Calisthenics Instructor Certification Chapter 3: Bones of the Human Body

Chapter 3: Bones of the Human Body

1. Overview of the Skull

The skull (cranium) is divided into two major regions:

  1. Neurocranium (Braincase)
  2. Viscerocranium (Facial skeleton)

The neurocranium protects the brain and consists of eight bones:

  • Single (unpaired): Frontal, Occipital, Sphenoid, Ethmoid
  • Paired: Temporal (2), Parietal (2)

2. Bones of the Neurocranium

2.1 Frontal Bone

  • Location: Anterior surface, forms the forehead.
  • Key features: Supraorbital ridges, frontal sinuses.

2.2 Parietal Bones (2)

  • Location: Superior and lateral surfaces of the neurocranium.
  • Articulations: Meet at the sagittal suture; join the frontal bone via the coronal suture, and the occipital bone via the lambdoid suture.

2.3 Occipital Bone

  • Location: Posterior surface of the skull.
  • Key features: Foramen magnum (spinal cord passage), occipital condyles (articulation with C1).

2.4 Temporal Bones (2)

  • Location: Lateral surfaces of the base and sides of the skull.
  • Key feature: External acoustic meatus (ear canal).

2.5 Sphenoid Bone

  • Location: Floor of the neurocranium, central wedge-shaped bone.
  • Key features: Sella turcica (pituitary gland housing), greater and lesser wings.

2.6 Ethmoid Bone

  • Location: Roof of the nasal cavity, between the two orbits.
  • Key features: Cribriform plate (olfactory nerve foramina), perpendicular plate (nasal septum).

3. Cranial Sutures

Cranial bones are joined by immovable fibrous joints called sutures:

Suture Name Bones Connected
Sagittal Right and left parietal bones
Coronal Parietal bones with the frontal bone
Lambdoid Parietal bones with the occipital bone

4. Development of the Infant Skull

  • At birth: Consists of 7 major bones separated by membranous gaps (fontanelles and sutures).
  • Functions:
    1. Molding during birth: Flexibility allows the head to pass through the birth canal.
    2. Rapid growth: Accommodates the brain’s rapid expansion in the first two years (brain reaches ~75% adult volume by age 2).

4.1 Fontanelles (“Soft Spots”)

Two larger membranous gaps where three sutures converge:

  1. Posterior Fontanelle
    • Located at the junction of the sagittal and lambdoid sutures.
    • Typically closes by 3 months of age.
  2. Anterior Fontanelle
    • Located at the junction of the sagittal and coronal sutures (top/front of skull).
    • Closes between 6 months and 2 years of age.

5. Internal Structure & Sinuses

  • Each cranial bone comprises two compact (dense) layers sandwiching a spongy (diploë) layer.
  • In some regions, the compact layers are separated to form air-filled cavities called paranasal sinuses.
    • Clinical note: Inflammation of these sinuses by infection leads to sinusitis, most commonly involving the maxillary and ethmoid sinuses.

6. Viscerocranium (Facial Skeleton)

The viscerocranium forms the skeleton of the face. It comprises fourteen bones, subdivided into:

  1. Nasal capsule (7 bones)
    • Inferior nasal conchae (2)
    • Nasal bones (2)
    • Lacrimal bones (2)
    • Vomer (1)
  2. Jaw complex (7 bones)
    • Maxillae (upper jaws) (2)
    • Zygomatic bones (cheekbones) (2)
    • Palatine bones (2)
    • Mandible (lower jaw) (1)

6.1 Inferior Nasal Conchae

  • Location: Project from the lateral walls of the nasal cavity.
  • Function: Create narrow air passages to warm, humidify, and filter inhaled air.

6.2 Nasal Bones

  • Location: Form the bridge (dorsum) of the nose.
  • Articulation: Meet at the midline and connect to the frontal bone superiorly and frontals of the maxillae laterally.
  • Clinical note: Common site of nasal fractures in facial trauma.

