THE RESPIRATORY SYSTEM
The thoracic cavity of mammals is lined with a thin membrane, the pleura, in the form of two pleural sacs, one on either side of the median plane. The costal pleura covers the body wall, the diaphragmatic pleura the anterior surface of the diaphragm, while the pulmonary pleura is reflected over the outer surfaces of the lungs. The two pleural sacs meet in the median plane to form the mediastinal septum. The smooth surfaces of the pleura reduce the friction between the organs of the thorax.
The heart has already been removed. Clear away the muscle on the ventral side of the neck, exposing the trachea (or windpipe) and esophagus. The larynx, to be described later, is at the anterior end of the trachea. The thyroid gland is an H-shaped gland of internal secretion lying on the trachea just back of the larynx. The isthmus, which corresponds to the transverse bar of the H, runs across the ventral surface of the esophagus, joining the two lateral lobes of the gland. Throughout its length the trachea is in contact with the esophagus. The latter is dorsal to the larynx, but gradually swerves to the left until it passes into the thorax on the left side of the trachea. The heart and the vessels connected with it are ventral to both esophagus and trachea. The trachea bifurcates posteriorly, forming the two primary bronchi. Each bronchus leads to a lung. Observe the cartilaginous rings imbedded in the walls of the trachea and bronchi. They keep the lumen open for the free passage of air. Explain why the dorsal wall of the trachea is more flexible than the ventral.
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