THE SKELETAL SYSTEM

THE SKELETAL SYSTEM

Bone

• Functions of bones  
• Types of bones  
• Bone structure  
• The microscopic structure of bone 

Axial skeleton

• Skull
• Vertebral column 
• Thoracic cage 

Appendicular skeleton

• Shoulder girdle and upper limb 
• Pelvic girdle and lower limb

Joints

• Fibrous joints 
• Cartilaginous joints 
• Synovial joints 

The main synovial joint of the limbs

• Shoulder joint 
• Elbow joint 
• Proximal and distal radioulnar joints  
• Wrist joint 
• Joints of the hands and fingers 
• Hip joint 
• Knee joint 
• Ankle joint 
• Joints of the feet and toes 

Bone:-
Bone is living material, made up of minerals, consisting of living tissue and non-living substances. Bone is a dynamic tissue that continues to be built, broken down and rebuilt in a process called bone remodeling.


Functions of bones:-  

• The functions of bones include:
• 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 hemopoiesis, the production of blood cells in red bone marrow mineral storage, especially calcium phosphate – the mineral reservoir within the bone is essential for the maintenance of blood calcium levels, which must be tightly controlled.

Types of bones  

1. long bone 
2. short bones 
3. flat bones 
4. irregular bones 
5. sesamoid bones.

Bone structure  

1. Long bones These bones are often curved to assist with strength; they are longer and wider than other bones and consist of a shaft and a variable number of extremities (endings). The femur, tibia, fibula,  humerus, ulna, and radius are examples of long bones.

2. Short bones Can be described as cube-shaped and are approximately the same size length and width. Their primary function is to provide support and stability with little movement. Examples of short bones are the carpals and tarsals – the wrist and foot bones. These bones consist of only a thin layer of the compact, hard bone with cancellous bone on the inside with relatively large amounts of bone marrow.

3. Flat bones are strong, flat plates of bone whose key function are to provide protection to the body’s vital organs as well as being a base for muscular attachment.
A prime example of a flat bone is:-

Scapula (shoulder blade)
The sternum (breast bone)
Cranium (skull)
Os coxae (hip bone) pelvis and ribs are also classed as flat bones

4. Irregular bones These bones do not fall into any other category, because of their non-uniform shape. Examples of these are the vertebrae, sacrum, and mandible (lower jaw). They consist chiefly of cancellous bone, with a thin outer layer of compact bone.

5. Sesamoid bones These types of bones are mostly short or irregular bones, embedded in a tendon. The patella (knee cap) is the most obvious example of this and sits within the patella or quadriceps tendon. Other types of sesamoid bones are the pisiform (smallest of the carpals) and the two small bones at the base of the first metatarsal. Sesamoid bones are usually present in a tendon where it passes over a joint; this provides protection for the tendon.

The microscopic structure 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 an 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:

• osteoblast 
• osteocyte 
• osteoclast

Osteoblasts:-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 the bone is growing, repairing or remodeling, e.g:
• in the deeper layers of the periosteum
• in the centers of ossification of immature bone
• at the ends of the diaphysis adjacent to the epiphyseal cartilages of long bones
• at the site of a fracture.

As they deposit new bone tissue around themselves. They eventually become trapped in tiny pockets in the growing bone, and differentiate into osteocytes.

Osteocytes:- 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.

Osteoclasts:- These cells break down bone, releasing calcium and phosphate. They are very large cells with up to 50 nuclei, which have formed from the fusion of many monocytes. Osteoclasts are found in areas of the bone where there is active growth, repair or remodeling, e.g.:
• under the periosteum, maintaining bone shape during growth and to remove excess callus formed during the healing of fractures.
• round the walls of the medullary canal during growth and to canalize callus during healing.

Axial skeleton
The axial skeleton comprises

• Skull - includes bones of the cranium, face, and ears (auditory ossicles). 
• Hyoid – U-shaped bone or complex of bones located in the neck between the chin and larynx.
• Vertebral column – includes spinal vertebrae. 
• Thoracic cage – includes ribs and sternum (breast bone). 

