Unit 1: Bone PDF

UNIT 1. BONE Bone is a metabolically active connective tissue that provides structural support, facilitates movement, and protects vital organs. Bone is a living tissue that makes up the body’s skeleton. Bone provides the structure for our bodies. The a...

UNIT 1. BONE Bone is a metabolically active connective tissue that provides structural support, facilitates movement, and protects vital organs. Bone is a living tissue that makes up the body’s skeleton. Bone provides the structure for our bodies. The adult human skeleton is made up of 206 bones. Functions of bones Providing the body framework Giving attachment to muscles and tendons Allowing movement of the body as a whole and of parts of the body, by forming joints that are moved by muscles Forming the boundaries of the cranial, thoracic and pelvic cavities, and protecting the organs they contain Haemopoiesis, the production of blood cells in red bone marrow. Mineral storage, especially calcium phosphate – the mineral reservoir within bone is essential for maintenance of blood calcium levels, which must be tightly controlled. Types of bones Bones are classified as long, short, irregular, flat and sesamoid. Long bones: These consist of a shaft and two extremities. As the name suggests, these bones are longer than they are wide. Most long bones are found in the limbs; examples include the femur, tibia and fibula. Short, irregular, flat and sesamoid bones: These have no shafts or extremities and are diverse in shape and size. Examples include: short bones – carpals (wrist) irregular bones – vertebrae and some skull bones flat bones – sternum, ribs and most skull bones sesamoid bones – patella (knee cap) Bone structure: i. Long bones These have a diaphysis (shaft) and two epiphyses (extremities). The diaphysis is composed mainly of compact bone with a central medullary canal, containing fatty yellow bone marrow. The epiphyses consist of an outer covering of compact bone with spongy (cancellous) bone inside. The diaphysis and epiphyses are separated by epiphyseal cartilages, which ossify when growth is complete. Long bones are almost completely covered by a vascular membrane, the periosteum, which has two layers. The outer layer is tough and fibrous, and protects the bone underneath. The inner layer contains osteoblasts and osteoclasts, the cells responsible for bone production and breakdown (see below), and is important in repair and remodelling of the bone. ii. Short, irregular, flat and sesamoid bones These have a relatively thin outer layer of compact bone, with spongy bone inside containing red bone marrow. They are enclosed by periosteum except the inner layer of the cranial bones where it is replaced by dura mater. Composition of Bone: Bone is a strong and durable type of connective tissue. Its major constituent (65%) is a mixture of calcium salts, mainly calcium phosphate. This inorganic matrix gives bone great hardness, but on its own would be brittle and prone to shattering. The remaining third is organic material, called osteoid, which is composed mainly of collagen. Collagen is very strong and gives bone slight flexibility. The cellular component of bone contributes less than 2% of bone mass. Bone cells There are three types of bone cell: 1. Osteoblast: These bone-forming cells are responsible for the deposition of both inorganic salts and osteoid in bone tissue. They are therefore present at sites where bone is growing, repairing or remodeling. 2. Osteocyte: These are mature bone cells that monitor and maintain bone tissue, and are nourished by tissue fluid in the canaliculi that radiate from the central canals. 3. Osteoclast: These cells break down bone, releasing calcium and phosphate. Osteoclasts are found in areas of the bone where there is active growth, repair or remodeling. CLASSIFICATION BASED ON ITS LOCATION: The bones of the skeleton are divided into two groups: the axial skeleton and the appendicular skeleton. The axial skeleton consists of the skull, vertebral column, ribs and sternum. Together the bones forming these structures constitute the central bony core of the body, the axis. The appendicular skeleton consists of the shoulder and pelvic girdles and the limb bones. A. AXIAL SKELETON: 1. Skull: The skull rests on the upper end of the vertebral column and its bony structure is divided into two parts: the cranium and the face.  i. Cranium The cranium is formed by a number of flat and irregular bones that protect the brain. The bones of the cranium are: 1 frontal bone 1 occipital bone 2 parietal bones 1 sphenoid bone 2 temporal bones 1 ethmoid bone ii. Face The skeleton of the face is formed by 13 bones in addition to the frontal bone 2 zygomatic (cheek) bones 1 maxilla 2 palatine bones 2 nasal bones 2 inferior conchae 2 lacrimal bones 1 mandible 1 vomer Fontanelles of the skull At birth, ossification of the cranial sutures is incomplete. The skull bones do not fuse earlier to allow for moulding of the baby’s head during childbirth. Where three or more bones meet there are distinct membranous areas, or fontanelles. The two largest are the anterior fontanelle, not fully ossified until the child is between 12 and 18 months old, and the posterior fontanelle, usually ossified 2–3 months after birth. Functions of the skull The various parts of the skull have specific and different functions: the cranium protects the brain the bony eye sockets protect the eyes and give attachment to the muscles that move them the temporal bone protects the delicate structures of the inner ear the sinuses in some face and skull bones give resonance to the voice the maxilla and the mandible provide alveolar ridges in which the teeth are embedded the mandible, controlled by muscles of the lower face, allows chewing. 