6.3 Lacrimal Bones

  • Location: Small, thin plates in the medial walls of the orbits.
  • Key feature: Lacrimal fossa houses the lacrimal sac—part of the tear drainage pathway.

6.4 Vomer

  • Location: Forms the posteroinferior portion of the nasal septum.
  • Function: Contributes to the separation of the left and right nasal cavities.

6.5 Maxillae (Upper Jaws)

  • Location: Paired bones that fuse at the midline (intermaxillary suture).
  • Contributions:
    • Orbit floor
    • Nasal cavity walls
    • Hard palate (anterior two-thirds)
  • Key feature: Houses the maxillary sinuses (largest paranasal sinuses).

6.6 Zygomatic Bones (Cheekbones)

  • Location: Lateral to the orbits, forming the cheeks and part of the zygomatic arch.
  • Articulation: Connect with the maxilla, temporal bone, sphenoid, and frontal bone.

6.7 Palatine Bones

  • Location: Posterior part of the hard palate and a portion of the nasal cavity floor.
  • Function: Complete the separation between the oral and nasal cavities.

6.8 Mandible (Lower Jaw)

  • Unique feature: Only movable bone of the skull (via the temporomandibular joints).
  • Key landmarks:
    • Body: Horizontal portion bearing the lower teeth
    • Rami (2): Vertical projections with condylar and coronoid processes
    • Mental protuberance: Chin prominence

7. Clinical Correlations

  • Nasal fractures: Often involve the nasal bones and may impair airway patency.
  • Lacrimal obstruction: Blockage of the lacrimal sac can lead to chronic tearing (epiphora).
  • Cleft palate: Failure of palatine bones to fuse properly results in an opening between oral and nasal cavities.
  • TMJ disorders: Affect the mandible’s condylar articulation—manifesting as pain, clicking, or restricted movement.

8. Thoracic Skeleton

The thoracic skeleton forms the protective cage around the heart, lungs, and great vessels. It consists of:

  1. Twelve thoracic vertebrae (posteriorly)
  2. Sternum (anteriorly)
  3. Twelve pairs of ribs (laterally)

Note: In rare congenital variants, a 13th rib or additional ossicles may be present.

8.1 Sternum

The sternum (breastbone) is a flat, sword-shaped bone located on the anterior chest wall. It comprises three parts:

Part Description
Manubrium Superior “handle,” with the jugular (suprasternal) notch at its top and the clavicular notches on each side.
Body (Gladiolus) Central, elongated portion articulating with costal cartilages of ribs 2–7.
Xiphoid Process Small, cartilaginous tip that ossifies in adulthood. Serves as an attachment for the linea alba and diaphragm.
  • Sternal (Angle of Louis):
    Formed at the junction of the manubrium and body (rib 2 level). Palpable landmark used to count ribs.

8.2 Ribs

Each rib is a curved, flat bone with the following features:

  • Head: Articulates with the vertebral bodies
  • Neck: Short segment just lateral to the head
  • Tubercle: Articulates with the transverse process of the vertebra
  • Body (Shaft): Thin, flattened portion ending in the costal cartilage
  • Costal Groove: Inferior border of the shaft, housing intercostal nerves and vessels

Classification of Ribs

Type Ribs Anterior Attachment
True (Vertebrosternal) 1–7 Directly to the sternum via their own costal cartilages
False (Vertebrochondral) 8–10 Indirectly to the sternum via the costal cartilage of the rib above (costal arch)
Floating (Vertebral) 11–12 Free anterior ends; no sternal attachment

Clinical note:

  • Rib fractures often occur in the middle shaft (most vulnerable region).
  • Flail chest: Multiple adjacent rib fractures can produce a segment that moves paradoxically during respiration.

8.3 Thoracic Vertebrae (T1–T12)

  • Each thoracic vertebra has costal facets on the body and transverse processes for rib articulation.
  • The spinous processes are long and project downward, overlapping adjacent vertebrae.
  • The vertebral bodies increase progressively in size from T1 to T12 to bear greater load.

