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4. The Thoracic Cavity
The heart and lungs are situated in the thorax, the walls of which afford them protection. The heart lies between the two lungs, and is enclosed within a fibrous bag, the pericardium, while each lung is invested by a serous membrane, the pleura. The skeleton of the thorax, and the shape and boundaries of the cavity, have already been described (page 117). The Cavity of the Thorax.—The capacity of the cavity of the thorax does not correspond with its apparent size externally, because (1) the space enclosed by the lower ribs is occupied by some of the abdominal viscera; and (2) the cavity extends above the anterior parts of the first ribs into the neck. The size of the thoracic cavity is constantly varying during life with the movements of the ribs and diaphragm, and with the degree of distention of the abdominal viscera. From the collapsed state of the lungs as seen when the thorax is opened in the dead body, it would appear as if the viscera only partly filled the cavity, but during life there is no vacant space, that which is seen after death being filled up by the expanded lungs. The Upper Opening of the Thorax.—The parts which pass through the upper opening of the thorax are, from before backward, in or near the middle line, the Sternohyoideus and Sternothyreoideus muscles, the remains of the thymus, the inferior thyroid veins, the trachea, esophagus, thoracic duct, and the Longus colli muscles; at the sides, the innominate artery, the left common carotid, left subclavian and internal mammary arteries and the costocervical trunks, the innominate veins, the vagus, cardiac, phrenic, and sympathetic nerves, the greater parts of the anterior divisions of the first thoracic nerves, and the recurrent nerve of the left side. The apex of each lung, covered by the pleura, also projects through this aperture, a little above the level of the sternal end of the first rib. The Lower Opening of the Thorax.—The lower opening of the thorax is wider transversely than from before backward. It slopes obliquely downward and backward, so that the thoracic cavity is much deeper behind than in front. The diaphragm (see page 404) closes the opening and forms the floor of the thorax. The floor is flatter at the center than at the sides, and higher on the right side than on the left; in the dead body the right side reaches the level of the upper border of the fifth costal cartilage, while the left extends only to the corresponding part of the sixth costal cartilage. From the highest point on each side the floor slopes suddenly downward to the costal and vertebral attachments of the diaphragm; this slope is more marked behind than in front, so that only a narrow space is left between the diaphragm and the posterior wall of the thorax.
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