SKELETAL SYSTEM

Introduction

This system has circa 206 bones, plus associated structures like tendons and ligaments, and it
serves many important functions. Since bones are rigid, they support the body, give it its shape,
and protect its vital organs against injury. Bones provide points for muscle attachments, and,
together with the movable joints, they form a system of levers upon which muscles act to produce
contraction and thus body movement. A joint is a place where bones meet, or articulate. Bones also
function as a site for mineral storage (mainly calcium and phosphorus) and blood cell formation
(haematopoiesis). Tendons and ligaments are strong bands of fibrous connective tissue that attach,
or link, muscles to bones (tendons), and bones to bones (ligaments).

Skeleton

The skeleton has two parts: the axial skeleton and the appendicular skeleton.

The axial skeleton includes the skull, the hyoid bone, the vertebral column, and the thorax. Its
components are aligned along the axis of the body.

The appendicular skeleton includes the bones of the upper and lower extremities (or limbs), the
pectoral (or shoulder) girdle, and the pelvic (or hip) girdle. Its components are outside the body
main axis.



Bones

There are four main categories of bones: long, short, flat and irregular.

Long bones form the extremities (arms and hands, legs and feet) and consist of a diaphysis, two
bulbous epiphyses (sing. epiphysis), articular cartilage and periosteum. Periosteum is a dense
fibrous membrane of connective tissue covering the surface of bones (except at the joint ends where
the bone is covered by cartilage) that contains numerous blood and lymph vessels, and nerves. Dense
and hard compact bone is the main building material of long bones and in their hollow centre they
contain yellow bone marrow. The epiphyses are mostly formed by spongy, or cancellous, bone
material.

Short bones are of irregular, cubic shape and they are found in the wrists and ankles of the upper
and lower limbs, respectively. Their core is typically made up of spongy bone.

Flat bones provide broad surfaces for attachment of muscles and protection for internal organs.
Examples of flat bones are bones of the skull, ribs, shoulder blades, the breastbone and pelvis.

Irregular bones are all the others; for instance vertebrae (bones of the spine), and bones of the
ear and face. They are of various shapes and sizes.

The cavities of the spongy bone in some flat and irregular bones contain red bone marrow, a kind of
connective tissue in which blood cells are produced.

Ossification is the medical term for bone growth. Bone is a living tissue that is being renewed
throughout life and three types of bone cells participate in this process: osteoblasts
(bone-building cells), osteocytes (mature bone cells), and osteoclasts (bone-destroying cells).


The Skull

The skull (or cranium) is the bony framework of the head. Bones of the skull comprise cranial bones
and facial bones. Cranial bones are flat, rounded (curved), and fused together to protect the brain
which is situated in the brain case (or cranium in its restricted sense). The six cranial bones are
the frontal (a), parietal (b), temporal (c), sphenoid (d), occipital (e), and ethmoid (f). These
bones meet at zigzag lines called sutures which are in fact immoveable joints.

Of the bones of the face, the upper jaw (or maxilla; g), the lower jaw (or mandible; h), the
zygomatic bone (or cheek bone), and the nasal bone are the most prominent. The maxilla, zygomatic
and nasal bones are two in number (right and left), and the mandible is the only separate and
moveable bone of the skull. Besides the brain case, a number of other cavities are found in the
skull: eye-sockets (or orbits), the nasal cavity, and sinuses.


The Vertebral Column

The vertebral column (also called spinal column, spine, or backbone) is composed of 33 vertebrae
(sing. vertebra). Its main function is the support of the skull and trunk (or torso) and protection
of the spinal cord (or chord) located in the spinal canal. The spine also provides attachment sites
for the ribs, and muscles of the trunk. In adults, three groups of vertebrae and two sets of fused
bones form the backbone: seven cervical (neck) vertebrae, twelve thoracic (upper back) vertebrae,
and five lumbar (lower back) vertebrae. Five (or sometimes four) fused vertebrae form the sacrum
(sacral vertebrae) and from three to five fused small vertebrae form the coccyx (or tail bone;
coccygeal vertebrae). The first cervical vertebra that articulates with the skull is called the
atlas, the second cervical vertebra is called the axis. The atlas allows the head to nod, the axis
allows it turn.

A typical vertebra consists of a body, arch and processes (or projections). The prominent upward
projection of the axis is called the odontoid process due to its toothlike shape. The 26 moveable
vertebrae are separated by intervertebral discs which function as shock absorbers. These discs
contain a lot of water the content of which decreases with age, resulting in loss of height.

