The Skeletal System: Axial and Appendicular Skeleton Study Notes

The Skeletal System

I. Divisions of the Skeletal System

The human skeletal system is divided into two main parts: the axial skeleton and the appendicular skeleton. Together, they provide structure, protection, and support for the body, as well as facilitate movement.

• Axial skeleton: Consists of bones arranged along the longitudinal axis of the body. It is composed of 80 bones, including the skull, hyoid bone, vertebral column, sternum, and ribs.

• Appendicular skeleton: Composed of 126 bones, including the pectoral girdles, upper extremities, pelvic girdle, and lower extremities.

A. The Axial Skeleton

II. Skull

The skull is a complex structure composed of 22 bones, which protect the brain and form the structure of the face.

• Cranial bones (8): Form the cranial cavity that houses the brain.

• Facial bones (14): Form the structure of the face.

• Sutures: Immovable joints found only between skull bones.

• Fontanels: Membrane-filled spaces between cranial bones in infants, allowing for flexibility during birth and brain growth.

• Major functions of fontanels:

  • Enable the fetal skull to modify its size and shape as it passes through the birth canal.

  • Permit rapid growth of the brain during infancy.

• Cranial bones include: frontal, parietal (2), temporal (2), occipital, sphenoid, ethmoid.

• Facial bones include: nasal (2), maxillae (2), zygomatic (2), mandible, lacrimal (2), palatine (2).

III. Hyoid Bone

The hyoid bone is a unique U-shaped bone in the neck that does not articulate with any other bone. It supports the tongue and provides attachment for muscles of the pharynx and neck.

• Function: Assists in swallowing and speech.

IV. Vertebral Column

The vertebral column, or spine, is composed of 26 bones and provides the main support for the trunk. It protects the spinal cord and allows for flexible movement.

• Regions of the vertebral column:

  • Cervical vertebrae (7)

  • Thoracic vertebrae (12)

  • Lumbar vertebrae (5)

  • Sacrum (5, fused)

  • Coccyx (4, fused)

• Intervertebral discs: Fibrocartilaginous pads between vertebrae that permit movement and absorb shock.

• Normal curves: Four curves (two primary—thoracic and sacral; two secondary—cervical and lumbar) provide strength, support, and balance.

• Structure of a vertebra:

  • Body (centrum)

  • Vertebral arch

  • Seven processes

• Regions differ in size and shape: Cervical, thoracic, lumbar, sacrum, coccyx.

V. Thorax

The thorax refers to the entire chest. The thoracic skeleton consists of the sternum, ribs, costal cartilages, and bodies of the thoracic vertebrae.

• Ribs: 12 pairs

  • True ribs (1–7): Have direct cartilage connection to the sternum.

  • False ribs (8–12): Either have indirect or no cartilage connection to the sternum.

  • Floating ribs (11–12): No cartilage connection to the sternum.

• Function: Protects vital organs in the chest and upper abdomen; provides support for the shoulder girdle and upper extremities.

VI. Disorders: Homeostatic Imbalances

Several disorders can affect the bones of the axial skeleton, impacting their structure and function.

• Osteoporosis: Decrease in bone mass and strength due to reduced hormone output. Bone resorption outpaces bone formation.

• Osteomalacia: Bones are inadequately mineralized, leading to softening.

• Paget's disease: Characterized by massive osteoclast resorption and extensive bone formation, resulting in weak, deformed bones.

• Scoliosis: Lateral bending of the vertebral column. Causes include congenital defects, paralysis, poor posture.

• Kyphosis: Exaggeration of the thoracic curve. Causes include rickets, poor posture, osteoporosis.

• Lordosis: Exaggeration of the lumbar curve. Causes include pregnancy, obesity, poor posture, rickets.

• Spina bifida: Congenital defect where the laminae fail to unite at the midline, causing partial or complete paralysis.

B. The Appendicular Skeleton

I. Introduction to the Appendicular Skeleton

The appendicular skeleton consists of 126 bones and is responsible for facilitating movement and supporting the limbs.

• Pectoral (shoulder) girdles (2x2)

• Upper extremities (2x30)

• Pelvic (hip) girdle (2)

• Lower extremities (2x30)

II. Pectoral (Shoulder) Girdle

The pectoral girdle attaches the bones of the upper extremities to the axial skeleton, allowing for a wide range of motion.

• Components: Clavicle and scapula.

• Function: Provides attachment for upper limb muscles and allows shoulder joint movement.

III. Upper Extremity

Each upper extremity contains 30 bones, including the humerus, forearm bones, and bones of the hand.

• Humerus: Longest and largest bone of the upper extremity.

• Forearm: Contains the ulna (little finger side) and radius (thumb side).

• Hand: 27 bones (8 carpals, 5 metacarpals, 14 phalanges).

• Phalanges: 14 in each hand (three in each finger, two in the thumb).

IV. Pelvic (Hip) Girdle

The pelvic girdle consists of two coxal (hip) bones, providing strong and stable support for the lower extremities.

• Each coxal bone: Composed of three separate bones at birth—ilium, pubis, ischium.

• Acetabulum: Depression where the three bones fuse, forming the socket for the hip joint.

• Pelvis: Formed by the two coxal bones, sacrum, and coccyx.

V. Lower Extremity

The lower extremities are composed of 60 bones (30 in each leg), supporting the weight of the body and enabling movement.

• Femur: Longest and heaviest bone in the body, just distal to the pelvic girdle.

• Patella: Kneecap, a sesamoid bone that increases leverage for the quadriceps femoris muscle.

• Tibia and fibula: Bones of the leg.

• Foot: 26 bones (7 tarsals, 5 metatarsals, 14 phalanges).

• Ankle: Formed by seven tarsal bones.

• Phalanges in toes: Same arrangement as fingers and thumb.

• Arches of the foot: Two non-rigid arches support body weight, distribute weight, and provide leverage while walking.

VI. Female and Male Skeletons

There are notable anatomical differences between male and female skeletons, especially in the pelvic region.

• Male bones: Generally larger and heavier; joint surfaces are larger.

• Muscle attachment points: More well-defined in males due to larger muscle mass.

• Pelvic girdle differences: Female pelvis is broader and shallower to facilitate childbirth.

• Pubic arch angle: Less than 90 degrees in males; more than 90 degrees in females.

Table: Comparison of Male and Female Pelvis

Feature	Male Pelvis	Female Pelvis
Size	Larger, heavier	Smaller, lighter
Shape	Narrow, deep	Broad, shallow
Pubic Arch Angle	< 90 degrees	> 90 degrees
Muscle Attachment Points	More defined	Less defined
Function	Support body weight	Facilitate childbirth