Human Skeleton

Human Skeleton

Without the skeletal system, you would be unable to perform movements such as walking or grasping. The framework of bones and cartilage that protects organs and allows movement is called skeletal (skeletos= dried up) system.

The specialized branch of medicine that deals with the preservation and restoration of the skeletal system, joints, and associated structures is called orthopedics (ortho = correct or straighten, pais = child).

There are two type of skeletal system

Exoskeletons are external, as is typical of many invertebrates; they enclose the soft tissues and organs of the body.

Endoskeletons are internal, as is typical of many vertebrates; they are usually surrounded by skin and musculature, though they often enclose vital organs.

Skeletons may or may not be mineralized- human skeletons are calcified, while shark skeletons are cartilaginous - and may be jointed for flexibility and motility or rigid for structural strength. The average adult human skeleton has around 206 bones. These bones meet at joints, the majority of which are freely movable. The skeleton also contains cartilage for elasticity. Ligaments are strong strips of fibrous connective tissue that hold bones together at joints, thereby stabilizing the skeleton during movement.

Functions of skeletal system

• Bones give shape to the body.
• Various skeletal muscles are attached to the bones. These are voluntary muscles. So contraction of these muscles is responsible for the movement at the joints.
• Bones give support and protection to various vital organs like brain in skull, heart and lungs in thoracic cage.
• In bone marrow red and white blood cells are formed.
• Bones take part in metabolism of calcium.

Division of the skeletal system

1. Axial skeleton
2. Appendicular skeleton

Axial skeleton

It consists of the bones that lie around the axis:

1. Skull bones
2. Hyoid bone
3. Ribs
4. Sternum (breastbone)
5. Vertebrae (bones of the backbone)

Appendicular skeleton

It consists of following

1. Bones of the upper limb
2. Bones of the lower limb
3. Girdles

Types of bones

Long Bones

 
They have greater length than width and consist of a shaft and variable number of extremities (ends). They are slightly curved for strength. A curved bone absorbs the stress of the body weight at several different points so the stress is evenly distributed. Long bones consist mostly of compact bone tissue, which is dense and has few spaces. They also contain considerable amount of spongy bone tissue, which has large spaces. Long bones includes those in the thigh (femur), leg (tibia and fibula), toes (phalanges), arm (humerus), forearm (ulna and radius), and fingers (phalanges).

Short bones

They are small bones somewhat cube-shaped and nearly equal n length and width. They are spongy bone except at the surface, where there is a thin layer of compact bone. Examples are the wrist (carpal) and ankle (tarsal) bones.

Flat bones

They are flat and composed of two nearly parallel plates of compact bone enclosing a layer of spongy bone. Flat bones afford considerable protection and provide extensive area for muscle attachment. These include the cranial bone (which protect the brain) and the sternum and ribs (which protect the organs in the thorax), and the shoulder blades (scapulae).

Irregular bones

They have irregular and complex shape. They also vary in the amount of spongy and compact bone present. Examples are backbone (vertebrae), and certain facial bones.

Two additional types of bones are not included in this classification by shape but instead are classified by location.

SuturalorWormain bones– These are small bones located within the joints (sutures) of certain cranial bones.

Seamoid bones– These are small bones that are embedded in tendons where considerable pressure develops, for instance, in the thumb and great toe. Examples are kneecaps (patellae).

The Skull

The skull is a bony framework which protects the brain; contain 22 bones, rests on the superior end of the vertebral column. It consists of two parts:

1. Cranium
2. Facial skeleton

Skull Lateral Side

Cranium

It consists of eight bones. Its upper surface is known as vault of the skull and lower surface the base of the skull. Vault is smooth on its outer surface but inner surface has many depressions and ridges for accommodating brain and base of the skull contain many openings for passage of nerves and blood vessels. The bones of the cranium are

        Bones                         Numbers

1. Occipital bone                        1
2. Parietal bones                        2
3. Frontal bone                           1
4. Temporal bones                     2
5. Ethmoid bone                        1
6. Sphenoid bone                      1

Occipital bone

It forms the posterior and inferior part of the cranium. It contain foramen magnum. Through this foramen medulla oblongata join the spinal cord. On the sides of foramen magnum there are occipital condyles which articulate with atlas (1st vertebra).

