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) Yellow marrow, which give rise to white blood cells ii) 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 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 s 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. There are three major categories of joints in our body :- Immovable, partially movable and freely movable. 1. Immovable s: In this type of joint no movement is possible between the two bones. Ex. Skull bones 2. Partially Movable s: 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 s: In this type of joint, varying degrees of movement are possible between the two bones forming the joint. Immovable Partially movable Freely movable Gliding Pivot Hinge Ball and Socket
Freely Movable joints are categorised into four major types: Hinge Ball and Socket Pivot Gliding A movable joint that allows movement in one direction only. A joint in which the round end of a bone fits into the cavity of another bone. A joint in which movement is limited to rotation. A freely moving joint in which the movement is limited to gliding motions. Elbow, Knee and Finger Shoulder and hip The joint where neck meets the skull Ankle and Wrist 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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