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.
Sinuses 
Sinuses containing air are present in the sphenoid, ethmoid, maxillary and frontal bones. They all communicate with the nasal cavity and are lined with ciliated mucous membrane. They give resonance to the voice and reduce the weight of the skull, making it easier to carry.

Cranium 
The cranium is formed by a number of flat and irregular bones that protect the brain. It has a base upon which the brain rests and a vault that surrounds and covers it. The periosteum lining the inner surface of the skull bones forms the outer layer of the dura mater.
In the mature skull, the joints (sutures) between the bones are immovable. The bones have numerous perforations (e.g. foramina, fissures) through which nerves, blood and lymph vessels pass. The bones of the cranium are:

• 1 frontal bone 
• 2 parietal bones 
• 2 temporal bones
• 1 occipital bone 
• 1 sphenoid bone 
• 1 ethmoid bone

1 frontal bone 
The frontal bone is the single bone that forms the forehead. At its anterior midline, between the eyebrows, there is a slight depression called the glabella.

2 Parietal Bone
The parietal bone forms most of the upper lateral side of the skull. These are paired bones, with the right and left parietal bones joining together at the top of the skull. Each parietal bone is also bounded anteriorly by the frontal bone, inferiorly by the temporal bone, and posteriorly by the occipital bone.

2 Temporal BoneThe temporal bone forms the lower lateral side of the skull

1 Occipital Bone

The occipital bone is the single bone that forms the posterior skull and posterior base of the cranial cavity.

1 Sphenoid Bone
The sphenoid bone is a single, complex bone of the central skull. It serves as a “keystone” bone because it joins with almost every other bone of the skull. The sphenoid forms much of the base of the central skull and also extends laterally to contribute to the sides of the skull

1 Ethmoid BoneThe ethmoid bone is a single, midline bone that forms the roof and lateral walls of the upper nasal cavity, the upper portion of the nasal septum, and contributes to the medial wall of the orbit. On the interior of the skull, the ethmoid also forms a portion of the floor of the anterior cranial cavity.

Face Face The skeleton of the face is formed by 13 bones:-
• 2 zygomatic (cheek) bones 

• 1 maxilla 

• 2 nasal bones 

• 2 lacrimal bones
• 1 vomer 

• 2 palatine bones 

• 2 inferior conchae 

• 1 mandible

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.

Cervical vertebrae

• These are the smallest vertebrae. The transverse processes have a foramen through which a vertebral artery passes upwards to the brain. The first two cervical vertebrae, the atlas, and the axis are atypical. 
• The first cervical vertebra (C1), the atlas, is the bone on which the skull rests. Below the atlas is the axis, the second cervical vertebra (C2). 
• The atlas is essentially a ring of bone, with no distinct body or spinous process, although it has two short transverse processes. It possesses two flattened facets that articulate with the occipital bone; these are condyloid joints and they permit nodding of the head. 
• The axis sits below the atlas and has a small body with a small superior projection called the odontoid process (also called the dens, meaning tooth). This occupies part of the posterior foramen of the atlas above and is held securely within it by the transverse ligament. The head pivots (i.e. turns from side to side) on this joint. 
• The 7th cervical vertebra, C7, is also known as the vertebra prominent. It possesses a long spinous prominence terminating in a swollen tubercle, which is easily felt at the base of the neck

Thoracic vertebrae

• The 12 thoracic vertebrae are larger than the cervical vertebrae because this section of the vertebral column has to support more bodyweight. The bodies and transverse processes have facets for articulation with the ribs.

Lumbar vertebrae

• These are the largest of the vertebrae because they have to support the weight of the upper body. They have substantial spinous processes for attachment of the muscles of the lower back.

Sacrum 

• This consists of five rudimentary vertebrae fused to form a triangular or wedge-shaped bone with a concave anterior surface. The upper part, or base, articulates with the 5th lumbar vertebra. 

