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• The skeletal system is also known as the الجهاز الهيكلي

Function of Skeletons

• Skeletons have three major functions.
• Skeletons provide support
• Skeletons facilitate movement
• Skeletons provide protection of internal organs

Types of Skeletons

• Hydrostatic skeleton
• Exoskeleton
• Endoskeleton

Hydrostatic Skeleton

• The hydrostatic skeleton consists of fluid held under pressure in a closed body compartment.
• It protects body parts by cushioning them from shock.
• It provides body shape, which can be changed by contracting muscles in the body wall.
• The hydrostatic skeleton provides support for muscle action.
• Earthworms, hydras, and jellies have hydrostatic skeletons.
• Fluid fills the coelom to support the body shape and movement.

Exoskeleton

• The exoskeleton is a rigid external skeleton that can be hard or leathery.
• The exoskeleton of arthropods is made of chitin.
• The shells of mollusks are exoskeletons made of calcium carbonate.

Endoskeleton

• Invertebrate endoskeletons consist of hard or leathery supporting elements situated among the soft tissues.
• Most echinoderms like sea urchins and sea stars, have an endoskeleton of hard plates beneath their skin.
• Vertebrate endoskeletons consist of cartilage or a combination of cartilage and bone.

Vertebrates Endoskeleton: Cartilage

• Cartilage is a type of vertebrate endoskeleton.
• Chondrocytes are cartilage-producing cells.
• Cartilage contains an abundant extracellular matrix.
• The cartilage matrix contains fibers made of collagen & elastin, and a jellylike ground of complex sugar molecules.
• Cartilage contains 60-80% water
• Cartilage has no nerves or vessels.

Types of Cartilage

• Three types of cartilage: Hyaline, Elastic and Fibrocartilage

Hyaline Cartilage

• Hyaline cartilage is flexible and resilient.
• Chondrocytes appear spherical in hyaline cartilage.
• Hyaline cartilage contains lacuna, a cavity in the matrix holding chondrocytes.
• Collagen is the only fiber in this cartilage.
• Found at the end of long bones, trachea, nose and larynx, and in the knee joints

Elastic Cartilage

• Elastic cartilage is highly bendable.
• The matrix of elastic cartilage has more elastic fibers than collagen fibers.
• It is found in the epiglottis and outer ear.

Fibrocartilage

• Fibrocartilage resists compression and tension.
• Rows of think collagen fibers alternate with rows of chondrocytes in the matrix.
• It is found in intervertebral discs.

Bone

• Bones contain 25% water, 25% protein fibers, and 50% mineral salts.
• Bones provides support, protection, and allows movement.
• Bones also act as mineral bank for storage of calcium and phosphorus.
• Blood production, hemopoiesis occurs in red bone marrow.
• The adipose tissue in yellow bone marrow stores energy.

Bone Cells

• Bone cells live in a matrix of flexible protein fibers and hard calcium salts.
• Protein fibers resist cracking and calcium salts resist compression.
• Osteoblasts are young bone-forming cells that allows for new bone formation.
• Osteocytes are mature bone cells that maintain the structure of bone.
• Osteocytes are the most abundant cell type of bone.
• Osteoclasts dissolve bone, and bone is resorbed by them.

Long Bones

• Long bones are in the limbs.
• Each bone is made of shaft of bone called diaphysis
• The end of the bone is called epiphysis
• The wall of bones consists of dense tissue (compact bones).
• The central canal is filled with yellow marrow that stores fat
• Spongy bone contains the red marrow that produces blood cells
• A joint is where two bones meet.

Compact Bone

• Osteocytes need contact with blood vessels in order to get nutrients and eliminate waste.
• The bone forms a ring pattern around Haversian canals, that contain blood vessels and nerves.
• This system is called Harvesion system or Osteon

Spongy Bone

• Spongy or cancellous bone is at the ends of the long bones (epiphysis).
• It is surrounded by the compact bone.
• Spongy bone forms branching bony plates with spaces filled with red marrow, a site of hemopoiesis.

Human Skeleton

• The human skeleton consists of 206 bones.
• The human skeleton is divided into the axial and appendicular skeleton

Axial Skeleton

• The axial skeleton supports the axis, or trunk, of the body.
• It consists of the skull, backbone, and, in most vertebrates, a rib cage.

Appendicular Skeleton

• Appendicular skeleton supports the paired appendages.
• It consists of bones of the pectoral girdle (shoulder), upper limbs, pelvic girdle, and lower limbs in humans.

Axial Skeleton; Skull & Vertebrae

• The skull contains cranial bones, and facial bones.
• The vertebrae consists of: 7 cervical, 12 thoracic, 5 lumbar and sacrum and coccyx.

Vertebrae

• There are 26 vertebrae
• The vertebral column forms a dorsal backbone protecting the spinal cord.
• Disks between the vertebrae act as padding and shock absorbers.
• The vertebral column serves directly or indirectly as the anchor for all other bones of the body.

Ribs and Sternum

• The rib cage consists of 12 pairs of ribs.
• All ribs connect directly to thoracic vertebrae in the back, and most connect directly or indirectly by cartilage to the sternum in the front.
• Lower ribs are called floating ribs because they are not attached to the sternum.
• Numbered according to attachment to thoracic vertebrae
• Ribs 1-7 are attached to the sternum.
• Ribs 8-10 attach to 7 (indirectly to sternum)
• Ribs 11, 12 are "floating"

Appendicular Skeleton

• The appendicular skeleton contains bones of the pectoral and pelvic girdles and the appendages attached, totaling 126 bones.
• The pectoral (shoulder) girdle and its appendages (arms and hands) are specialized for flexibility.
• The pelvic (hip) girdle and its appendages (legs and feet) are specialized for strength.

Pectoral Girdle and Appendages (Arm, and Hand)

• Individual parts of the pectoral girdle are loosely linked by ligaments.
• It divided into the clavicle, connecting to the sternum in front.
• The Scapula lies in the upper back.
• The humerus fits into the socket of the scapula, providing little stability for shoulder dislocation.
• The humerus joins the radius and ulna at the elbow.
• The wrist consists of 8 carpals, the hand contains 5 metacarpals, and the digits are formed by 14 phalanges.

Pelvic Girdle, Leg, and Foot

• The pelvic girdle has two large, heavy hip bones: the innominate, known as ilium/ischium/pubis, and the sacrum.
• The femur is in the thigh and constitutes the largest bone of the body.
• The Tibia is the shinbone .
• The fibula is on the outer part of the lower leg.
• Leg contains 7 tarsals (ankle), 5 metatarsals (foot), and 14 phalanges (toe bones).

Joints

• Joints of the skeleton link bones together
• Joints are classified by how much movement they allow.
• Fibrous joints are immovable (e.g., the bones of the cranium/skull are sutured together).
• Cartilaginous joints are slightly movable (e.g., the epiphyseal plate, rib/sternum joint and intervertebral joints).
• Synovial joints are freely movable joints (e.g., in the knee and elbow).