Bones: Structure and Function

• "Skeleton" originates from Greek and Latin, signifying "dried framework."
• Bones consist of living tissues.
• Bones function in muscle attachment, protect soft tissues, produce blood cells, store inorganic salts, bear weight, and allow movement.

Bone Classification by Shape

• Long Bones: elongated with expanded ends (e.g., arm and leg bones).

• Short Bones: roughly equal in length and width (e.g., wrist and ankle bones).

• Sesamoid (Round) Bones: small bones within tendons (e.g., patella).

• Flat Bones: plate-like with broad surfaces (e.g., ribs, scapula, some skull bones).

• Irregular Bones: varied shapes (e.g., vertebrae, some facial bones).

• Epiphyses: expanded bone ends forming joints.

• Articular Cartilage: hyaline cartilage covering epiphyses.

• Diaphysis: the bone shaft.

• Periosteum: dense connective tissue covering bone, continuous with ligaments and tendons.

• Medullary Cavity: hollow chamber within the diaphysis.

• Endosteum: thin cellular layer lining the medullary cavity.

• Bone Marrow: connective tissue filling the medullary cavity.

• Bone shape is directly related to its function.

• Bony processes provide sites for tendon and ligament attachment.

• Compact Bone: tightly packed tissue forming the diaphysis wall.

• Spongy Bone: cancellous bone filling the epiphyses, reducing weight.

• Trabeculae: branching bony plates within spongy bone.

• Osteocytes: bone cells located in lacunae chambers.

• Lacunae: chambers housing osteocytes.

• Lamellae: concentric matrix circles in compact bone.

• Central (Haversian) Canals: cylindrical tubes within lamellae, containing blood vessels and nerves.

• Canaliculi: small canals for nutrient and gas exchange.

• The bone matrix comprises collagen and inorganic salts, mainly calcium phosphate.

• Collagen offers strength and resilience.

• Inorganic salts give hardness and crush resistance.

• Osteons (Haversian Systems): organized units in compact bone.

• Perforating (Volkmann’s) Canals: connect central canals, sharing blood supply.

• Spongy bone lacks osteons, containing osteocytes and matrix within trabeculae.

Bone Functions

• Bones provide shape to the head, face, thorax, and limbs.

• Pelvis and lower limbs support body weight.

• Skull bones protect the eyes, ears, and brain.

• Rib cage and shoulder girdle protect the heart and lungs.

• Pelvic girdle protects lower abdominal and reproductive organs.

• Bones work with muscles for movement.

• Hematopoiesis: blood cell formation.

• Blood cell formation initially occurs in the yolk sac.

• Blood cells are later produced in the liver, spleen, and bone marrow.

• Red Marrow: functions in red blood cell, white blood cell, & platelet formation; found in spongy bone of skull, ribs, sternum, clavicles, vertebrae, and pelvis.

• Yellow Marrow: stores fat, occupies most adult bone cavities.

• Bones store calcium salts, mainly calcium phosphate.

• Calcium is required for muscle contraction, nerve impulse conduction, and blood clotting.

• Calcium in bone serves as a reservoir for body calcium.

• Osteoclasts release calcium from bone when blood calcium levels are low, regulated by parathyroid hormone.

• Osteoblasts deposit bone matrix when blood calcium levels are high, stimulated by hormone calcitonin.

• Bones also store magnesium, sodium, potassium, and carbonate ions.

• Bones accumulate harmful metallic elements like lead, radium, and strontium.

• Bones form by replacing existing connective tissues.

• Ossification: bone formation from connective tissue.

• Intramembranous Bones: originate within connective tissue layers.

• Endochondral Bones: form from a hyaline cartilage model.

• Intramembranous bones develop from undifferentiated connective tissue.

• Osteoblasts deposit bony matrix, forming spongy bone.

• Osteocytes: osteoblasts encased in matrix.

• Periosteum: derived from membranous tissue outside bone.

• Osteoblasts under periosteum form compact bone over spongy bone.

• Endochondral bones: develop as hyaline cartilage models, replaced by bone tissue.

• Primary Ossification Center: spongy bone formation in diaphysis.

• Osteoclasts: bone-resorbing cells breaking down calcified matrix.

• Osteoblasts: bone-building cells depositing new bone.

• Epiphyseal Plate: cartilaginous cells allowing bone lengthening.

• Medullary Cavity: formed by osteoclasts eroding bone inside diaphysis.

• Bone Remodeling process involves bone resorption and deposition throughout life.

• Bone remodeling is controlled by hormones and regulates blood calcium levels.

• Vitamin D is needed for calcium absorption.

• Growth hormone stimulates cartilage cell division in the epiphyseal plate.

• Sex hormones stimulate epiphyseal plate ossification, ending height growth.

• Physical exercise stresses bone, increases thickness and strength.

• The skeleton includes the axial and appendicular portions.

• Axial Skeleton: supports and protects head.

• Appendicular Skeleton: bones of limbs and anchoring girdles.

• Pectoral Girdle: clavicle and scapula.

• Upper Limbs: humerus, ulna, radius, carpals, metacarpals, phalanges.

• Pelvic Girdle: two hip bones (coxal).

• Lower Limbs: femur, patella, tibia, fibula, tarsals, metatarsals, phalanges.

The Skull

• The skull consists of 22 bones: 8 cranial, 14 facial, and 6 middle ear bones.
• Sutures: immovable joints connecting skull bones (except lower jaw).