Coccyx

• This consists of the four-terminal vertebrae fused to form a very small triangular bone, the broad base of which articulates with the tip of the sacrum.

Functions of the vertebral column These include:

• Collectively the vertebral foramina form the vertebral canal, which 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.
• The 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.

Thoracic cage 

The thoracic cage (rib cage) forms the thorax (chest) portion of the body. It consists of the 12 pairs of ribs with their costal cartilages and the sternum. The ribs are anchored posteriorly to the 12 thoracic vertebrae (T1–T12). The thoracic cage protects the heart and lungs.

Sternum

The sternum is the elongated bony structure that anchors the anterior thoracic cage. It consists of three parts: 

The manubrium 

Body

Xiphoid process

• The manubrium is the wider, superior portion of the sternum. The top of the manubrium has a shallow, U-shaped border called the jugular (suprasternal) notch. This can be easily felt at the anterior base of the neck, between the medial ends of the clavicles. The first ribs also attach to the manubrium.
• The elongated, central portion of the sternum is the body. The manubrium and body join together at the sternal angle, so-called because the junction between these two components is not flat, but forms a slight bend. 
• The inferior tip of the sternum is the xiphoid process. This small structure is cartilaginous early in life but gradually becomes ossified starting during middle age.

Ribs

Each rib is a curved, flattened bone that contributes to the wall of the thorax. The ribs articulate posteriorly with the T1–T12 thoracic vertebrae and most attach anteriorly via their costal cartilages to the sternum. There are 12 pairs of ribs. The ribs are numbered 1–12 in accordance with the thoracic vertebrae.

Appendicular skeleton

• Shoulder girdle and upper limb 
• Pelvic girdle and lower limb 

shoulder girdle and upper limb The upper limb forms a joint with the trunk via the shoulder (pectoral) girdle.

shoulder girdleShoulder girdle The shoulder girdle consists of two clavicles and two scapulae.
Clavicle 

The clavicle is an S-shaped long bone. It articulates with the manubrium of the sternum at the sternoclavicular joint and forms the acromioclavicular joint with the acromion process of the scapula. The clavicle provides the only bony link between the upper limb and the axial skeleton.
Scapula 

The scapula is a flat triangular-shaped bone, lying on the posterior chest wall superficial to the ribs and separated from them by muscles.

The upper limb

The upper limb is divided into three regions. These consist of 

• Arm, located between the shoulder and elbow joints 
• Forearm, which is between the elbow and wrist joints
• Hand, which is located distal to the wrist

There are 30 bones in each upper limb. 

• The humerus is the single bone of the upper arm, and the ulna (medially) and the radius (laterally) are the paired bones of the forearm. 
• The base of the hand contains eight bones, each called a carpal bone, and the palm of the hand is formed by five bones, each called a metacarpal bone. 
• The fingers and thumb contain a total of 14 bones, each of which is a phalanx bone of the hand.

Humerus

This is the bone of the upper arm. The head sits within the glenoid cavity of the scapula, forming the shoulder joint. Distal to the head are two roughened projections of bone, the greater and lesser tubercles, and between them, there is a deep groove, the bicipital groove or intertubercular sulcus, occupied by one of the tendons of the biceps muscle. 

The distal end of the bone presents two surfaces that articulate with the radius and ulna to form the elbow joint.

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. They articulate with the humerus at the elbow joint, the carpal bones at the wrist joint and with each other at the proximal and distal radioulnar joints.


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.

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.

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.

Pelvic girdle and lower limb

The lower limb forms a joint with the trunk at the pelvic girdle.
The pelvic girdle  

The pelvic girdle is formed from two innominate (hip) bones. 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 the femur. 
• The ilium is the upper flattened part of the bone and it presents the iliac crest, the anterior curve of which is called the anterior superior iliac spine. The ilium forms a synovial joint with the sacrum, the sacroiliac joint, a strong joint capable of absorbing the stresses of weight-bearing and which tends to become fibrosed in later life. 
• The pubis is the anterior part of the bone and it articulates with the pubis of the other hip bone at a cartilaginous joint, the symphysis pubis. 
• The ischium is the inferior and posterior part. The rough inferior projections of the ischia, the ischial tuberosities, bear the weight of the body when seated. 
• The union of the three parts takes place in the acetabulum.

