RESPIRATORY ORGANS. 



255 



membranes which vibrate by the passage of air, as do the vocal 

 cords of mammals. Most common is the broncho-tracheal syrinx, 

 in which the last rings of the trachea are united to form a reso- 

 nating chamber, the tympanum, while folds of membrane, internal 

 and external tympanic membranes (not to be confused with the simi- 

 larly named structure in the ear, p. 187), extend into the cavity from the 

 median and lateral wall of each bronchus. In some cases there is also 

 an internal skeletal element (pessulus) which bears a semilunar mem- 

 brane on its lower surface. In many birds this type of syrinx is 

 often asymmetrical (fig. 263) and is ex- 

 panded into a (usually) bony resonat- 

 ing vesicle. In the tracheal type of 

 syrinx the lateral port ons of the last 

 tracheal rings disappear and the mem- 

 brane which closes the gap forms the 

 vibratile part. In the bronchial syrinx 

 the membranes occur between two suc- 

 cessive rings of each bronchus, each ring 

 being concave toward its fellow. By a 

 shortening of the bronchial wall these 

 membranes are forced as folds into the 

 tube. In all types of syrinx there are 

 muscles attached to trachea and bronchi , 

 which, by moving these parts, alter 

 the tension of the folds, thus changing 

 the note. 



In the mammals the trachea is elon- 

 gate (shortest in the whales and sire- FIG 263. Syrinx of canvas-back 



duck, Aythya, laid open (Princeton 



nians, dividing in the latter immedia- 915). b, bronchi; p, pessulus; t, tra- 

 tely behind the cricoid into the two chea; ty ' ty m P anum - 

 bronchi), and the cartilage rings are usually incomplete dorsally, 

 the gaps being closed by membrane. This structure allows the tube to 

 remain open under ordinary conditions and yet allows it to give when 

 food is passing down the oesophagus, just dorsal to it. In the cetacea 

 and sirenia the tracheal cartilages are sometimes spirally arranged. 



Lungs. 



The morphology of the lungs may be understood by following their 

 development in the mammals and then describing their modifications 