Parietal bone

They are two in number. They join and form the roof and lateral sides of the cranium. Inner surface shows deep furrows for lodgment of cranial arteries. Especially a large furrow in the center for lodging middle meningeal artery. Rupture of this artery can cause collection of the blood and formation of clot which compresses the soft brain tissue. This usually results into unequal size of pupil of the eye.

Frontal bone

It forms the upper and anterior part (forehead) of the cranium. It forms part of the orbital cavities (eye sockets) and the prominent ridges above the eyes, the supraorbital margins. Just above the supraorbital margins, within the bone, are two air-filled cavities or sinuses lined with ciliated mucous membrane, which open into the nasal cavity.

Temporal bone

They are two in number and form the lower part of the lateral sides of cranium. Each bone consists of following parts-

1. Squamous part
2. Mastoid part
3. Petrous part

Squamous part

It projects upwards. Temporalis muscle is attached to it. Its zygomatic process projects forward to join zygomatic bone. Posterior and inferior to this process lies the external auditory canal.

Mastoid part

It lies posteriorly and projects downwards as mastoid process. To its outer surface stern-mastoid muscle is attached. Mastoid process contains air spaces known as mastoid air cells. Particularly large space is tympanic antrum which id continuous with the cavity of the middle ear (tympanic cavity).

Petrous part

It is wedge shaped part present at the base of the skull. It contain cochlea the organ for hearing.

Ethmoid bone

It is a light, spongelike bone located on the midline in the anterior part of the cranial floor medial to the orbits. It is anterior to the sphenoid and posterior to the nasal bones. The ethmoid bone forms-

• Part of the anterior portion of the caranial floor.
• The medial wall of the orbits.
• The superior portion of the nasal septum, a portion that divides the nasal cavity  into right and left sides.
• Most of the sidewall of the nasal cavity. 

The ethmoid is a major supporting structure of the nasal cavity.

Sphenoid bone

It lies at the middle part of the base of the skull. This bone is called the keystone of the cranial floor because it articulates anteriorly with the frontal bone, laterally with the temporal bones, and posteriorly with the occipital bone. The shape of the sphenoid resembles a bat with outstretched wings. It consists of body, two greater wings, and two lesser wings. 

Body has a depression- sellaturcica for lodging pituitary gland.

All these bones of the skull are joined together by immovable joints called sutures.
Principal sutures –

1. Coronal suture – Between frontal and two parietal bones.
2. Sagittal suture – Between two parietal bones.
3. Lambdoid suture – Between occipital and two parietal bones.
   

All the sutural joints are immovable except the temporomandibular joint i.e. articulation of mandible (lower jaw) with the temporal bone which is a bi-condylar type of joint.

Facial bones

The face shape of the face changes dramatically during first two years after birth. Growth of the face ceases at about 16 years of age. Face is formed of 14 bones. All these bones except the mandible are joined by immovable joints (sutures).

            Name of bones            number

1. Nasal bones                          2   
2. Palatine bones                      2   
3. Lacrimal bones                      2   
4. Zygomatic (cheek) bones      2
5. Vomer                                    1
6. Inferior turbinate bones         2
7. Maxillae                                 2
8. Mandible bone                       1

Skull Front Side

Nasal bones 

These are two small flat, bones meet at the midline and form part of the bridge of the nose. The major portion of the nose consists of cartilage.

Palatine bones

These are two small L-shaped bones. The horizontal parts unite to form the posterior part of the hard palate and the perpendicular parts project upwards to form part of the lateral walls of the nasal cavity. At their upper extremities they form part of the orbital cavities.

