THE RESPIRATORY SYSTEM. 213 
each bronchus, are the special agents of song. They sound as 
does an oboe or any other reed instrument. Their action is 
modified by special muscles which act on adjacent parts. These 
are called evtrinsic muscles if they pass between the respiratory 
tube and parts external to it. They are called intrinsic muscles 
if they only pass from one part of the respiratory tube (trachea 
and bronchi) to another. Generally there are two pairs of 
“extrinsic ” muscles, passing from the clavicles, or the sternum, 
Fig. 164. 


Muscues or Syrinx or RAVEN. 
A. Front view of syrinx. B. Side view. 
a & f, Inferior bifurcations of the lateral tracheal muscle (tracheo-lateralis) 
going to the third bronchial half-ring; }, shorter muscle (broncho- 
trachealis brevis) inserted into posterior end of second bronchial half- 
ring; ¢, posterior common laryngeal muscle—bronchialis posticus, going 
from last tracheal ring to hinder end of second bronchial half-ring ; 
e, anterior ditto—bronchialis anticus, from last tracheal ring to fore 
ends of first and second bronchial half-rings ; d, relaxor of tympani- 
form membrane (sterno-trachealis) going from the side of the trachea 
to the sternum ; g, membranous inner walls of the bronchi. 
to the trachea. In addition to these there is commonly one 
pair or there may be five or six pairs of ‘intrinsic ” muscles, 
passing down from the trachea to the bronchial rmgs. These 