9. Vertebral Column (Spine)

The vertebral column is the central, posterior supporting axis of the body. It comprises 33–34 vertebrae, arranged into five regions, and separated by intervertebral discs. Viewed laterally, it exhibits four natural curvatures.

9.1 Regions & Counts

Region Abbreviation Number of Vertebrae Notes
Cervical C1–C7 7 Supports the skull; allows head movement
Thoracic T1–T12 12 Articulates with ribs
Lumbar L1–L5 5 Bears the bulk of body weight
Sacral S1–S5 (fused) 5 (fused as one) Forms the posterior pelvic wall
Coccygeal Co1–Co4/5 4–5 (fused as one) “Tailbone”

True vs. False Vertebrae:

  • True (Moving): Cervical, thoracic, and lumbar vertebrae.
  • False (Fixed): Sacral and coccygeal vertebrae fuse into the sacrum and coccyx, respectively.

9.2 Spinal Curvatures

Viewed from the side, the spine has four curves:

  1. Cervical Lordosis (concave posteriorly)
  2. Thoracic Kyphosis (convex posteriorly)
  3. Lumbar Lordosis
  4. Sacrococcygeal Kyphosis
  • Hyperkyphosis: Excessive thoracic curve (“hunchback”)
  • Hyperlordosis: Excessive lumbar curve; can be transient in late pregnancy
  • Scoliosis: Lateral curvature seen on frontal view, often in adolescence

9.3 Vertebral Anatomy

Each true vertebra shares a common plan:

  • Vertebral Body: Weight-bearing, separated by an intervertebral disc
  • Vertebral Arch: Forms the vertebral foramen, through which the spinal cord passes
  • Processes:
    • Spinous Process: Projects posteriorly
    • Transverse Processes (2): Lateral projections for muscle and rib (thoracic) attachments
    • Articular Processes (4): Superior and inferior facets for vertebral articulation

Adjacent vertebrae form intervertebral foramina, transmitting the spinal nerves.

9.4 Intervertebral Discs

  • Structure:
    • Annulus Fibrosus: Tough, outer fibrocartilaginous ring
    • Nucleus Pulposus: Gelatinous core providing shock absorption
  • Disc Herniation:
    • Excess pressure can rupture the annulus, allowing the nucleus to protrude into the spinal canal.
    • Results in radiculopathy (e.g., sciatica) or myelopathy, depending on location.

Disc Degeneration Spectrum:
Protrusion → Prolapse → Extrusion → Sequestration → Stenosis → Degeneration → Disappearance

Imaging: CT and MRI clearly delineate disc pathologies.

10. Pelvic Girdle (Pelvis)

The pelvis (pelvic girdle) is formed by:

  • Two hip bones (os coxae or innominate bones), which connect:
    • Anteriorly at the pubic symphysis
    • Posteriorly with the sacrum

10.1 Sacrum

  • Formed by the fusion of five sacral vertebrae into a single, robust bone.
  • Articulates superiorly with L5 and laterally with each hip bone at the sacroiliac joints.

10.2 Hip Bone (Os Coxae)

Each hip bone comprises three fused elements:

Component Description
Ilium Superior, fan-shaped portion forming the iliac crest and contributing to the acetabulum roof.
Ischium Posteroinferior portion bearing the ischial tuberosity (“sit bone”).
Pubis Anteromedial portion; the two pubic bones meet at the pubic symphysis.
  • Together, the two hip bones and sacrum form the bony pelvis, enclosing the pelvic cavity.

10.3 Pubic Symphysis

  • A cartilaginous joint uniting the left and right pubic bones.
  • Allows minimal movement, increasing slightly during childbirth.

10.4 Acetabulum

  • The deep socket on the lateral aspect of each hip bone.
  • Articulates with the femoral head to form the hip (coxal) joint.

