ICSE Class 9 Biology Chapter 12 Skeleton Movement and Locomotion

Skeleton - Movement And Locomotion

Syllabus - Skeleton: Movement And Locomotion

Functions of human skeleton

Axial and Appendicular Skeleton

Types of joints with reference to their location - immovable joints, slightly movable joints and freely movable (hinge joint, ball and socket joint, gliding joint, pivot joint.)

Most animals are characterized by a definite shape and the capacity for movement of the different parts of the body. These movements also help the animal in locomotion, i.e. movement from place to place. These functions are performed by the skeleton and the muscles together.

12.1 Functions Of Human Skeleton

The skeleton in our body serves six main purposes:

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. Imagine if you did not have small pieces of bones (phalanges) in your fingers, would you be able to hold a pen in your hand?

An example of shape and support. Just hold your ear, try to fold it and then leave it free. It resumes its shape. Now feel its interior, it is tough but flexible. This is due to the cartilage skeleton inside it, which is giving it support as well as the shape.

2. Protection. Several delicate and important organs are well protected by a casing of bones. For example, the skull protects the brain, the vertebral column (backbone) protects the spinal cord, the ribs protect the heart and lungs, and so on.

3. Movement. Many bones are joined to each other in a manner that one bone can be moved on another. These movements are brought about by muscles which originate on one bone and are inserted on another.

4. Leverage. Some bones and joints form levers that increase the speed and distance of movement by a muscle. For example, contraction of only a few centimetres by the biceps muscle swings the hand swiftly through an arc of more than a foot (Fig.12.14).

5. Formation of blood cells. Certain types of blood cells, including red and white blood cells are formed in the tissue of the central hollow space or the 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.

Cartilages are the supporting and connecting structures. For example, the cartilage supporting the projecting external ears and the tip of the nose.

Ligaments bind the bones together.

12.2 Bone

Bone is the chief component of our skeleton. It consists of organic and inorganic material. The inorganic part constitutes nearly two-thirds of the entire bone substance, and includes mainly the compounds of calcium and phosphorus. If a bone is placed in weak hydrochloric acid, the mineral part is removed from it (gets dissolved) and the remaining organic framework is left behind. Such a bone is called decalcified (calcium removed) bone and it is soft and flexible which can even be tied into a knot.

Classification Of Bone On The Basis Of Shape

Shapewise bones are classified into: long, short, flat and irregular bones, as given in the above scheme.

Long bones - Consist of a shaft with a knob at each end. Shaft is thick - walled, tubular, lined with yellow marrow. The ends are spongy e.g., bones of arms, legs, and chest (ribs)

Short bones - Box like, spongy bone, show little movement, e.g. ankle, wrist.

Flat bones - Composed of two or more parallel plates of compact bone enclosing spongy bone e.g. shoulder blade, skull, sternum

Irregular bones - Complex shape, vary in amount of bone tissue, compact and spongy e.g. facial bones vertebral bones.

On the other hand, if a bone is strongly heated its organic matter is destroyed (oxidised) and only the mineral part (or ash) will remain. Such a bone turns brittle and quickly breaks. With age, as in old people, the organic part of the bone is reduced and the bones become more fragile taking much time in rejoining after a fracture.

Progress Check

Fill in the blanks:

(i) Our skeleton consists of bones, _____, and _____.

(ii) _____ is the chief component of our skeleton.

(iii) Shapewise the bones can be classified as _____, _____ _____ and _____.

(iv) The central hollow part of the long bones is known as _____.

(v) A bone becomes soft and flexible when placed in dilute _____.

Teacher's Note

The skeleton gives shape to our body and protects vital organs. Think about how bones support you when you carry your school bag or play sports.

12.3 The Human Skeleton (Fig. 12.2)

The human skeleton altogether consists of 206 bones including three very tiny bones in each ear. Table 14.1 (p. 126) gives a detailed regionwise break-up of the total number of bones in the human skeleton. The skeleton has two main divisions: the axial skeleton which includes the basic central framework of the body and the appendicular skeleton which includes the bones of the limbs and girdles.

12.3.1 Axial Skeleton

The axial skeleton consists of the skull, the vertebral column, the ribs and the sternum.

A. SKULL is the skeleton of the head. It contains two parts. The upper top part the cranium (or the brain-box) is made of eight bones which are so joined to each other that they are permanently fixed. The other part of the skull forms the face which contains a total of fourteen bones. The upper and lower jaws are also formed by some of these bones. The back part of the cranium contains a large hole, the foramen magnum, through which the spinal cord after emerging from the brain continues behind into the backbone.

B. VERTEBRAL COLUMN (Fig. 12.3A). The vertebral or spinal column is popularly called the backbone. It is composed of a total of 33 ring-like bones called vertebrae. These are divided into five

Teacher's Note

Your backbone allows you to bend, twist, and maintain posture throughout the day. Without it, everyday activities like sitting and reaching would be impossible.

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