Lacrimal bones

These two small bones are posterior and lateral to the nasal bones and form part of the medial walls of the orbital cavities. These are the smallest bones of the face. Each is pierced by a foramen for the passage of the nasolacrimal duct that carries the tears form the medial canthus of the eye to the nasal cavity.

Zygomatic (cheek) bones

They form the prominences of the cheeks and part of the floor and lateral walls of the orbital cavities. They articulate with the frontal, maxilla, sphenoid, and temporal bones.

Vomer

The vomer is a roughly triangular bone that forms the inferior and posterior part of the nasal septum. It articulates with the septal cartilage of the nasal septum that divides the external nose into right and left sides. Its superior border articulates with the perpendicular plate of the ethmoid bone.

Inferior turbinate bone

The two inferior nasal conchae or turbinate are scroll-like bones that forms a part of the lateral wall of the nasal cavity and project into the nasal cavity inferior to the superior and middle nasal conchae of the ethmoid bone. The superior and middle conchae are parts of the ethmoid bone. The conchae collectively increase the surface area in the nasal cavity, allowing inspired air to be warmed and humidified more effectively.

Maxillae (upper jaw bone)

This originates as two bones, but fusion takes place before birth. The maxilla forms the upper jaw, the anterior part of the roof of the mouth, the lateral walls of the nasal cavity and part of the floor of the orbital cavities. The alveolar ridge, or process, projects downwards and carries the upper teeth. On each side is a large air sinus, the maxillary sinus, lined with ciliated mucous membrane and with openings into the nasal cavity.

Mandible bone

The mandible or lower jaw bone, is the largest, strongest facial bone. it is the only skull bone that moves. It originates as two parts: a curved body with the alveolar ridge containing the lower teeth and a ramus, which projects upwards almost at right angle to the posterior end of the body. At the upper end the ramus divides into the condylar process which articulates with the temporal bone to form the temporomandibular joint and the coronoid process, which gives attachment the muscles and ligaments that close the jaw. The point where the ramus joins the body is the angle of the jaw.

Hyoid bone This is an isolated horseshoe-shaped bone lying in the soft tissues of the neck just above the larynx and below the mandible. It does not articulate with any other bone, but is attached to the styloid process of the temporal bone by ligaments. It supports the larynx and gives attachment to the base of the tongue.

Functions of the skull

1. The cranium protects the delicate tissues of the brain.
2. The bony eye sockets provide the eyes with some protection against injury and give attachment to the muscles that move the eyes.
3. The temporal bone protects the delicate structures of the ear.
4. Some bones of the face and the base of the skull give resonance to the voice because they have cavities called sinuses, containing air. The sinuses have tiny opening into the nasal cavity.
5. The bones of the face form the walls of the posterior part of the nasal cavities. They keep the air passages open, facilitating breathing.
6. The maxilla and the mandible provide alveolar ridges in which the teeth are embedded.
7. The mandible is the only movable bone of the skull, and chewing food is the result of coordinated activity of certain muscles of the face, the muscles of mastication, which move the mandible.

Thorax

The term thorax refers to the entire chest. The thorax is a bony cage formed by the sternum, costal cartilages, ribs and the bodies of the thoracic vertebrae. The thoracic cage is narrower at its superior end and broader at its inferior end. The thoracic cage encloses and protects the organs. It also provides support for the bones of the shoulder girdle and upper limbs.

Sternum (Breast bone)

This flat bone is located in the middle of the anterior thoracic wall. During thoracic surgery, it may be split in the midsagittal plane to allow surgeons access to the thymus gland, heart and great vessels of the heart.

Rib Cage

The sternum consists of three portions:

Manubrium stern

It is the upper tri-angular part on the sides it has clavicular notches which articulate with clavicle bones on two sides. These clavicular notches are separated by suprasternal notch. On the sides of manubrium first pair of ribs is attached. 2nd pair of the ribs is attached at the junction of manubrium with the body.