The pelvis

The pelvis is formed by the hip bones, the sacrum, and the coccyx. It is divided into upper and lower parts by the brim of the pelvis, consisting of the promontory of the sacrum and the iliopectineal lines of the innominate bones. The greater or false pelvis is above the brim and the lesser or true pelvis is below. 

Differences between male and female pelves 

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 more shallow and rounded and is generally roomier.

The lower limb The 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 neck extends outwards and slightly downwards from the head to the shaft and most of it is within the capsule of the hip joint. 

The posterior surface of the lower third forms a flat triangular area called the popliteal surface. The distal extremity has two articular condyles, which, with the tibia and patella, form the knee joint. The femur transmits the weight of the body through the bones below the knee to the foot.

Tibia (shin bone)

The tibia is the medial of the two bones of the lower leg. The proximal extremity is broad and flat and presents two condyles for articulation with the femur at the knee joint. The head of the fibula articulates with the inferior aspect of the lateral condyle, forming the proximal tibiofibular joint. The distal extremity of the tibia forms the ankle joint with the talus and the fibula. The medial malleolus is a downward projection of bone medial to the ankle joint.

Fibula

The fibula is the long slender lateral bone in the leg. The head or upper extremity articulates with the lateral condyle of the tibia, forming the proximal tibiofibular joint, and the lower extremity articulates with the tibia and projects beyond it to form the lateral malleolus. This helps to stabilize the ankle joint.

Patella (knee cap) 

This is a roughly triangular-shaped sesamoid bone associated with the knee joint. Its posterior surface articulates with the patellar surface of the femur in the knee joint and its anterior surface is in the patellar tendon, i.e. the tendon of the quadriceps femoris muscle.

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 calcaneus forms the heel of the foot. The other bones articulate with each other and with the metatarsal bones.

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.

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.

Joints

A joint is a site at which any two or more bones articulate or come together. Joints allow flexibility and movement of the skeleton and allow attachment between bones.
Fibrous joints 
The bones forming these joints are linked with tough, fibrous material. Such an arrangement often permits no movement. For example, the joints between the skull bones, the sutures, are completely immovable, and the healthy tooth is cemented into the mandible by the periodontal ligament. The tibia and fibula in the leg are held together along their shafts by a sheet of fibrous tissue called the interosseous membrane. This fibrous joint allows a limited amount of movement and stabilizes the alignment of the bones.
Cartilaginous joints 
These joints are formed by a pad of tough fibrocartilage that acts as a shock absorber. The joint may be immovable, as in the cartilaginous epiphyseal plates, which in the growing child links the diaphysis of a long bone to the epiphysis. Some cartilaginous joints permit limited movement, as between the vertebrae, which are separated by the intervertebral discs, or at the symphysis pubis, which is softened by circulating hormones during pregnancy to allow for expansion during childbirth.
Synovial joints 
Synovial joints are characterized by the presence of a space or capsule between the articulating bones. The ends of the bones are held close together by a sleeve of fibrous tissue and lubricated with a small amount of fluid. Synovial joints are the most moveable of the body.

The main synovial joint of the limbs

Shoulder joint 
This ball and socket joint is the most mobile in the body and consequently is the least stable and prone to dislocation, especially in children. It is formed by the glenoid cavity of the scapula and the head of the humerus and is well padded with protective bursae. The capsular ligament is very loose inferiorly to allow for the free movement normally possible at this joint. The glenoid cavity is deepened by a rim of fibrocartilage, the glenoidal labrum, which provides additional stability without limiting movement. 

