EESPIRATOEY SYSTEM 407 



2. The lungs are a pair of spongy bodies, attached to the 



dorsal and dorso-lateral walls of the anterior part of 

 the thorax. They are covered ventrally by the pleura, 

 an anterior continuation of the peritoneum. Their 

 inner edges lie close together, separated from each 

 other in the median plane by the projecting centra 

 of the thoracic vertebrae, and by the dorsal aorta. 



Remove the pleura from the ventral surface of one lung ; 

 and separate the lung from the thoracic wall, noting its close 

 attachment to this wall, and the indentations of its surface 

 caused by the ribs. 



3. The syrinx, or 'lower larynx,' is the organ of voice 



of the pigeon, and is formed by the dilated hinder 

 end of the trachea, and the commencement of the 

 bronchi. 



Cut across the trachea about an inch in front of its bifur- 

 cation. Make two lateral incisions, with scissors, along the 

 sides of the trachea and bronchi, and turn down the ventral 

 wall. 



i. The tympanum is the cavity of the lower end 

 of the trachea and commencement of the 

 bronchi. 



ii. The membrana tympaniformis interna is the 

 mucous membrane lining the inner wall of the 

 commencement of each bronchus. 



iii. The membrana semilunaris is a delicate vertical 

 fold of mucous membrane, extending forwards 

 into the tympanum from the angle of bifurca- 

 tion of the trachea : by its vibration the voice 

 is produced. 



iv. The pessulus is a slender bar of cartilage 

 running across the tympanum, in the sub- 

 stance of the membrana semilunaris, from 

 the dorsal to the ventral surface. 



