The heart is a hollow muscular organ, situated behind the sternum and costal cartilages, and between the lungs, two-thirds being on the left of the middle line. It is cone-shaped ; the base is upwards and backwards, a little below the level of the second costal cartilage ; the apex is directed downwards, forwards, and to the left between the fifth and sixth ribs, 1.5

inches below the nipple. Like the lungs, the heart is enclosed in a double serous membrane, called the pericardium, one layer of which, the visceral portion, is closely attached to the heart, and covers the great bloodvessels for about 1.5 inches. The parietal portion is continuous with and reflected over the visceral portion, so that it envelops it and the heart. It secretes a small quantity of serous fluid ; the internal surfaces are smooth and polished, and continually glide on each other as the heart contracts.

The interior of the heart is lined by a delicate membrane, called the endocardium.

Relations.—In front the heart is partly covered by the lungs, more so during inspiration; behind, the bronchi, oesophagus, and descending aorta ; on either side the pleura and the phrenic nerves and vessels.

(For cavities, etc., see Chapter IV.)

The œsophagus passes down the neck, close behind the trachea, into the hinder part of the thorax; it connects the pharynx with the stomach, into which it opens on passing through the diaphragm ; it extends from the sixth cervical to< the tenth or eleventh dorsal vertebra. It corresponds to the cervical and dorsal curves of the spine. It has three coats— an external muscular, an internal mucous, with an areolar coat between. The surface of the mucous coat has numerous papillae, and is covered by epithelium.

Relations.-—In front, trachea, arch of aorta, left carotid and left subclavian arteries, left bronchus, and the posterior surface of the pericardium ; behind, the vertebral column ; on the sides it is covered by the pleurae ; the vena azygos major lies on the right and the descending aorta on the left. The right pneumo-gastric nerve passes down behind and the left pneumo-gastric in front.

The thoracic duct commences in the abdomen, on a level with the second lumbar vertebra, in a dilatation called the receptaculum chyli. It enters the thorax on the right side of the aorta, through the same opening in the diaphragm ; it passes up in front of the vertebral column, and out through the thoracic inlet ; it inclines to the left, and ascends to the seventh cervical vertebra, when it arches outwards and downwards, and joins the internal jugular vein at its junction with the subclavian vein.