2. Vertebral column: There are 26 bones in the vertebral column. Twenty-four separate vertebrae extend downwards from the occipital bone of the skull; then there is the sacrum, formed from five fused vertebrae, and lastly the coccyx, or tail, which is formed from between three and five small fused vertebrae. The vertebral column is divided into different regions. The first seven vertebrae, in the neck, form the cervical spine The next 12 vertebrae are the thoracic spine, The next five the lumbar spine, the lowest vertebra of which articulates with the sacrum. Each vertebra is identified by the first letter of its region in the spine, followed by a number indicating its position. For example, the topmost vertebra is C1, and the third lumbar vertebra is L3. Cervical vertebrae: These are the smallest vertebrae Thoracic vertebrae: The 12 thoracic vertebrae are larger than the cervical vertebrae because this section of the vertebral column has to support more body weight Lumbar vertebrae: These are the largest of the vertebrae because they have to support the weight of the upper body Functions of the vertebral column provides strong bony protection for the delicate spinal cord lying within it the pedicles of adjacent vertebrae form intervertebral foramina, one on each side, providing access to the spinal cord for spinal nerves, blood vessels and lymph vessels Numerous individual bones with their intervertebral discs allow movement of the whole column Support of the skull The intervertebral discs act as shock absorbers, protecting the brain Formation of the axis of the trunk, giving attachment to the ribs, shoulder girdle and upper limbs, and the pelvic girdle and lower limbs. 3. Thoracic cage: The thorax (thoracic cage) is formed by the sternum anteriorly, twelve pairs of ribs forming the lateral bony cages, and the twelve thoracic vertebrae. Sternum (breast bone, Fig. 16.29): This flat bone can be felt just under the skin in the middle of the front of the chest. 4. Ribs The 12 pairs of ribs form the lateral walls of the thoracic cage. They are elongated curved bones that articulate posteriorly with the vertebral column. Anteriorly, the first 7 pairs of ribs articulate directly with the sternum and are known as the true ribs. The next three pairs articulate only indirectly. In both cases, costal cartilages attach the ribs to the sternum. The lowest two pairs of ribs, referred to as floating ribs, do not join the sternum at all, their anterior tips being free.  B. APPENDICULAR SKELETON The appendicular skeleton consists of the: shoulder girdle with the upper limbs pelvic girdle with the lower limbs 1.1 Shoulder girdle The shoulder girdle consists of two clavicles and two scapulae. 1. Clavicle (collar bone) The clavicle is an S-shaped long bone. The clavicle provides the only bony link between the upper limb and the axial skeleton. 2. Scapula (shoulder blade) The scapula is a flat triangular-shaped bone, lying on the posterior chest wall superficial to the ribs and separated from them by muscles. 1.2 The upper limb i. Humerus This is the bone of the upper arm. The distal end of the bone presents two surfaces that articulate with the radius and ulna to form the elbow joint. ii. Ulna and radius These are the two bones of the forearm. The ulna is longer than and medial to the radius and when the arm is in the anatomical position, i.e. with the palm of the hand facing forward, the two bones are parallel. iii. Carpal (wrist) bones There are eight carpal bones arranged in two rows of four. From outside inwards they are: proximal row: scaphoid, lunate, triquetrum, pisiform distal row: trapezium, trapezoid, capitate, hamate. These bones are closely fitted together and held in position by ligaments that allow a limited amount of movement between them. The bones of the proximal row are associated with the wrist joint and those of the distal row form joints with the metacarpal bones. iv. Metacarpal bones (bones of the hand) These five bones form the palm of the hand. They are numbered from the thumb side inwards. The proximal ends articulate with the carpal bones and the distal ends with the phalanges. v. Phalanges (finger bones) There are 14 phalanges, three in each finger and two in the thumb. They articulate with the metacarpal bones and with each other, by hinge joints. 2. Pelvic girdle and lower limb: The lower limb forms a joint with the trunk at the pelvic girdle. 2.1 The pelvic girdle The pelvis is the term given to the basin-shaped structure formed by the pelvic girdle and its associated sacrum. Innominate (hip) bones. Each hip bone consists of three fused bones: the ilium, ischium and pubis. On its lateral surface is a deep depression, the acetabulum, which forms the hip joint with the almost-spherical head of femur. The pelvis: The pelvis is formed by the hip bones, the sacrum and the coccyx. Differences between male and female pelvis: The shape of the female pelvis allows for the passage of the baby during childbirth. In comparison with the male pelvis, the female pelvis has lighter bones, is shallower and more rounded and is generally roomier. 2.2 The lower limb i. Femur (thigh bone): The femur is the longest and heaviest bone of the body. The head is almost spherical and fits into the acetabulum of the hip bone to form the hip joint. The femur transmits the weight of the body through the bones below the knee to the foot. ii. Tibia (shin bone): The tibia is the medial of the two bones of the lower leg. iii. Fibula: The fibula is the long slender lateral bone in the leg. iv. Patella (knee cap): This is a roughly triangular-shaped sesamoid bone associated with the knee joint. v. Tarsal (ankle) bones: The seven tarsal bones forming the posterior part of the foot (ankle) are the talus, calcaneus, navicular, cuboid and three cuneiform bones. The talus articulates with the tibia and fibula at the ankle joint. The other bones articulate with each other and with the metatarsal bones. vi. Metatarsals (bones of the foot: These are five bones, numbered from inside out, which form the greater part of the dorsum (sole) of the foot. At their proximal ends they articulate with the tarsal bones and at their distal ends, with the phalanges. The enlarged distal head of the 1st metatarsal bone forms the ‘ball’ of the foot. vii. Phalanges (toe bones): There are 14 phalanges arranged in a similar manner to those in the fingers, i.e. two in the great toe (the hallux) and three in each of the other toes.