged into two transverse rows of four bones each:

  • Proximal Row (Lateral to Medial): Scaphoid (boat-shaped; articulates with the radius), Lunate (crescent-shaped), Triquetrum (triangular; articulates with the fibrocartilage articular disc of the ulna), Pisiform (pea-shaped sesamoid bone resting on the anterior triquetrum).

  • Distal Row (Lateral to Medial): Trapezium (articulates with metacarpal $I$ of the thumb), Trapezoid (wedge-shaped), Capitate (largest, rounded carpal), Hamate (possesses an anterior hook-like process, the hamulus).

  • Metacarpals ($5$ Long Bones): Numbered $I$ to $V$ from lateral (thumb) to medial (little finger); each possesses a proximal base, a diaphysis (body), and a distal rounded head forming the knuckles.

  • Phalanges ($14$ Bones of the Digits):

  • Digits $II$ through $V$ each contain three phalanges: proximal, middle, and distal phalanx.

  • Digit $I$ (the thumb, or pollex) contains only two phalanges: proximal and distal phalanx.

• Clinical Relevance: Wrist Fractures:

  • The wrist is the most frequently injured skeletal region of the upper limb, commonly resulting from a fall onto an outstretched hand (FOOSH injury).

  • Colles Fracture: A fracture of the distal radius causing posterior displacement of the distal fragment, producing a characteristic "dinner fork" deformity; frequently accompanied by an ulnar styloid fracture.

  • Scaphoid Fracture: The most frequently fractured carpal bone; susceptible to avascular necrosis due to limited retrograde arterial perfusion.

  • Demographics: Wrist fractures peak in children and adults aged $60\text{–}69$ years; in individuals over age $70$, incidence shifts predominantly to hip fractures because individuals fail to extend their arms during falls.

The Pelvic Girdle and Lower Limb

• Structure of the Pelvis:

  • Composed of the two pelvic bones (coxal bones or os coxae) and the sacrum.

  • Pelvic Brim: A continuous oval bony ridge formed by the sacral promontory, arcuate lines, pectineal lines, and pubic crests; subdivides the pelvis into two regions:

  • Greater (False) Pelvis: Broad space situated superior to the pelvic brim, bounded laterally by the iliac fossae; forms part of the lower abdominal cavity.

  • Lesser (True) Pelvis: Space located inferior to the pelvic brim; encloses the pelvic cavity.

  • Pelvic Apertures:

  • Pelvic Inlet: Superior opening into the true pelvis circumscribed by the pelvic brim.

  • Pelvic Outlet: Inferior boundary of the true pelvis bounded by the pubic arch, ischial spines, ischial tuberosities, and coccyx.

• The Coxal Bone (Hip Bone / Os Coxae):

  • Formed by the childhood fusion of three separate bones: the ilium, ischium, and pubis; all three contribute to the deep lateral cup-shaped socket, the acetabulum (which articulates with the femoral head).

  • Ilium:

  • Largest, most superior bone of the coxal complex.

  • Ala: Flared superior wing providing an internal hollow, the iliac fossa.

  • Iliac Crest: Superior border of the ala terminating anteriorly at the anterior superior iliac spine (ASIS) and posteriorly at the posterior superior iliac spine (PSIS).

  • Inferior Spines: Anterior inferior iliac spine (AIIS) and posterior inferior iliac spine (PIIS) situated directly below the superior spines.

  • Greater Sciatic Notch: Deep posterior indentation through which the sciatic nerve passes into the thigh.

  • Arcuate Line: Bony ridge on the internal surface defining the iliac segment of the pelvic brim.

  • Gluteal Lines: Three ridges on the lateral surface (posterior, anterior, and inferior gluteal lines) providing origin boundaries for the gluteal muscles.

  • Ischium:

  • Forms the posteroinferior third of the coxal bone.

  • Ischial Body: Contributes to the posteroinferior margin of the acetabulum.

  • Ischial Spine: Sharp medial projection separating the greater sciatic notch from the lesser sciatic notch.

  • Ischial Tuberosity: Roughened, thickened inferolateral region that bears body weight in the seated position.

