The Human En Do Skeleton

8/2/2019 The Human En Do Skeleton

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•The Human Endoskeleton



Human endoskeleton

The skeleton in man refers to the hard,

supportive connective tissue around which

the organism is built. The skeleton includesall the bones of the body, the joints formed

by the attachment of the bones to one

another, connective tissues and cartilagewhich surround the bones and ligaments

that connect bone to bone.



The Human Skeleton



As in all other vertebrates, humans have aninternal skeleton which is surrounded by muscles

and skin. Such an internal skeleton is called anendoskeleton. In human beings the skeletonconsists of more than 200 different kinds of bonewhich are joined together in various ways to form

a rigid framework. The skeleton can be divided into two main parts,

viz. the axial skeleton and the appendicularskeleton. The major components of each are

represented in the following table:



Skeleton

AXIAL SKELETON APPENDICULAR

SKELETON

Skull Pectoral (shoulder) girdle andupper limbs

Vertebral column, ribs and

breastbone

Pelvic (hip) girdle and lower

limbs



I.The Axial Skeleton

The axial skeleton forms the central axis of the

body. It consists of the skull, the vertebral

column, the ribs and the sternum orbreastbone.



1.The Skull

The skull consists of 28 different bones (including the ossiclesof the ear). The bonesof the skull can bedivided into two maingroups: the cranium

which encloses andprotects the brain andthe facial bones.



The Cranium

The cranium consists of eight flat bones which are rigidly

attached to each other with dentate sutures (joints with teeth-

like protrusions). They envelop and protect the brain. The

frontal bone forms the forehead and portions of the eye sockets(or orbits). The occipital bone, at the base of the skull contains

a large opening, called the foramen magnum, through which

the spinal cord passes. On each side of the opening is the

occipital condyle, - two round protuberances, - by means of

which the skull articulates with the first neck (or cervical)vertebra (the atlas). The organs of hearing are situated in the

temporal bone, one on each side. The openings leading into

these organs can also be seen on each side.



The Facial Bones

The facial skeleton consists of fourteen irregular bones,

which are all (with the exception of the lower jawbone)

firmly attached to the cranium by means of sutures. They

include the nasal bones, the two jawbones and the cheek

bones. The lower jaw articulates with the temporal bone

part of the cheek bone, just in front of the ear. This allows

for the necessary movement of the lower jaw when food is

bitten off and chewed. Both upper and lower jaws havealveolar pockets into which teeth fit.



The teeth are embedded in sockets in the ridges of theupper and lower jaw bones. Three regions can be

distinguished in a tooth:The root which are embedded inthe alveolar pocket of the jaw. The root is firmly attachedto the jaw by a surrounding layer of cement and strongconnective tissue. The neck is the area where the root(s)and crown meet. The crown projects above the gum. It is

covered with a hard, white layer of enamel. The largestpart of the tooth consists of dentine which is a hardersubstance than ordinary bone. The dentine surrounds thecavity which extends from the root to the crown. Bloodcapillaries and nerves enter the cavity at a small opening in

the tip of the root.



2.The Vertebral Column

The vertebral column forms the central part of the skeleton. It supports the skull and protectsthe spinal cord. It also serves as attachment forthe ribs, the pectoral and pelvic girdles. Thevertebral column consists of separate bones, thevertebrae. The different vertebrae are arrangedabove each other. Because the separate vertebraeare attached to each other by means of fibrouscartilaginous discs they form a flexible column.

Each vertebra has articular surfaces above andbelow, which allow articulation movementbetween them.



The vertebral column of

33 vertebrae is dividedinto five regions according to their positionand structure. The fiveregions consist of: Seven

cervical (neck) vertebrae,Twelve thoracic (chest)vertebrae, Five lumbarvertebrae, Five fusedsacral vertebrae, and Four

fused vertebrae.



3.The Ribs

Twelve pairs of ribs articulate with the 12 vertebrae of the

thoracic region. The ribs are flat, narrow bones with a

distinctive bow-shaped curve. Each rib consists of a head

or capitulum, a small tubercle (which is a short distance

back from the head) and the shaft. The head of the rib

articulates with the semi-circular articulating facets formed

by the centra of two successive thoracic vertebrae. The

tubercle fits into and articulates with the articulating facets

on the transverse process. The first seven ribs on each sideare joined to the breastbone by bars of hyaline cartilage

(called costal cartilage in this region).



