110. The air-vessel of the lungs, termed the bronchus, together with the blood-vessels and nerves, enter the organ at its flattened side, not exactly in the middle, but rather towards the upper and back part. This portion is termed the root of the lung. 111. The lungs are attached to the neck by the trachea (fig. LX. 4), the continuation of which forms the bronchus; to the spinal column by the pleura, and to the heart by the pulmonary vessels (fig. LX. 3, _d_): their remaining portion is free and unattached. 112. In the living body, the lungs on each side completely fill the cavity of the chest, following passively the movements of its walls, and accurately adapting themselves to its size, whether its capacity enlarge in inspiration, or diminish in expiration, so that the external surface of the lung (the pulmonary pleura) is always in immediate contact with the lining membrane of the walls of the cavity (the costal pleura); consequently, during life, there is no cavity, the chest being always completely full. [Illustration: Fig. LX. _a._ The cut edges of the ribs, forming the lateral boundaries of the cavity of the thorax. _b._ The diaphragm, forming the inferior boundary of the thorax, and the division between the thorax and the abdomen. _c._ The cut edges of the abdominal muscles, turned aside, exposing the general cavity of the abdomen. 1. The cut edge of the pericardium turned aside. 2. The heart. 3. The great vessels in immediate connexion with the heart. 4. The trachea, or wind-pipe. 5. The lungs. 6. The liver. 7. The stomach. 8. The large intestine. 9. The small intestines. 10. The urinary bladder.] 113. The anterior surface of the pericardium, the bag which envelopes the heart, lies immediately behind the sternum, and the cartilages of the second, third, fourth, and fifth ribs, covered at its sides by the pleura, and firmly attached below to the diaphragm (fig. LX. 1). 114. Surrounded by its pericardium, within the mediastinum, the heart is placed nearly in the centre of the chest, but its direction is somewhat oblique, its apex being directly opposite to the interval between the fifth and sixth ribs on the left side (fig. LX. 2); while its basis is directed upwards, backwards, and towards the right (fig. LX. 2). That portion of its surface which is presented to view on opening the pericardium is convex (fig. LX. 2); but its opposite surface, namely, that which rests upon the part of the pericardium which is attached to the diaphragm, is flattened (fig. LX. 1). It is fixed in its situation partly by the pericardium and partly by the great vessels that go to and from it. But under the different states of expiration and inspiration, it accompanies, in some degree, the movements of the diaphragm; and in the varied postures of the body, the heart deviates to a certain extent from the exact position here described.