Skeletal System Bones are living organs made up of several different types of tissue Bone tissue Cartilage Dense connective tissue Blood Nervous tissue They perform many different types of functions: Points of attachment for muscles Protection of soft tissues House blood producing cells

Bone Structure For longer bones, there are two distinct external parts Epiphysis – The expanded end portions of the bone These are where joints are formed They are covered in a thin layer of cartilage called the articular cartilage Diaphysis – the thinner portion that extends between the two epiphyses. Tough tissue called the periosteum completely encloses the bone except for where the articular cartilage is found. This periosteum helps to form and repair bone tissue.

The diaphysis is made up of compact bone which has no spaces This compact bone has a hollow medullary cavity which houses the marrow where new blood cells are produced. The epiphysis is made up of spongy bone and a small amount of compact bone. The spongy bone has irregular spaces which help to reduce the weight of the bone. Osteocytes are found in lacunae (pits) and form concentric rings of new bone material around a central canal. This ring structure is called an osteon The central canals are only found in compact bone and have blood vessels and nerves in them.

Bone Development and Growth During the first few weeks of prenatal development, parts of the skeleton begin to develop. They form by replacing existing types of connective tissue. Intramebranous bones which are found in the skull form from thin layers of membranous tissue. When the bone begins to develop, specialized cells differentiate into Osteoblasts which make the hardened bone. These osteoblasts are called osteocytes when they have completely surrounded themselves with the bony matrix.

The other type of bone is called an endochondral bone These develop from cartilaginous structures that look like the future bones. Bone formation begins at the primary ossification center which is located in the center of the diaphysis. The periosteum forms around the primary ossification center which will provide blood vessels and osteoblast cells. Cartilage is replaced by osteoblasts which secrete the bony matrix around themselves. The epiphyses remain cartilaginous and continue to grow for some time. Eventually secondary ossification centers arise on the epiphyses The epiphyseal plates remain between the primary and secondary ossification centers. (Also called the “Growth Plates”) If damage occurs to the epiphyseal plates, the bone may stop growing. Eventually the epiphyseal plates are ossified

Reparing a Fracture

Skeletal Organization There are 206 bones in the normal human skeleton It is divided up in to two main sections The Axial skeleton consists of the skull, vertebral column, and the thoracic cage. The Appendicular skeleton consists of the upper and lower limbs, and the bones that anchor those limbs.

The Skull Primary job of the skull is to protect the brain. It is composed of 22 different bones which all interlock. The mandible, or jaw bone, is the only movable bone in the skull. Baby skulls have two “soft spots” called fontaneles which are places where the ossification of the intramembranous bones has not finished.

Vertebral Column Extends from the skull to the pelvis Consists of 26 bones 7 cervical vertebrae (found in the neck) 12 thoracic vertebrae (middle of the back) 5 lumbar vertebrae (lower back) 1 sacrum (formed from 5 fused vertebrae) 1 coccyx (tailbone – formed from 4 fused vertabrae) Each vertebra is separated from those next to it by intervertebral discs which are made of cartilage Each vertebra also has a vertebral foramen which houses the spinal cord. Together these vertebral foramen make up the vertebral canal.

Thoracic Cage This includes the ribs, thoracic vertebrae, sternum, and costal cartilage Ribs Usually 24 ribs, one pair for each thoracic vertebrae First 7 are called true ribs because they reach the sternum directly Next 5 are false ribs because they do not reach sternum directly Last 2 are called floating ribs because they do not reach sternum at all. Sternum is also called the breastbone Made up of 3 sections Manubrium – top section Body – middle and biggest section Xyphoid process – lowest section

Pectoral Girdle Made up of two clavicles and two scapulae Clavicles are the collarbones Scapulae are shoulder blades These are movable and act as attachments for clavicles and the humerus (upper arm bone)

Upper Limb Made up of 5 different groups of bones Humerus is the upper arm bone Radius and Ulna are the two bones of the forearm When shaking hands, the Ulna is “underneath” the radius. Carpals are tiny bones that make up most of the wrist Metacarpals are longer bones that make up the palm of the hand Your knuckles are the distal ends of these bones Phalanges are the finger bones. Three phalanges per finger. (Thumb only has 2)

Pelvic Girdle

Lower Limb Femur is the thigh bone and the longest in the body. Tibia and Fibula are the two bones of the shin. Tibia is the bigger of the two and is located on the same side of the shin as the big toe. The kneecap, or patella, is located between the femur and the tibia/fibula. Tarsal bones are small and located between the ankle and the first metatarsals. Metatarsals are longer bones that make up the instep of the foot. Distal ends of these bones make up the ball of the foot. The Phalanges make up the toes. Each toe has 3 phalanges. (The big toe has only 2)

Major Joint Types Joints are connections between bones some are movable, some are not Fibrous Joints are joints that have limited or no movement. Located in the skull Cartilaginous Joints have cartilage pads between the bones. Limited movement Found between the vertebrae Synovial Joints most joints in the body Composed of cartilage capsules with synovial fluid in them The synovial membrane becomes inflamed and thickened in rheumatoid arthritis The cartilage gradually disintegrates in osteoarthritis

Fibrous Joints

Synovial Joint

Three Common Synovial Joint Types Ball and Socket Joint – bone with a ball shaped head articulates with a bone that has a cup shaped socket. Allow greatest degree of movement Shoulder and hip joints Gliding joint – nearly flat of slightly curved Allow for sliding or twisting movements Wrist joints Hinge Joints – Convex surface of one bone fits into a concave surface on another. Allows for movement in only one direction. Elbow and phalange joints