The first seven pairs of ribs arereferred to as true ribs. Thecartilages of the 8th, 9th and

10th ribs are joined to the costalcartilage of the rib immediatelyabove (i.e. to the costalcartilage of the 7th rib). Thesethree pairs of ribs are known asvertebrochondral ribs. Thelast two pairs of ribs have freeends which are not attached tothe sternum at all. They arefloating ribs. Thevertebrochondral ribs and the

floating ribs are collectivelyknown as false ribs. The ribs(together with their muscles)play an important role in thebreathing mechanism of a

mammal.

Diagram to illustrate the attachment of

the ribs to the thoracic vertebrae

and sternum.



4.The Sternum (Breastbone)

The sternum is a long, flat, dagger-shaped bone.

It is about 15 - 18 cm long and is found in the

center of the chest region. The broad upper end

supports the collar bones. The first seven pairs of ribs are attached to the articulating facets on the

sides of the sternum. The 12 thoracic vertebrae,

the 12 pair of ribs and the sternum forms the

thorax which protects the delicate and vitalorgans of the thorax, viz. the heart and lungs.



II.THE APPENDICULAR

SKELETON The appendicular skeleton consists of the girdles

and the skeleton of the limbs. The upper

(anterior) limbs are attached to the pectoral(shoulder) girdle and the lower (posterior) limbs

are attached to the pelvic (hip) girdle.



1.The Pectoral (Shoulder) Girdle

The Pectoral girdle consists of two shoulder

blades (scapulae) and two collar bones

(clavicles). These bones articulate with oneanother, allowing some degree of

movement.



2.Shoulder Blades (Scapulae)

The shoulder blade is a flat triangular bone whichstretches from the shoulder to the vertebral column at theback. On the back side it has a bony ridge for theattachment of the muscles. The bony ridge forms a

prominent projection, the acromion, above the shoulder joint. Beneath the collar bone and just on the inside of theshoulder joint, is another bony projection of the shoulderblade, the coracoid process, which also serves for theattachment of muscles. The upper outer corner of the

shoulder blade ends in the glenoid cavity into which fitsthe head of the upper arm bone, forming a ball and socket

joint.



3.Collar Bones (Clavicles)

Each collar bone is rod-shaped and roughly S-shaped. It lies

horizontally andarticulates with the upperend of the breastbone,right in the middle andfront, just above the first

rib. The lateral endarticulates with theacromium.

The Pectoral

Girdle.



Collar bones serve as a

support for the shoulder

blades in front and keep

the shoulder blades back

so that the arms can hang

freely at the sides of thebody. They prevent the

pectoral girdles from

getting out of joint easily

and ample movement of the shoulders.



4.The Upper Limbs

The skeleton of theupper limbs or armmay be divided into

five main regions: anupper arm bone, theforearm (radius andulna), the wrist, the

palm of the hand andthe fingers.



5.The Pelvic (Hip) Girdle

The pelvic girdle consists of two large, sturdy hip bones.Each hip bone consists of three fused bones namely theilium, ischium and the pubis. The ilium is the largest of the three and forms the upper part of the hip bones. Thesacrum fits like a wedge posteriorly between the two hip

bones. The sacrum has a large, flat articular surface oneach side for articulation with the ilia. The ischium formsthe inferior part of the hip bone and the pubis the central infront. The two pubic bones are attached in the middle, onthe front side by a symphysis which consists of

fibrocartilage and ligaments, the pubic symphysis. Thetwo hip bones and the sacrum form a complete bony ring,the pelvis . On the outer side of the point where the fusedbones meet, there is a deep hip socket into which the head

of the femur fits.



The pelvic girdle forms astrong support for the

attachment of the limbs.Strong muscles of theback, the legs and thebuttocks are attached toit. It protects some of the

internal organs. In femalesit forms a strong basin-likestructure for supportingand protecting thedeveloping foetus duringchild-bearing. The

Pelvic

Girdle.



6.The Lower Limbs or Legs

The skeleton of the lower limb may be divided

into five main regions: the upper leg (thigh), the

lower leg, the ankle, the arch of the foot and thetoes.



The Upper Leg or Thigh

The upper leg has a single long bone, the femur and is the longest bone in the body. The head of the femur is turned slightly inwards and has alarge, rounded portion which articulates in theacetubulum, forming a ball-and-socket joint. Atits distal end, the femur widens to form two large

knobs (condyles) which form the hinged knee joint with the main long bone (tibia) of the lowerleg. On the anterior side of these two condyles,there is an articular surface against which the

kneecap (patella) slides. The patella is a small,triangular, flat bone which develops on the tendonof the thigh muscle and is attached by ligaments tothe tibia. This enables movement in the knee joint.



