Download Skeletal System Std. VIII The skeleton in our body serves following

Skeletal System
Std. VIII
The skeleton in our body serves following functions :
1. Support and shape : The skeleton provides a support or framework to all the soft
parts and gives the body and its parts a definite shape.
2. Protection : Several delicate and important organs are well protected by a casing of
bones.
3. Movement : Many bones are joined to each other in a manner that one bone can be
moved on another.
4. Leverage : Some bones and joints form levers that increase the speed and distance of
movement by a muscle.
5. Formation of blood cells :Formation of red and white blood cells occur in the bone
marrow of some of the long bones.
6. The bones are a storehouse of calcium and phosphorus for the rest of the body.
Constituents of skeleton :
Our skeleton consists of bones, cartilages and ligaments.
Bones comprise the hard framework of the body.
Cartilage is the supporting and connecting structure.
Ligaments bind the bones together.
Bone : Bone is the chief component of our skeleton. It consists of mainly the compounds of
calcium and phosphorus.
Structure of a typical large bone :
It is a highly calcified, hard and rigid connective tissue. It is very strong and can withstand
severe stresses. It consists of bone cells arranged in the form of concentric rings embedded in
round matrix in which collagen fibres and mineral salts (calcium and phosphorus) are
deposited.
.
A long bone has a hollow cavity in the middle which is filled with bone marrow. Bone
marrow is of two types.
i)
ii)
Yellow marrow, which give rise to white blood cells
Red marrow, which produces red blood cells
Human Skeleton : Human skeleton consists of 206 bones including three very tiny bones in
each ear.
The skeleton has two main divisions :
a) The axial skeleton which includes the basic central framework of the body.
b) The appendicular skeleton which includes the bones of the limbs and girdles.
A) Axial Skeleton : The axial skeleton consists of the skull, the vertebral column, the ribs
and the sternum.
Axial
Skelton
iii) Skull
Sternum
mmm
Rib Cage
Vertebral
Column
a) Skull:
i) Skull forms the protective bone of the head.
ii) It consists of two parts. Upper top part, the cranium(or brain box) is made up of eight
bones which are fixed to each other. The other part of the skull forms the face which contains
a total of fourteen bones.
b) Vertebral Column: Vertebral column is dorsally placed. It extends from the base of
the skull and constitutes the main framework of the trunk. Our vertebral column is
formed by 26 serially arranged ring-like bones called vertebrae. Each vertebra has a
central hollow portion (neural canal) through which the spinal cord passes.
The vertebral column is differentiated into following regions starting from the skull:
1. cervical (7) (neck region),
2. thoracic (12) (chest),
3. lumbar (5) (Middle back),
4. sacrum (1-fused) (lower back)and
5. coccyx (1-fused) (tail)
The number of cervical vertebrae are seven in almost all mammals including human
beings. The vertebral column protects the spinal cord, supports the head and serves as
the point of attachment for the ribs and musculature of the back.
Rib Cage: There are 12 pairs of ribs. Each rib is a thin flat bone connected dorsally to
the vertebral column and ventrally to the sternum. Sternum or a breast bone is a flat
bone on the ventral midline of thorax.
First seven pairs of ribs are called true ribs. Dorsally, they are attached to the thoracic
vertebrae dorsally and ventrally connected to the sternum. The 8th, 9th and 10th pairs of ribs
are not directlyattached to the sternum but join the seventh rib with the help of a cartilage.
These are called false ribs. Last 2 pairs (11th and 12th) of ribs are not connected ventrally to
the sternum and are therefore, called floating ribs. Thoracic vertebrae, ribs and sternum
together form the rib cage.
Appendicular Skeleton: The bones of the limbs alongwith their girdles constitute the
appendicular skeleton. Each limb is made of 30 bones.
• Forelimb :The bones of the hand (fore limb) are humerus in the upper arm, radius and ulna
in the lower arm, carpals (wrist bones – 8 in number), metacarpals (palm bones – 5 in
number) and phalanges (digits – 14 in number).
• Hind-limb :Hind limb consists of Femur (thigh bone – the longest bone), tibia and fibula
in the shank, tarsals (ankle bones – 7 in number), metatarsals (5 in number) and phalanges
(digits – 14 in number) are the bones of the legs (hind limb). A cup shaped bone called
patella cover the knee ventrally (knee cap).
Girdles: Pectoral and Pelvic girdle bones help in the articulation of the upper and the lower
limbs respectively with the axial skeleton.
Pectoral girdle
Each girdle is formed of two halves. Each half of pectoral girdle (shoulder girdle)consists of
a clavicle (collar bone) and a scapula. Scapula is a large triangular flat bone which lies on the
upper ribs on the back of the thorax. Its outer apex bears a large somewhat cup-shaped
glenoid cavity into which fits the round upper head of the humerus, and closed to this joint
the shoulder-blade has a small raised part to which a long and curved collar-bone (Clavicle)
is attached. The other end of the collar bone is joined to the upper part of the breast bone.
Pelvic girdle(Hip girdle)is a large trough-shaped part formed by two hip bones that are
joined medially to the sacrum. On each side, it bears a large cup-shaped articular
cavity(acetabulam) into which fits the large round head of the thigh bone. The hip girdle not
only gives the support to the skeleton of the hind limbs but also protects and supports the
abdominal organs.
JOINTS
Joints are essential for all types of movements involving the bony parts of the body.
The point at which two separate bones meet is called a Joint.
There are three major categories of joints in our body :- Immovable, partially movable and
freely movable.
1. Immovable Joints: In this type of joint no movement is possible between the two bones.
Ex. Skull bones
2. Partially Movable Joints: In this type of joint only very little movement occurs between
the two bones. Ex. The joint between a rib and sternum, joints between vertebrae
3. Freely Movable Joints: In this type of joint, varying degrees of movement are possible
between the two bones forming the joint.
Joint
Immovable
Freely movable
Partially movable
Joint
Joint
Gliding Joint
Joint
Pivot Joint
Hinge Joint
Ball and Socket
Joint
Freely Movable joints are categorised into four major types:
Hinge Joint
A movable joint that allows movement in
one direction only.
Ball and Socket A joint in which the round end of a bone fits
into the cavity of another bone.
Joint
Elbow Joint, Knee Joint
and Finger Joint
Shoulder and hip Joint
Pivot Joint
A joint in which movement is limited to
rotation.
The joint where neck
meets the skull
Gliding Joint
A freely moving joint in which the
movement is limited to gliding motions.
Ankle and Wrist Joint
Freely movable joints are also called as synovial joints as they have lubricating fluid known
as synovial fluid which avoids friction.
Muscles :
Functions:
The muscles in the body provide the means of all movements.
They cover the skeletal framework and also give shape to the body.
Muscles help to maintain body posture while sitting, standing or walking.
Each muscle usually has two ends – a fixed end where the muscle originates and a movable
end which pulls some other part. This movable end is drawn out to form a tough structure the
tendon which is attached to the bone. Muscles can only contract and relax, they cannot
lengthen.
Antagonistic muscles:A structure which has been moved by a muscle cannot return to its
original position without the action of another muscle. Such muscles causing opposing
movements are called antagonistic muscles.
Ex. The biceps muscle of the upper arm bends the lower arm over the upper arm.
Straightening of the lower arm is brought about by the triceps muscle. Therefore, these two
muscles are antagonistic.
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