Explain about the HEART & its position & structure of heart.

Introduction:-
The heart is roughly cone – shaped hollow muscular organ. It is about 10 cm long & is about the size of the owner’s fist. Its weighs about 225g in women and is heavier in men about 310 g.
Position:- The heart lies in the thoracic cavity in the mediastinum . It lies obliquely, a little more to the left than the right and presents a base above, and an apex below. The apex is about 9 cm to the left of the midline at the level of the 5th intercostal space, i.e a little below the nipple and slightly nearer the midline. The base extends to the level of the 2ndrib.
Organs associated with the heart:-
(1) Inferiorly –the apex rests on the central tendon of the diaphragm
(2) Superiorly –the great blood vessels , i.e the aorta, superior vena cava , pulmonary artery and pulmonary veins
(3) Posteriorly-the oesophagus , trachea , left and right bronchus, descending aorta, inferior vena cava and thoracic vertebrae
(4) Laterally –the lungs
(5) Anteriorly:- the sternum , ribs and intercostal muscles .
Structure:- The heart is composed of three layers of tissue-
–Pericardium
— Myocardium
— Endocardium
(1) Pericardium:- It is outer layer of heart . It is made up of two sacs. The outer sac consists of fibrous tissue and the inner of a continuous double layer of serous membrane.
The two layers of pericardium are-
– Outer layer (parietal pericardium)
– Inner layer (visceral pericardium)
(2) Myocardium:- The myocardium is composed of specialised cardiac muscle found only in the heart. It is not under voluntary control but, like skeletal muscle, cross –stripes are seen on microscopic examination.
(3) Endocardium:- It is inner most layer of heart. This lines the chambers and valves of the heart. It is a thin, smooth, glistening membrane that permits smooth flow of blood inside the heart. It consists of flattened epithelial cells, and it is continuous with the endothelium lining the blood vessels.