  • Ischial Ramus: Slender anterior process that fuses with the inferior pubic ramus.

  • Pubis:

  • Forms the anterior portion of the coxal bone.

  • Composed of a pubic body, a superior pubic ramus (which helps form the acetabulum), and an inferior pubic ramus.

  • The two pubic bodies articulate at the anterior midline joint, the pubic symphysis, separated by a fibrocartilaginous interpubic disc.

  • The pubic rami and ischial ramus encircle the obturator foramen, a large aperture closed in life by the fibrous obturator membrane except for a small neurovascular canal.

• Pelvic Sexual Dimorphism:

• Greater Pelvis: Wider and shallower in females with more flared iliac crests; narrower and deeper in males.

• Pelvic Inlet: Broad, open, and oval-shaped in females; narrow, restricted, and heart-shaped in males.

• Pelvic Outlet: Wider and roomier in females with greater transverse diameters; narrower in males.

• Pubic Arch / Subpubic Angle: Measures between $90^\circ\text{–}100^\circ$ in females (broad U-shape); measures between $60^\circ\text{–}70^\circ$ in males (sharp V-shape).

• Sacrum: Wider, shorter, and less curved anteriorly in females; longer, narrower, and more curved in males.

• Coccyx: Straighter and more movable in females; curved anteriorly and more rigid in males.

• Acetabula: Positioned further apart and directed more anteriorly in females; positioned closer together and directed laterally in males.

• Ischial Tuberosities: Point outward laterally in females; point inward medially in males.

  • The Femur and Patella (Thigh):

• Femur: The longest, heaviest, and strongest bone in the body:

  • Head: Spherical proximal epiphysis articulating with the acetabulum; features a central depression, the fovea capitis, anchoring the ligamentum teres femoris.

  • Neck: Constricted bridge connecting the head to the shaft; angled laterally at approximately $125^\circ$.

  • Trochanters: Massive traction processes for muscle attachment. The greater trochanter is positioned laterally; the lesser trochanter is situated posteromedially. They are united anteriorly by the intertrochanteric line and posteriorly by the prominent intertrochanteric crest.

  • Diaphysis: Features a longitudinal posterior ridge, the linea aspera, which bifurcates distally into medial and lateral supracondylar lines.

  • Distal Epiphysis: Flared medial condyle and lateral condyle articulate with the tibial plateaus; separated posteriorly by the deep intercondylar fossa. Lateral and medial epicondyles flank the condyles. The anterior distal surface forms the smooth patellar surface.

• Patella (Kneecap):

  • Triangular sesamoid bone enclosed within the quadriceps femoris tendon.

  • Possesses a broad, flat proximal base and a pointed inferior apex.

  • Its apex attaches to the patellar ligament, which inserts into the tibial tuberosity.

  • The smooth posterior surface possesses medial and lateral articular facets for articulation with the femoral patellar surface.

• The Leg (Crus: Tibia and Fibula):

  • Tibia (Medial / Shinbone):

  • The broad, primary weight-bearing bone of the leg.

  • Proximal end features the medial condyle and lateral condyle, separated on the plateau by the irregular intercondylar eminence.

  • Tibial Tuberosity: Rough anterior midline projection distal to the condyles where the patellar ligament inserts.

  • Diaphysis: Distinct triangular shaft possessing a sharp anterior border (anterior crest, or "shin").

  • Distal Epiphysis: Expands into a broad articular surface for the talus, terminating medially in a prominent process, the medial malleolus.

  • Fibula (Lateral):

  • Slender, non-weight-bearing strut bone (supports only approximately one-sixth of lower extremity load).

  • Proximal head articulates with the lateral condyle of the tibia at the proximal tibiofibular joint.

  • Distal end expands into the lateral malleolus, which forms the lateral ankle prominence and articulates with the lateral surface of the talus.

• The Ankle, Foot, and Arches:

  • Tarsals ($7$ Short Bones):

  • Proximal Group: Talus (dome-shaped superior trochlea articulates with the tibia and fibula; only tarsal involved in the ankle joint), Calcaneus (largest tarsal, forming the heel; receives the calcaneal/Achilles tendon posteriorly), Navicular (flattened, boat-shaped bone situated between the talus and the cuneiforms).

