THE RESPIRATORY ORGANS 59 



from the clavicle or the syrinx and run along 

 the entire length of the trachea. The sterno- 

 tracheal muscles are shorter and more easily 

 recognised as they pass from the sternum to 

 the trachea. The trachea ends by dividing 

 into the right and left bronchi ; each bronchus 

 entering the ventral surface of a lung. 



The interior of the trachea is lined by a 

 delicate mucous membrane, the surface of 

 which is covered by ciliated, columnar epithelial 

 cells (Fig. 30). Small saccular glands, lined by 

 a clear epithelium, occur at frequent intervals. 

 The cartilage of which the tracheal ring is com- 

 posed is of the hyaline variety. 



The syrinx, or broncho-tracheal larynx, is 

 indicated by a lateral compression where the 

 trachea divides into the two bronchi. Between 

 the two bronchial openings is a ridge — gener- 

 ally ossified — supporting a crescentic or semi- 

 lunar membrane. On each side of this is an 

 elastic membrane (internal tympaniforrn niem- 

 brane) projecting into the entrance to the 

 bronchus, opposite a similar membrane {external 

 tympaniforrn membrane) attached to the outer 

 wall of the tube. The presence of the two mem- 

 branes converts the entrance to the bronchus 

 into a slit ; the membranes themselves being 



