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- ae oe ee 
nians, dividing in the latter immedia- 915). 8, bronchi; p, pessulus; ¢, tra- 
tely behind the cricoid into the two ‘°/% tympanum. 
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 cesophagus, 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 