Looking at it from the side, the backbone has the form of a long S: there are four curves and two
basic types of curvature. The congenital (inborn) backward (posterior) curvature of the thoracic
and sacral spine is called kyphosis, whilst the natural forward (anterior) curvature affecting the
cervical and lumbar spine develops during infancy and childhood, respectively; it is called
lordosis. Scoliosis – one or more lateral curves of the spinal column – may also occur.


 The Thorax

The thorax (or rib cage) is primarily built up of twelve pairs of ribs that articulate with the
thoracic vertebrae posteriorly and the sternum (or breast bone) anteriorly.  The 24 ribs are long,
flat, curved bones that form, together with the sternum and thoracic vertebrae, a protective cage
for the heart, lungs, and other internal organs. They give the chest its shape. There are three
kinds of ribs: true ribs are the upper seven pairs; they attach directly to the breast bone. False
ribs (the next three pairs below) are only indirectly attached to the sternum; in fact, they are
interconnected by their common cartilage (called costal) and attached to the lowest pair of true
ribs. The last two pairs of ribs are called floating ribs, as they are not connected to either the
sternum or other ribs in front. The spaces between the ribs (intercostal spaces) are filled with
muscles that help you breathe.


The Upper Extremities

The upper limbs are a part of the appendicular skeleton. They are attached to the axial skeleton by
the pectoral girdle on each side, the bones of which include two scapulae (or shoulder blades;
sing. scapula) and two clavicles (or collar bones). The scapula is located in the upper back and
articulates with it by muscles and with the humerus in the shoulder joint. The clavicle is
connected to the sternum and the whole shoulder girdle is fixed by muscles.

Beginning in the shoulder joint, the (upper) arm (or brachium) extends down to the elbow joint and
is formed by a long single bone called the humerus. The upper epiphysis, or end, of the humerus is
characterised by its hemispherical smooth joint head, fitting in the shoulder joint socket. The
forearm begins at the elbow. The forearm is composed of two long bones, the radius on the thumb
side, and the ulna on the little finger side.

The forearm articulates with the hand at the wrist (or carpus). The wrist consists of eight small
bones arranged in two irregular rows called carpal bones (or just carpals). They are tightly bound
together by ligaments. The five metacarpal bones (or just metacarpals) that form the framework of
the palm project from the carpus. The bones of the fingers are referred to as phalanges (sing.
phalanx). They articulate directly with the metacarpals and are three in number in each of the
fingers, and two in the thumb. The individual names of the fingers are as follows: forefinger (or
index finger), middle finger, ring finger, and little finger (or pinkie).


The Lower Extremities

The lower limbs, appendages of the axial skeleton, are attached to the trunk by the pelvic girdle.
It is composed of two coxal (or hip) bones (also called coxae, sing. coxa, or innominate bones). In
adults, each of the coxae consists of three fused bones: the pubis (or pubic bone), ilium and
ischium. Together with the sacrum (pl. sacra) and coccyx (pl. coccyges), segments of the vertebral
column, the coxal bones form the pelvis (pl. pelves). The pelvis is a bowl-shaped ‘basin’ of bones,
broader in women, that protects and supports many internal, abdominal organs (or viscera, sing.
viscus), and the spinal column.

The pelvic girdle articulates with the femora (sing. femur), or thigh bones, at the hip joint. The
thigh bone is the longest, strongest and heaviest bone in the body, and one of the 30 bones that
constitute a leg (an arm also consists of 30 bones), or lower extremity. The thigh (or upper leg)
is the region of a leg above the knee (or knee joint), below the knee there is the shin (the
anterior portion) and calf (pl. calves; the posterior portion). The framework of the lower leg is
formed by the tibia (or shin bone) in front, and the fibula at the back. These two long bones
articulate with the thigh bone at the knee which is protected anteriorly by the patella (or
knee-cap).

The feet (sing. foot) extend from the ankle downwards, so they are the inferior ends of the lower
limbs. They bear the whole weight of the body and the strength of the foot bones corresponds to
that fact. The seven tarsal bones (or just tarsals) are the bones of the ankle and the proximal
region of the foot (they are the equivalent of the carpals in the hand). Amongst these, the
calcaneus (pl. calcanei), or heel bone, and the talus (pl. tali), or ankle bone, are the largest
and most prominent. The tarsal bones together with the metatarsal bones (or just metatarsals) form
the instep, or the major arch of the foot; the bottom of the foot is called the sole. The toes,
like the fingers of the hand, consist of bones called phalanges. The big toe has only two of them.

A human ankle joint. The inner and outer bulges at the ankle are formed by the ends of the tibia
(the inner bulge) and fibula (the outer bulge). (Reproduced by permission of Photo Researchers,
Inc.)