Body

Long narrow portion. It has notches on each side for attachments of 3rd, 4th, 5th, 6th, and 7th costal cartilages. At the junction between manubrium and body of the sternum, there is a prominence because of the pad of cartilage and it is known as angle of Louis or Ludwig. It corresponds with the level of the second rib.

Xiphoid process

Lower part. It is cartilaginous in young person and gets ossified in older persons. Diaphragm, rectus abdominal muscles and lineal Alba are attached to xiphoid.

Ribs

Twelve pairs of ribs make up the sides of the thoracic cavity. The ribs increase in length from the first through seventh, then decrease in length to the twelfth rib. The first through seventh pairs of ribs have a direct anterior attachment to the sternum by a strip of hyaline cartilage called costal cartilage. These ribs are called true (vertebra - sternal) ribs. 

The remaining five pairs of ribs are referred to as false ribs because their costal cartilages either attached indirectly to the sternum or do not attach to the sternum at all. The cartilages of the eighth, ninth and tenth pairs of ribs attach to each other and then to the cartilages of the seventh pair of ribs. These false ribs are called vertebra - chondral ribs. The eleventh and twelfth pairs of ribs are false ribs designated as floating (vertebral) ribs.

Rib is a long bone and has two ends anterior and posterior and a shaft. Posterior end has head, neck and tubercle. There are facets on the head and tubercle which get attached to facets present on the sides and on the transverse processes of the thoracic vertebrae. Anterior end of each rib is attached to a costal cartilage. Shaft has inner and outer surfaces. It is thin and flat. Inner surface is marked by a groove known as subcostal groove in which intercostal nerves and vessels lie.

Shaft of the rib is curved and ribs slope downwards as they came from posterior to anterior side. Posterior end of the rib is more or less fixed whereas anterior end is more movable. During respiration movements of the ribs occur mostly due to elasticity of the costal cartilages.

Rib

Intercostal spaces

Space between each two ribs is called as intercostal space. These spaces vary in size. They contain intercostal muscles attached to ribs above and below. 

There are two types of intercostal muscles – external intercostal and internal intercostal. External intercostal muscle in each space originates from the lower border of the rib above, and is inserted into upper border of the rib below. It runs obliquely downwards and forwards.
The internal intercostal muscle also has same position but it runs obliquely downwards and backwards.

Vertebral column

The vertebral column is a flexible and made up of a number of vertebrae, together with the sternum and ribs form the skeleton of the trunk of the body. Whereas the spinal column consists of bone, the spinal cord consists of nervous tissue. The vertebral column is a strong flexible rod that bends anteriorly, posteriorly, and laterally and rotates.

Vertebral column

It encloses and protects the spinal cord, supports the head and serves as a point of attachment for the ribs and the muscles of the back. Between vertebrae are openings called intervertebral foramina. The nerves that connect the spinal cord to various part of the body pass through these opening. 

There are total 33 vertebrae out of which 24 are separate bones. 

Remaining is fused to form two bones.

The vertebrae are named and grouped according to the region in which they are present as follows:-

• Cervical vertebrae - They are seven in number and are present in the neck region.
• Thoracic vertebrae – They are present in the thoracic region and are twelve in number.
• Lumber vertebrae – They are five in number and are present in the lumber region.
• Sacral vertebrae - There are five sacral vertebrae which are fused and form the bone known as sacrum.
• Coccygeal vertebrae – They are four in number and they are fused to form the bone known as coccyx, (tail bone).
  

The cervical, thoracic and lumber vertebrae are separate bones and are called movable vertebrae, whereas vertebrae forming sacrum and coccyx are called fixed vertebrae. 

Spinal Column

Characteristic of a typical vertebra

Vertebrae in different regions of the vertebral column vary in size and shape.

The body is the thick, disc-shaped anterior portion that is the weight-bearing part of a vertebra. Its superior and inferior surface is roughened for the attachment of the fibrocartilaginous intervertebral discs. The anterior and lateral surfaces contain nutrient foramina for blood vessels.

