THE SYRINX OF GALLUS DOMESTICUS 167 



OBSERVATIONS 



In this investigation Galliis domesticus was used for two 

 reasons. First, adults are accessible at all seasons of the year. 

 Second, developmental stages are easily obtained and controlled. 



I. ANATOMY OF THE ADULT SYRINX 



1 . Gross anatomy 



Birds differ from mammals in possessing two larynges, an 

 upper and a lower. The latter, known also as the syrinx, was 

 described by Johannes Mliller as occurring in three different 

 positions. When the organ is located at the lower end of the 

 trachea but above the bifurcation, he designated it 'syrinx 

 trachealis.' If found in the bronchi below the bifurcation, 

 'syrinx bronchiahs.' But if the lower end of the trachea and 

 the upper portions of the bronchi both took part in its formation 

 he called it 'syrinx tracheo-bronchialis' (fig. 2). The syrinx of 

 the domestic chicken belongs to the last type. 



In the domestic chicken, the upper larynx (fig. 1) is located 

 behind the tongue and on the floor of the pharynx. It is rudi- 

 mentary when compared with that of other animals and is ap- 

 parently incapable of producing voice. From the upper larynx 

 the trachea courses caudad, ventral to the oesophagus, passes 

 to the left of the ingluvies and pierces the pleura between the 

 limbs of the furcula to enter the so-called thoracic cavity. Shortly 

 after entering this cavity, the trachea bifurcates into the two 

 primary bronchi. In the region of the bifurcation a great modi- 

 fication of the cartilaginous and intercartilaginous elements 

 takes place. Here (fig. 2) a well developed bone, known as the 

 cross-piece, or pessulus, lies transversely in the dorso-ventral 

 plane between the two bronchi. Its dorsal and ventral extremi- 

 ties are greatly enlarged. The caudal end of the trachea is 

 compressed. It is composed of rings which are enlarged and 

 closely related to each other. The four most caudal rings are 

 imperfectly fused. Immediately caudad to this fusion are four 



