Appendicular Skeleton

Overview of the Appendicular Skeleton

Total Bone Count: The appendicular skeleton is composed of approximately $126$ bones.
Primary Subdivisions: It consists of the pectoral (shoulder) girdle, the pelvic (hip) girdle, and the upper and lower limbs.

The Pectoral Girdle and Upper Limbs

Functions and Connections:
- Serves as a major site for muscle attachment.
- The only direct connections between the axial skeleton and the appendicular skeleton are via the clavicle and the scapulae.
Clavicle (Collarbone):
- Position: Anteriorly located; acts as a brace.
- Attachment Sites: Serves as an important site for muscle attachment.
- Origin: Originates at the manubrium of the sternum.
- Articulation: Articulates with the scapula at the acromion.
- Morphology: The acromial end of the bone is broader than the sternal end.
- Resilience: Although the clavicle is fragile, it usually heals without the need for a cast.
Scapula (Shoulder Blade):
- Position: Posteriorly located.
- Morphology: The body of the scapula is triangular in shape.
- Glenoid Cavity: A cup-shaped depression that serves as the socket into which the humerus fits.
- Acromion: The articulation at the acromion constitutes the second joint of the girdle.
- Muscle Attachment: Provides critical surface area for muscle attachment.
Upper Limbs: The Arm (Brachium):
- Range: Extends from the shoulder to the elbow.
- Composition: Consists of a single bone, the humerus.
- Humerus Characteristics:
  - The humerus is a site of blood formation.
  - Deltoid Tuberosity: A raised bump serving as the attachment point for the deltoid muscle.
  - Radial Groove: A depression that holds the radial nerve.
  - Ulnar Nerve: Known colloquially as the "funny bone."
Upper Limbs: The Forearm (Antebrachium):
- Range: Extends from the elbow to the wrist.
- Composition: Consists of two bones: the ulna and the radius.
- Ulna:
  - Positioned medially to the radius (aligned with the pinkie finger).
  - Morphology is roughly triangular.
  - Interosseous Membrane: A fibrous sheet of tissue that connects the lateral margin of the ulna to the radius.
  - Head: The head of the ulna is located near the pinkie. The ulna is the only bone that "stands on its head," meaning the thin end (head) is by the wrist.
- Radius:
  - Positioned along the side of the thumb.
  - The large end of the bone is located along the thumb side at the wrist.
  - Articulations: Articulates in the ulnar notch; the head of the radius is located near the humerus.

The Wrist and Hand

Carpal Bones (Wrist):
- Composed of $8$ individual bones.
- Capitate Bone: The largest of the carpal bones.
- Pisiform Bone: Specifically identified as a sesamoid bone.
Hand Structure:
- Metacarpals: Consists of $5$ bones.
- Phalanges: Consists of a total of $14$ bones.
- Pollex (Thumb): The thumb contains only $2$ phalanges.
- Remaining Digits: Each of the other four fingers contains $3$ phalanges.

The Pelvic Girdle and Lower Limbs

Primary Functions:
- Attaches the lower limbs to the axial skeleton.
- Transmits the weight of the upper body to the lower limbs.
- Supports the visceral organs of the pelvic region.
Stability vs. Mobility:
- The pelvic girdle lacks the high degree of mobility found in the pectoral girdle.
- Conversely, it is significantly more stable than the pectoral girdle.
Anatomy of the Pelvis:
- The term "pelvis" refers collectively to the Os Coxae, the Sacrum, and the Coccyx.
- Hip Bones (Os Coxae): Also known as the innominate bones; they provide sites for muscle attachment.
Components of the Os Coxae:
- Ilium: The large, fan-like upper portion. It is the only part of the hip bone that articulates with the axial skeleton. It contains the Greater Sciatic Notch, which holds the sciatic nerve.
- Ischium: The "back of the circle," referred to as the "sit on bones." It contains the Lesser Sciatic Notch, which holds blood vessels, nerves, and small muscles.
- Pubis: The forward portion of the hip forming the front of the circle. It contains the Pubic Symphysis, which is made of fibrocartilage.
Acetabulum: The socket where the ilium, ischium, and pubis all meet; it accepts the head of the femur.
True vs. False Pelvis:
- False Pelvis: Consists of the upper blades of the ilium only.
- True Pelvis: The lower portion in line with the coccyx. It involves all three hip bones and contains the pelvic cavity.