The tendon of the long head of the biceps muscle is held in the intertubercular (bicipital) groove of the humerus by the transverse humeral ligament. It extends through the joint cavity and attaches to the upper rim of the glenoid cavity.
Elbow joint This hinge joint is formed by the trochlea and the capitulum of the humerus, and the trochlear notch of the ulna and the head of the radius. It is an extremely stable joint because the humeral and ulnar surfaces interlock and the capsule is very strong. Extracapsular structures consist of anterior, posterior, medial and lateral strengthening ligaments, which contribute to joint stability.
Muscles and movements

Because of the structure of the elbow joint, the only two movements it allows are flexion and extension. The biceps are the main flexor of the forearm, aided by the brachialis; the triceps extend it.
Proximal and distal radioulnar joints Proximal and distal radioulnar joints The proximal radioulnar joint is a pivot joint formed by the rim of the head of the radius rotating in the radial notch of the ulna and is in the same capsule as the elbow joint. The annular ligament is a strong extracapsular ligament that encircles the head of the radius and keeps it in contact with the radial notch of the ulna. The distal radioulnar joint is a pivot joint between the distal end of the radius and the head of the ulna.
Muscles and movements

The forearm may be pronated (turned palm down) or supinated (turned palm up). Pronation is caused by the action of the pronator teres and supination by the supinator and biceps muscles.
Wrist joint This is a condyloid joint between the distal end of the radius and the proximal ends of the scaphoid, lunate and triquetrum. A disc of white fibrocartilage separates the ulna from the joint cavity and articulates with the carpal bones. It also separates the inferior radioulnar joint from the wrist joint. Extracapsular structures consist of medial and lateral ligaments and anterior and posterior radiocarpal ligaments.
Muscles and movements

The wrist can be flexed, extended, abducted and adducted.
Joints of the hands and fingers
There are synovial joints between the carpal bones, between the carpal and metacarpal bones, between the metacarpal bones and proximal phalanges and between the phalanges. Movement at the hand and finger joints is controlled by muscles in the forearm and smaller muscles within the hand. There are no muscles in the fingers; finger movements are produced by tendons extending from muscles in the forearm and the hand.
Hip joint

This ball and socket joint is formed by the cup-shaped acetabulum of the innominate (hip) bone and the almost spherical head of the femur. The capsular ligament encloses the head and most of the neck of the femur.
Muscles and movements. The lower limb can be extended, flexed, abducted, adducted, rotated and circumducted at the hip joint.
Knee joint This is the body’s largest and most complex joint. It is a hinge joint formed by the condyles of the femur, the condyles of the tibia and the posterior surface of the patella. The anterior part of the capsule is formed by the tendon of the quadriceps femoris muscle, which also supports the patella. Intracapsular structures include two cruciate ligaments that cross each other, extending from the intercondylar notch of the femur to the intercondylar eminence of the tibia. They help to stabilize the joint.
Muscles and movements 

Possible movements at this joint are flexion, extension, and a rotatory movement that ‘locks’ the joint when it is fully extended. When the joint is locked, it is possible to stand upright for long periods of time without tiring the knee extensors. The main muscles extending the knee are the quadriceps femoris, and the principal flexors are the gastrocnemius and hamstrings.
Ankle joint This hinge joint is formed by the distal end of the tibia and its malleolus (medial malleolus), the distal end of the fibula (lateral malleolus) and the talus. Four important ligaments strengthen this joint: the deltoid and the anterior, posterior, medial and lateral ligaments.
Muscles and movements 

The movements of inversion and eversion occur between the tarsal bones and not at the ankle joint. The ankle joint is wrapped in synovial sheaths and held close to the bones by strong transverse ligaments. They move smoothly within their sheaths as the joints move. In addition to moving the joints of the foot, these muscles support the arches of the foot and help to maintain balance.
Joints of the feet and toes Joints of the feet and toes There are a number of synovial joints between the tarsal bones, between the tarsal and metatarsal bones, between the metatarsals and proximal phalanges and between the phalanges. Movements are produced by muscles in the leg with long tendons that cross the ankle joint and by muscles of the foot.