The vertebral (neural) arch extends posteriorly from the body of the vertebra. It is formed by two short, thick processes, the pedicles which project posteriorly from the body to unite with the laminae. The laminae are the flat parts that join to form the posterior portion of the vertebral arch. The space that lies between the vertebral arch and body known as the vertebral foramen. The vertebral foramina of all vertebrae together form the vertebral (spinal) canal.

Seven processes arise from the each vertebral arch. At the point where a lamina and pedicle join, a transverse process extends laterally and on each side. A single spinous process (spine) projects posteriorly and inferiorly from junction of the laminae. These three processes serve as point of attachment for muscles. The remaining four processes form joint with other vertebrae above and below. The two superior articular processes of a vertebra articulate with the two inferior articular processes of the vertebra immediately superior to them. The two inferior articular processes of a vertebra articulate with two superior articular processes of the vertebra immediately below them.

Cervical vertebrae

These are the smallest vertebrae. The first two cervical vertebrae are atlas and axis, are atypical. Atlas is the first cervical vertebra on which the skull rests. The atlas is essentially a ring of bone, with no distinct body or spinous process. It has two short transverse processes. It possesses two flattened facets that articulate with the occipital bone. Axis is the second cervical vertebra.  The axis sits below the atlas, and has a small body with a small superior projection called the odontoid process. 

The seventh cervical vertebra is also known as the vertebra prominence. It possesses a long spinous prominence terminating in a swollen tubercle.

Thoracic vertebrae

They increase in size from above downwards and are larger than cervical vertebrae because this section of the vertebral column has to support more body weight. A typical thoracic vertebra has following characteristics.

• Body is heart-shaped and contains facets on the sides for attachments of the ribs.
• Neural arch is small.
• Transverse processes are thick and strong and they support the ribs. They have facets for the attachments of ribs.
• Spinous process is long and is directed downwards.

Lumbar vertebrae

They are the largest as the body is larger than the bodies of other vertebrae because they have to support the weight of the upper body. A typical lumbar vertebra has the following characteristics.

• Body is large and kidney shaped.
• Spinous process is broad and hatched shaped.
• Transverse processes are long and slender.
  

Last 5th lumbar vertebrae articulate with sacrum forming a limbo sacral joint.

Sacrum

It is formed by fusion of 5 sacral vertebrae. It is a triangular or wedge-shaped bone with a concave anterior surface, situated in the lower part of vertebral column with base lying above and apex lying below. The base articulates with 5th lumbar vertebrae. At the base the anterior edge forms sacral promontary. Sacral canal is the continuation of spinal canal. Walls of sacral canal are perforated for passage of sacral nerves. 

Spious processes are rudimentary and are seen on the posterior aspects. Anterior surface of the sacrum is concave and has 4 transverse ridges indicating the union of five sacral vertebrae. On the sides of these ridges there are apertures for passage of nerves. These apertures are called sacral foramina. The sides of the sacrum articulate with hip bone and the apex articulates with coccyx.

Coccyx

This consists of the four terminal vertebrae fused to form a very small triangular bone, the board of which articulates with the tip of the sacrum. In females, the coccyx points inferiorly; in males, it points anteriorly.

Functions of vertebral column

1. The vertebral foramina form the vertebral canal, which provides a strong bony protection to the spinal cord.
2. It gives support to the body.
3. 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.
4. The intervertebral discs act as shock absorbers during running, jumping so that brain and spinal cord are protected from shocks.
5. Because of the presence of inter-vertebral discs which act as buffers the column has flexibility which allows it to bend without breaking.
6. It supports the skull.
7. The numerous individual bones enable a certain amount of movement.
8. The vertebral column provides surface for the attachment of muscles which form strong posterior boundary for the cavities.
9. It supports the weight of the body.
10. It forms the axis of the trunk, giving attachment to the ribs, shoulder girdle and upper limbs and the pelvic girdle and lower limbs.