Cranial Bones

• Enclose/protect the brain, provide muscle attachments, contain paranasal sinuses.
• Frontal Bone: anterior part of skull above eyes; features include the supraorbital foramen and the frontal sinuses
• Parietal Bones: form the roof and sides of the skull; they join along the midline at the sagittal suture, and meet the frontal bone along the coronal suture
• Occipital Bone: back of skull, base of cranium; features include foramen magnum, occipital condyles; joins parietal bones at lambdoid suture.
• Temporal Bones: sides and base of cranium; features include external acoustic meatus, mandibular fossa, mastoid process, styloid process, and zygomatic process; connects to parietal bones at squamous suture
• Sphenoid Bone: base of cranium, sides of skull, orbits; features include sella turcica and sphenoidal sinuses.
• Ethmoid Bone: front of sphenoid; features include cribriform plates, crista galli, perpendicular plate, superior and middle nasal conchae, ethmoidal sinus

Facial Bones

• There are 13 immovable bones and a movable lower jawbone.

• Facial bones shape the face and attach jaw muscles.

• Maxillae: upper jaw, hard palate, eye orbit floors, nasal cavity sides; features include the maxillary sinuses, palatine and alveolar processes, and alveolar arch

• Palatine Bones: L-shaped, behind maxillae; form nasal cavity floor and lateral walls, posterior hard palate.

• Zygomatic Bones: cheekbones, lateral orbit walls; features include the temporal process, which joins the zygomatic process

• Lacrimal Bones: part of medial orbit walls.

• Nasal Bones: form nose bridge.

• Vomer Bone: portion of nasal septum.

• Infantile Skull: incompletely developed at birth.

• Fibrous Membranes: connect cranial bones in infants.

Vertebral Column

• Vertical axis from skull to pelvis.
• Supports head and trunk, protects spinal cord.
• Consists of vertebrae separated by intervertebral discs.
• Vertebrae connected by ligaments.
• Vertebrae consist of a body, pedicles, laminae, spinous process, vertebral foramen, transverse processes, superior and inferior articular processes and intervertebral foramina
• Sternum (Breastbone): anterior midline of thoracic cage.
• There are 3 portions; manubrium, body, and xiphoid process.
• Sternum articulates with clavicles.

Pectoral Girdle (Shoulder Girdle)

• Incomplete ring that supports upper limbs and is an attachment site for muscles.
• Composed of 2 scapulae (shoulder blades) and 2 clavicles (collarbones).
• Clavicles: S-shaped, brace scapulae, attach muscles.
• Scapulae: triangular bones on upper back.
• Acromion Process: articulation with clavicle is at the tip of the shoulder.
• Coracoid Process: attachment sites for muscles.
• Glenoid Cavity: articulates with humerus head.

Upper Limb

• Humerus: long bone from scapula to elbow; articulates with scapula and ulna and radius bones.
• Radius: lateral forearm bone (thumb side); has a disc-like head that rotates against humerus, has radial tuberosity and styloid process.
• Ulna: longer forearm bone, medial to radius; has a trochlear notch, olecranon process, coronoid process, head and styloid process.
• Hand: includes wrist, palm, fingers.
• There are 8 carpal bones, arranged into 2 rows of 4.

Pelvic Girdle

• Consists of 2 hip (coxal, pelvic, or innominate) bones.
• The hip bones articulate with each other anteriorly, and with sacrum posteriorly
• The pelvis consists of the sacrum, coccyx and pelvic girdle
• Supports trunk, attaches lower limbs, protects organs.
• Each coxal bone has an ilium, ischium, pubis.
• Ilium: largest, upper portion, includes iliac crest and sacroiliac joint.
• Ischium: L-shaped, posterior, supports sitting weight; features include the ischial tuberosity and ischial spine..
• Pubis: anterior portion, connects at pubic symphysis; other features include the pubic arch, obturator foramen, and pelvic brim
• Acetabulum: articulates with femur head. cup depression where the ilium, ischium, and pubis bones are fused.

Joints (Articulations)

• Functional junctions between bones.
• Joints enable movement, bind parts together, facilitate bone growth, and permit shape changes during childbirth.
• Immovable/Synarthrotic
• Slightly Moveable/Amphiarthrotic
• Freely Movable/Diarthrotic
• Fibrous, Cartilaginous, Synovial

Joint Types

• Fibrous Joints: dense connective tissue, mostly immovable (e.g., skull sutures).
• Cartilaginous Joints: hyaline or fibrocartilage; slightly movable (e.g., intervertebral discs).
• Synovial Joints: free movement, complex structure.
• Articular Cartilage: covers bone ends in joints.
• Joint Capsule: outer dense connective tissue, inner synovial membrane.
• Synovial Membrane: secretes lubricating synovial fluid.
• Menisci: fibrocartilage pads for shock absorption.
• Bursae: fluid-filled sacs aiding in movement.

Synovial Joint Shapes

• Ball-and-Socket (Spheroidal) Joint: widest motion range (e.g., shoulder, hip).
• Condylar (Ellipsoidal) Joint: back/forth and side/side movements, no rotation (e.g., metacarpals/phalanges).
• Plane (Gliding) Joint: sliding or twisting (e.g., wrist, ankle, vertebrae, sacroiliac joints).
• Hinge Joint: movement in one plane (e.g., elbow, phalanges).

Explore bone origins, living tissues, and diverse functions like muscle attachment and blood cell production. Learn about bone classification by shape, including long, short, and flat bones. Discover key bone structures such as epiphyses, diaphysis, and the medullary cavity.