The Lower Leg

The two bones of the lower leg are the tibia (shinbone) infront and the fibula behind. The tibia is the larger of thetwo and extends from the knee to the ankle. The upper endof the tibia has two articulating facets into which thecondyles of the femur fit to form the knee joint.

The lower end of the tibia articulates with one of the

tarsals to form the ankle joint. The fibula is smaller thanthe tibia and is situated on the outside and slightly behindit. The upper end articulates with the tibia but does notform part of the knee joint. The lower end forms part of theankle joint.



The Ankle

There are seven short, thick tarsal bones, the largest of

which is the heel bone (calcaneum), which presses firmlyonto the ground when one stands, walks or runs. The calf muscles are attached to the calcenum, allowing the heel tobe lifted during locomotion.

The Arch of the Foot

The arch is formed partly by some of the tarsals but mainlyby the five long metatarsals, which extends from thetarsals to the toes. The arch is modified for receiving theweight of the body.

The Toes There are fourteen short phalanges in the toes of each

foot. The big toe has two phalanges and the other toes

have three in each.



7.Joints

There are three types of joints:immovable, partly movable,and synovial. Immovable joints,like those connecting the cranialbones, have edges that tightly

interlock. Partly movable jointsallow some degree of flexibilityand usually have cartilagebetween the bones; example:vertebrae. Synovial jointspermit the greatest degree of flexibility and have the ends of bones covered with aconnective tissue filled withsynovial fluid; example: hip.

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The outer surface of thesynovial joints contains

ligaments that strengthen jointsand hold bones in position. Theinner surface (the synovialmembrane) has cells producingsynovial fluid that lubricates the

joint and prevents the two

cartilage caps on the bonesfrom rubbing together. Some

joints also have tendons (connective tissue linkingmuscles to bones). Bursae aresmall sacs filled with synovial

fluid that reduce friction in the joint. The knee joint contains 13bursae

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III. Bone structure and

composition Although bones vary greatly in

size and shape, they have

certain structural similarities.

Bones have cells embedded in a

mineralized (calcium) matrix

and collagen fibers. Compact

bone forms the shafts of long

bones; it also occurs on the

outer side of the bone. Spongy

bone forms the inner layer.

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Compact bone has a series of Haversian canals around whichconcentric layers of bone cells (osteocytes) and minerals occur.New bone is formed by the osteocytes. The Haversian canals

form a network of blood vessels and nerves that nourish andmonitor the osteocytes.

Spongy bone occurs at the ends of long bones and is less densethan compact bone. The spongy bone of the femur, humerus,and sternum contains red marrow, in which stem cells reproduce

and form the cellular components of the blood and immunesystem. Yellow marrow, at the center of these bones, is used tostore fats. The outer layer of the bones is known as theperiosteum. The inner layer of the periosteum forms new boneor modifies existing bone to meet new conditions. It is rich innerve endings and blood and lymphatic vessels. When fractures

occur, the pain is carried to the brain by nerves running through

the periosteum.

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Bone Growth Endochondral ossification is the process of converting the

cartilage in embryonic skeletons into bone. Cartilage isdeposited early in development into shapes resembling thebones-to-be. Cells inside this cartilage grow and begindepositing minerals.

The spongy bone forms, and osteoblasts attach and lay down themineral portions of spongy bone. Osteoclasts remove materialfrom the center of the bone, forming the central cavity of thelong bones. The perichondrium, a connective tissue, formsaround the cartilage and begins forming compact bone while theabove changes are occurring. Blood vessels form and grow into

the perichondrium, transporting stem cells into the interior.

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Two bands of cartilage

remain as the bone develops,one at each end of the bone.

During childhood, this

cartilage allows for growth

and changes in the shape of

bones. Eventually the

elongation of the bones stops

and the cartilage is all

converted into bone.





Bones continue to change as adults, to adapt to the

stresses generated by physical activity. Exercisecan increase the diameter and strength of bone;

inactivity can decrease them. Age is a factor:

osteoporosis is a disease that primarily affects

older, postmenopausal women. Increasing calcium

intake, reducing protein intake, exercise and low

doses of estrogen are effective treatments for

osteoporosis.





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The Human En Do Skeleton