Strictly speaking, there is only one ocean, the great body of salt water, which altogether covers about 72 % of the Earth's surface and surrounds the planet's great landmasses. But in more familiar terms, the one Great Ocean is divided into four principal parts, each of them known as an ocean. The Pacific covers about
18,13,00,000 sq. km. and is by far the
largest ocean, containing about 46% of the Earth's water.

The Atlantic is the second largest ocean,
containing about 23% of the world's
water. It is much narrower and only half
the size of the Pacific, covering about
8,22,17,000 sq. km. The Indian is the third-
largest ocean. Slightly smaller than the Atlantic, it covers about 7,34,26,500 sq. km. and holds 20% of the
world's water. The Arctic, laying within the Arctic Circle and
surrounding the North Pole, is the smallest ocean, with an area of
about 1,39,86,000 sq. km. containing 4% of the world's water. The
Great Southern or Antarctic Ocean, which circles Antarctica, is not
officially an ocean, but an extension of the southern portions of the
Pacific, Atlantic and Indian Oceans.

                     It is the recording of electrical activity of the heart. Electrocardiograph is theinstrument which is used to record the electrical current generated in the heart.By means of this instrument, the electrical current generated in the heart is conducted to remote parts of the body. The heart current can be recorded by connecting any two parts of the body with this instrument. The connections are called as leads.

Lead I : Right arm and left arm.
Lead Il: Right arm and left leg.
Lead IlI: Left arm and left leg

            Connections over the chest with indifferent electrodes are also sometimes used. The ECG recordings are disigned by letters P, Q, R, S, and T.

The wave P is caused by contraction of atria.The wave Q, R and S are produced by he contraction of ventricles.The wave T is produced by the relaxation of ventricles

  During disease of the heart, these waves are abnormal in shape and position.