Human Skeletal System

The Skeletal System

• The skeletal system is divided into two parts: Axial skeleton (80 bones) and Appendicular skeleton (126 bones)
• Functions of bones: framework, protection, movement, mineral homeostasis, production of blood cells, and storage

Classification of Bones

• Long bones: typically longer than wide, have a shaft with heads at both ends, contain mostly compact bone (e.g., femur, humerus)
• Short bones: generally cube-shaped, contain mostly spongy bone (e.g., carpals, tarsals)
• Flat bones: thin and flattened, usually curved, with thin layers of compact bone around a layer of spongy bone (e.g., skull, ribs, sternum)
• Irregular bones: irregular in shape, do not fit into other bone classification categories (e.g., vertebrae and hip)

Gross Anatomy of a Long Bone

• Diaphysis: the shaft, composed of compact bone
• Epiphysis: the ends of the bone, composed mostly of spongy bone

Divisions of the Skeletal System

• The Axial Skeleton: forms the longitudinal part of the body, divided into three parts (skull, vertebral column, and bony thorax)

Skull (Cranium)

• Consists of 22 bones
• Cranial bones: eight bones form the cranial cavity (e.g., frontal bone, parietal bones, temporal bones, occipital bone, sphenoid bone, ethmoid bone)
• Facial bones: 14 bones form the face (e.g., nasal bones, maxillae, zygomatic bones, mandible, lacrimal bones, palatine bones, inferior nasal conchae, and vomer)

Vertebral Column (Spine)

• Composed of a series of bones called vertebrae (adult = 26)
• Divided into four regions: cervical (7), thoracic (12), lumbar (5), and sacrum and coccyx
• Intervertebral discs: found between the bodies of adjacent vertebrae, form strong joints, permit various movements, and absorb vertical shock

Thorax

• Thoracic cage is formed by the sternum, ribs, costal cartilages, and thoracic vertebrae
• Functions: enclose and protect organs, provide support for upper limbs, and play a role in breathing

The Appendicular Skeleton

• Limbs (appendages)
• Pectoral girdle and pelvic girdle

Upper Limb

• The pectoral girdle consists of two bones: scapula and clavicle
• The free part has 30 bones: humerus, ulna, radius, carpals, metacarpals, and phalanges

Pectoral Girdle

• Composed of two bones: clavicle (collarbone) and scapula (shoulder blade)
• The clavicle is convex in shape anteriorly near the sternal junction
• The clavicle is concave anteriorly on its lateral edge near the acromion

Scapula

• Triangular in shape
• Most notable features include the spine, acromion, coracoid process, and glenoid cavity

Humerus

• Longest and largest bone of the free part of the upper limb
• The proximal ball-shaped end articulates with the glenoid cavity of the scapula
• The distal end articulates at the elbow with the radius and ulna

Forearm

• The forearm is formed by two bones: ulna and radius
• Ulna: longer, medial bone
• Radius: shorter, lateral bone

Hand

• The carpus (wrist) consists of 8 small bones (carpals)
• Metacarpals: 5 bones, numbered I-V, lateral to medial
• Phalanges: 14 bones, two in the thumb and three in each of the other fingers

Pelvic Girdle

• Composed of three pair of fused bones: ilium, ischium, and pubic bone
• The total weight of the upper body rests on the pelvis
• Protects several organs: reproductive organs, urinary bladder, and part of the large intestine

Femur (Thigh Bone)

• Longest, heaviest, and strongest bone in the body
• The head articulates with the acetabulum of the hip bone, forming the hip (coxal) joint
• Distally, the medial and lateral condyles articulate with the condyles of the tibia, forming the knee joint

Patella

• Largest sesamoid bone in the body
• Forms the patellofemoral joint
• Superior surface is the base
• Inferior, narrower surface is the apex

Tibia and Fibula

• Tibia: the larger, medial weight-bearing bone of the leg
• Fibula: the smaller, lateral, non-weight-bearing bone of the leg

Foot

• The foot is composed of tarsal bones, metatarsals, and phalanges
• Seven tarsal bones: talus, calcaneus, navicular, cuboid, and three cuneiforms
• Five metatarsals: base, shaft, and head
• 14 phalanges: two in the big toe and three in each of the other toes