THE PIGEON 179 



follow the probe with the scissors. Cut open the aorta and note 

 the three pocketlike semilunar valves which extend across its base. 

 Make a cross section of the heart in the middle of the two 

 ventricles and note their shape and the relative thickness of 

 their walls. 



Exercise 15. Draw a diagram showing the structure of the heart. 

 Exercise 16. Draw a diagram of the entire vascular system. 



The Respiratory System. This consists of the two lungs and 

 the air sacs in connection with them, and of the two bronchi and the 

 trachea, by which the lungs are placed in communication with 

 the outside world. 



The lungs are spongy bodies lying close against the dorsal body 

 wall and are covered ventrally by the pleura, which is a tough, 

 thickened portion of the peritoneum. The lungs are relatively 

 small in size and are held firmly in place between the pleura and 

 the dorsal body wall, so that they are capable of very little move- 

 ment. They are not saclike, as in the lower land vertebrates, 

 but spongy as in mammals, their interior being entirely filled 

 with air cells. They are joined with the heart by the pulmonary 

 veins and arteries,— the latter carrying venous blood away from 

 the heart to them, and the former carrying arterial blood from 

 them back to the heart. 



Entirely free the ventral surface of the lungs from the blood 

 vessels and other structures which may still be attached to them, 

 but do not injure the bronchi ; dissect the pleura from both lungs, 

 and the lungs away from the body wall. Note their intimate rela- 

 tion to this wall. Look for the openings into the air sacs. 



Separate the oesophagus and crop from their attachments with 

 the trachea and bronchi and turn them forward. Note the cartilag- 

 inous rings in the trachea and bronchi. 



Exercise 17. Draw a sketch of the lungs, with the trachea and bronchi. 



The bird has two larynges ; that is, two regions in its respiratory 

 system where the passage of air into and out of the lungs is con- 

 trolled by a special mechanism. One of these is at the anterior end 

 of the trachea and is homologous to the larynx of other verte- 



