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 medias- 

 tinal 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 secre- 

 tion 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, join- 

 ing 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 bifur- 

 cates 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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