Sexual Dimorphism in the Pelvis (Childbirth Modifications)

Skeletal Differences in Women:
- Width: The female pelvis is broader and lower than the male pelvis.
- Depth: It is shallower; the sacrum and coccyx are less curved. In males, these bones curve into the pelvic space.
- Weight: The female bones are lighter and smoother, largely because females generally lack the muscle mass of males.
- Shape: The female pelvis is rounder.
- Birth Canal: Both the Pelvic Inlet (upper lip of the space) and the Pelvic Outlet (lower lip of the space) are wider in females to accommodate a birth canal.
Pubic Angle Differential:
- Male: The pubic angle is less than $90$ degrees.
- Female: The pubic angle is more than $100$ degrees.
Hormonal Influence:
- Relaxin: A hormone produced during pregnancy that loosens the pubic symphysis to facilitate easier childbirth.

Lower Limbs: Thigh and Leg

Thigh (Upper Leg):
- Femur: The single bone of the thigh; it is the largest and heaviest bone in the body.
Leg (Lower Leg):
- Patella (Kneecap): Situated between the femur and the tibia.
  - Associated with the tendon of the Quadriceps Femoris.
  - The patella is cartilaginous at birth.
  - Ossification begins when a child starts walking, becoming noticeable between ages $2-3$ and finishing by puberty.
- Tibia (Shinbone):
  - The larger of the two lower leg bones.
  - The only lower leg bone that articulates with the femur.
  - Medial Malleolus: The projection on the inside of the "ankle bone."
  - Tibial Tuberosity: The bump located just under the knee.
- Fibula:
  - The smaller lower leg bone.
  - The head of the fibula articulates with the tibia, but the fibula has no contact with the femur.
  - It is used for muscle attachment and is not a weight-bearing bone.
  - Lateral Malleolus: The projection on the outside of the "ankle bone."

The Ankle and Foot

Tarsal Bones (Ankle):
- Composed of $7$ bones.
- Talus: A large tarsal bone that articulates with the tibia.
- Calcaneus: The largest of the tarsal bones; the heel bone.
- Weight Transfer Pathway: Tibia $\\rightarrow$ Talus $\\rightarrow$ Calcaneus.
- Achilles Tendon: Also known as the calcaneal or calcanean tendon; it runs along the back and allows a person to stand on their toes.
Foot Structure:
- Metatarsals: Consists of $5$ long bones.
- Phalanges: Consists of a total of $14$ bones.
- Hallux (Big Toe): The big toe contains only $2$ phalanges.
- Remaining Digits: Each of the other four toes contains $3$ phalanges.

Abnormalities and Clinical Conditions

Carpal Tunnel Syndrome: Inflammation of the tissues in the anterior wrist resulting in pain.
Dysplasia of the Hip: Occurs when the acetabulum forms incompletely or the hip joint ligaments are loose, causing the head of the femur to slip out of the joint.
Broken Hip: Specifically defined as a break in the neck of the femur.
Clubfoot (Congenital Talipes):
- A congenital defect where the soles of the feet face medially and the toes point inferiorly.
- Can be genetically induced or caused by an abnormal foot position during fetal development.
- Can be corrected via surgery if not too severe.
- More common in males than females.
- Notable individuals associated with this condition: Kristi Yamaguchi and Damon Wayans.
Potts Fracture: A break involving the distal ends of both the tibia and the fibula.
Colles Fracture: A break involving the distal ends of both the radius and the ulna.
Stress Fracture of the Foot: Often caused by poor arch support while running; running places significantly more stress on the foot than walking.
Flat Feet: A condition where the arches of the foot fall or fail to form. This places pressure on blood vessels and nerves, causing pain during extended walking. Individuals with flat feet are ineligible for military service (cannot get into the army).

Individual Variation and Age

Skeletal Changes: Changes in the skeleton begin as early as $3$ months of age and continue throughout a person's life.
Reference Materials: Consult Table $8-1$ and Table $8-2$ for essential details (students are required to know these).