THE MUSCLES OF THE AIR PASSAGES. 51 



depends, must also be l)rought to different degrees of 

 tension in its different parts. Striking from the list the 

 sterno-tracheales, all the muscles we have enumerated 

 for the true larynx are tensors, as it is evident their 

 contraction must stretch the tympanic membrane l:)y 

 lengthening the bronclii to which, as we have just said, 

 it is attached. On the other hand, if the sterno- 

 tracheales contract, they must stretch the windpipe, and 

 thus shorten the bronchi, and relax the tympanic mem- 

 brane, A glance at Fig. 13, where g is the tympanic 

 membrane, will at once make these points clear. 



Professor Owen found that " in many of the Volitores 

 there is a single pair of ' broncho-tracheales,' and a 

 single pair of short ventrieose ' bronchiales.' In Thamno- 

 pliilus each sterno-trachealis bifurcates to send a small 

 strip to the lower larynx, and the rest to the side of the 

 trachea, as nsual. In Furnai^ia the sterno-trachealis is 

 inserted into the upper end of a long appendage to the 

 upper bronchial half-ring. 



" The Parrot tribe have a single glottis bounded by 

 a lateral pair of vibratile membranes ; each membrane 

 connecting together, and occupying the interspace 

 between, the last tracheal and first bronchial rings. 

 These have each one margin concave, with the concavity 

 towards each other, and are movably joined together at 

 their fore and hind extremities. These half- rings expand, 

 and stand out from the end of the trachea. A narrow 

 muscle, 'tensor longus glottidis,' Fig. 105 [14], a, passes 

 from the side of the trachea to the upper (tracheal) 

 half-ring ; and, by raising it, makes tense the elliptical 

 elastic membrane : a broader ' tensor brevis glottidis,' ih., 

 b, passes from the lower rings of the trachea to the same 

 half-ring, diverging to its extremities ; a third narrow 

 muscle passes from the tracheal to the bronchial half- 



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