  • Distal Group: Medial cuneiform, intermediate cuneiform, lateral cuneiform, and the cuboid (lateral bone articulating proximally with the calcaneus).

  • Metatarsals ($5$ Long Bones): Numbered $I$ through $V$ from medial (great toe) to lateral; each consists of a proximal base, body, and distal head.

  • Phalanges ($14$ Bones of the Toes):

  • Toes $II$ through $V$ each possess three phalanges: proximal, middle, and distal.

  • The hallux (great toe, digit $I$) possesses only two phalanges: proximal and distal.

  • Arches of the Foot:

  • Support body weight and provide spring-like mechanical resilience during locomotion:

  • Medial Longitudinal Arch: Highest and most prominent arch; extends from the calcaneus, through the talus, navicular, three cuneiforms, to metatarsals $I\text{–}III$.

  • Lateral Longitudinal Arch: Lower arch extending from the calcaneus, through the cuboid, to metatarsals $IV\text{–}V$.

  • Transverse Arch: Runs coronally across the tarsometatarsal joints; formed by the cuboid, cuneiforms, and the bases of all five metatarsals.

Anatomical Mnemonics and Study Analogies

• Skull Mnemonic Systems:

  • Cranial Bones ("PEST OF 6"):

  • P: Parietal

  • E: Ethmoid

  • S: Sphenoid

  • T: Temporal

  • O: Occipital

  • F: Frontal

  • Facial Bones ("Virgil Is Now Making My Pet Zebra Laugh"):

  • V: Vomer

  • I: Inferior nasal concha

  • N: Nasal

  • M: Mandible

  • M: Maxilla

  • P: Palatine

  • Z: Zygomatic

  • L: Lacrimal

  • Bones of the Orbit ("Every Student Fancies Learning Zillions More Parts"):

  • E: Ethmoid

  • S: Sphenoid

  • F: Frontal

  • L: Lacrimal

  • Z: Zygomatic

  • M: Maxilla

  • P: Palatine

  • Paranasal Sinuses ("For Easier Sinus Memorization"):

  • F: Frontal

  • E: Ethmoidal

  • S: Sphenoidal

  • M: Maxillary

• Vertebral Mnemonic Systems and Visual Analogies:

  • Vertebral Counts by Meal Times:

  • "Breakfast at $7$:" $7$ cervical vertebrae.

  • "Lunch at 12$:" 12 thoracic vertebrae.\n * "Dinner at 5$:" $5$ lumbar vertebrae.

  • Vertebral Profiles:

  • Thoracic vertebra: Spinous and transverse processes resemble a giraffe's head ("thoRAcic giRAffe").

  • Lumbar vertebra: Spinous and transverse processes resemble a lumbering moose's head ("LUMBering moose").

• Appendicular Skeletal Mnemonics and Analogies:

  • Carpal Bones Proximal-to-Distal, Lateral-to-Medial:

  • "Stop Letting The People Touch The Cadaver's Hand:"

    • S: Scaphoid

    • L: Lunate

    • T: Triquetrum

    • P: Pisiform

    • T: Trapezium (remember: trapeziUM articulates with the thUMb)

    • T: Trapezoid

    • C: Capitate

    • H: Hamate

  • Tarsal Bones Proximal-to-Distal, Medial-to-Lateral:

  • "This College Needs Me In Lab Classes:"

    • T: Talus

    • C: Calcaneus

    • N: Navicular

    • M: Medial cuneiform

    • I: Intermediate cuneiform

    • L: Lateral cuneiform

    • C: Cuboid

  • Leg Bones Identity:

  • TIBia: Thick, Inner Bone.

  • FibuLA: LAteral bone.

  • Arm and Wrist Geometric Analogies:

  • The proximal trochlear notch of the ulna forms the letter U for Ulna.

  • The radius is located on the thumb side; making a "thumbs up" gesture is "rad" or "radical" (radial side).

  • The radial pulse is palpated over the lateral distal radius.

  • Patella translates in Latin to "little plate."

  • Navicular translates to "boat-shaped" (associated with navy).