180 VERTEBRATE ZOOLOGY 



brates ; the other is at its posterior end and is called the syrinx. 

 It is this latter organ which produces the voice of the bird. 



Study the structure of the syrinx. It consists of a chamber 

 called the tympanum, which is formed at the hinder end of the 

 trachea by the coalescence of the posterior tracheal and the an- 

 terior bronchial rings ; in this chamber is a vibrating membrane 

 called the semilunar membrane, which produces the voice. On 

 each lateral side of the trachea is the slender broncho-tracheal 

 muscle. 



Exercise 18. Draw the ventral aspect of the hinder end of the trachea 

 and the forward ends of the bronchi, showing accurately the shape 

 of the rings, on a scale of 3. 



The semilunar membrane is a delicate median, vertical projec- 

 tion extending into the tympanum from the meeting-point of the 

 medial walls of the bronchi. It is stiffened by a slender cartilage 

 called the pessulus, which lies in a dorsoventral position. 



On the trachea and the bronchi are a number of small muscles 

 that play a part in the production of the voice. These fall into 

 two groups : those which join the trachea with neighboring bones 

 and those which are confined to the trachea and bronchi. These 

 muscles vary much in different birds. In the pigeon the largest are 

 the tracheo-sternal muscles (a V-shaped pair which join the tra- 

 chea with the sternum and were seen when the great arteries issuing 

 from the heart were studied) and the broncho-tracheal muscles, 

 the lateral pair which were seen on the sides of the trachea. 



Open the tympanum by a lateral slit in the trachea and bronchus 

 and note the semilunar membrane. 



Exercise 19. Draw a semidiagrammatic sketch of the tympanum, 

 showing the semilunar membrane. 



Study the structure of the larynx. Dissect the anterior end of 

 the trachea, with the larynx and tongue, from the body. The 

 larynx is at the anterior end of the trachea ; its opening into the 

 pharynx is the glottis. Note the two long horns of the hyoid 

 bone which lie on either side of the glottis. The opening of the 

 glottis is bounded and supported on either side by the paired and 