Clinical Notes

  • Sacroiliac Joint: Highly stable but can transmit force between the spine and lower limbs; may become painful in inflammation or pregnancy.
  • Pelvic Differences by Sex:
    • Female pelvis—wider and shallower to facilitate childbirth.
    • Male pelvis—narrower and deeper, optimized for load-bearing.
  • Hip Joint Stability: The deep acetabular socket, labrum, and strong ligaments provide stability for weight-bearing and locomotion.

 

11. Skeleton of the Upper Limb

The upper limb skeleton consists of the following bones:

  • Pectoral girdle: Scapula and clavicle
  • Arm: Humerus
  • Forearm: Ulna and radius
  • Wrist (Carpus): Eight carpal bones
  • Hand: Five metacarpal bones and the phalanges of the fingers

11.1 Scapula (Shoulder Blade)

  • Shape & Location: A flat, triangular bone located on the posterior thoracic wall, spanning ribs 2–7.
  • Key Landmark:
    • Glenoid cavity: Shallow socket at the lateral angle that articulates with the humeral head to form the shoulder joint.
  • Clinical Note: The scapula’s mobility and muscle attachments allow a wide range of shoulder movements but make it susceptible to winging if the serratus anterior or long thoracic nerve is injured.

11.2 Clavicle (Collarbone)

  • Shape & Location: An S-shaped long bone lying horizontally above the first rib.
  • Articulations:
    • Medially: Manubrium of the sternum (sternoclavicular joint)
    • Laterally: Acromion of the scapula (acromioclavicular joint)
  • Function: Acts as a strut to hold the shoulder away from the thorax, transmitting forces from the upper limb to the axial skeleton.
  • Clinical Note: The clavicle is the most commonly fractured bone in the body, typically at its middle third.

11.3 Humerus (Arm Bone)

  • Regions:
    1. Proximal end: Rounded head that fits into the glenoid cavity; greater and lesser tubercles for muscle attachment.
    2. Shaft: Cylindrical proximally, triangular distally.
    3. Distal end: Two articular surfaces—trochlea (medial) for the ulna and capitulum (lateral) for the radius.
  • Clinical Note:
    • Proximal humerus fractures are common in elderly patients with osteoporosis.
    • Supracondylar fractures occur in children and may risk injury to the brachial artery or median nerve.

11.4 Ulna

  • Regions:
    1. Proximal end: Olecranon and coronoid processes form the trochlear notch that grips the humerus in the elbow joint; radial notch accommodates the head of the radius.
    2. Shaft: Prism-shaped proximally, cylindrical distally.
    3. Distal end: Styloid process of the ulna articulates with the wrist and radial head.
  • Function: Primary stabilizer of the forearm in the elbow joint.

11.5 Radius

  • Regions:
    1. Proximal end: Head of the radius articulates with the capitulum of the humerus and the radial notch of the ulna.
    2. Shaft: Prism-shaped.
    3. Distal end: Broad flat surface articulating with the carpal bones; radial styloid process for wrist ligament attachment.
  • Function: Main bone of forearm rotation (pronation and supination) against the ulna.
  • Clinical Note:
    • Distal radius fractures (“Colles’ fractures”) are frequent when patients fall on an outstretched hand.

11.6 Carpal Bones (Wrist)

Eight small bones arranged in two rows (proximal and distal):

Proximal Row (lateral→medial) Distal Row (lateral→medial)
Scaphoid Trapezium
Lunate Trapezoid
Triquetrum Capitate
Pisiform Hamate
  • Clinical Note:
    • Scaphoid fractures are the most common carpal fracture and risk non-union due to limited blood supply.
    • Carpal tunnel syndrome involves compression of the median nerve beneath the flexor retinaculum.

11.7 Metacarpals

  • Number: Five (one per digit).
  • Structure: Each has a base (articulates with distal carpals), shaft, and head (knuckle).
  • Function: Form the palm and transmit forces during grip and load-bearing.

11.8 Phalanges (Finger Bones)

  • Digits 2–5: Three phalanges each—proximal, middle, and distal.
  • Thumb (Digit 1): Two phalanges—proximal and distal.
  • Function: Enable fine motor tasks and gripping movements.

12. Skeleton of the Lower Limb

The lower limb skeleton comprises:

  • Pelvic girdle: Two hip bones (covered in §10)
  • Thigh: Femur
  • Knee cap: Patella
  • Leg: Tibia and Fibula
  • Ankle (Tarsus): Seven tarsal bones
  • Foot: Five metatarsals and phalanges

12.1 Femur (Thigh Bone)

  • General: The strongest and longest bone in the body; its length contributes to overall height.
  • Orientation: Slants obliquely downward and inward from hip to knee.
  • Regions:
    • Proximal end:
      • Head: Hemispherical, fits into the acetabulum (hip socket).
      • Neck: Narrow segment below the head.
      • Greater and lesser trochanters: Muscle attachment sites.
    • Shaft: Slightly bowed to distribute weight.
    • Distal end:
      • Medial and lateral condyles: Articulate with the tibia to form the knee joint.
      • Intercondylar fossa: Provides space for cruciate ligaments.

12.2 Patella (Kneecap)

  • Type: Largest sesamoid bone; embedded within the quadriceps tendon.
  • Shape: Triangular, with:
    • Base (superior): Broad surface for quadriceps tendon attachment.
    • Apex (inferior): Attaches via the patellar ligament to the tibial tuberosity.
    • Posterior surface: Two smooth facets (medial and lateral) that articulate with the femoral condyles.
  • Function: Increases the leverage of the quadriceps muscle and protects the knee joint.

12.3 Tibia (Shin Bone)

  • General: The larger, weight-bearing bone of the leg; second longest bone in the body.
  • Proximal end:
    • Medial and lateral condyles: Articulate with the femoral condyles.
    • Tibial plateau: Top surface that bears body weight.
    • Tibial tuberosity: Site of patellar ligament attachment.
  • Shaft: Triangular cross-section; anterior border forms the shin.
  • Distal end:
    • Medial malleolus: Prominence on the inner ankle, articulates with talus.
    • Fibular notch: For articulation with the fibula.

12.4 Fibula

  • General: Thin, non–weight-bearing bone located lateral to the tibia.
  • Regions:
    • Head: Proximal, articulates with the lateral tibial condyle (not the knee joint).
    • Neck: Narrow segment below the head.
    • Shaft: Slender, prismatic shape.
    • Distal end:
      • Lateral malleolus: Outer ankle projection, stabilizes the ankle joint.
  • Function: Provides sites for muscle attachment and forms the lateral boundary of the ankle.

12.5 Tarsal Bones (Ankle)

Seven bones arranged in three vertical rows:

Row Bones
Proximal Talus (articulates with tibia/fibula), Calcaneus (heel bone)
Intermediate Navicular (medial side)
Distal Cuboid (lateral), Medial Cuneiform, Intermediate Cuneiform, Lateral Cuneiform
  • Talus: Transmits body weight from tibia to foot.
  • Calcaneus: Largest tarsal; heel attachment for the Achilles tendon.

12.6 Metatarsals

  • Number: Five, one per digit (I–V).
  • Structure: Base (proximal), shaft, and head (distal).
  • Function: Form the arches of the foot and bear weight during stance and gait.

12.7 Phalanges (Toe Bones)

  • Digits II–V: Three phalanges each—proximal, middle, and distal.
  • Great toe (Hallux): Two phalanges—proximal and distal.
  • Function: Enable balance, propulsion, and fine adjustments during walking.

Clinical Notes

  • Femoral neck fractures: Common in the elderly, risk disrupting blood supply to the femoral head.
  • Patellar dislocation: Lateral displacement often injures the medial patellofemoral ligament.
  • Tibial stress fractures: Overuse injuries in runners; occur in the distal third.
  • Ankle sprains: Frequently involve the lateral ligaments near the lateral malleolus.
  • Flatfoot (pes planus): Collapse of the medial longitudinal arch involving tibialis posterior dysfunction.
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