2:>4 ANATOMY OF VERTEBRATES. 



elasticity of the membranes so put on tlie stretch : but there is a 

 direct 'relaxor' of the tympaniform membrane in the ' sterno- 

 trachealis/ ib. c?, which, passing from the side of the trachea to the 

 sternum, shortens the whole bronchus as it draws down the wind- 

 pipe. This is the most constant of all the muscles affecting the 

 lower larynx. It is reckoned by Savart as the sixth pair of vocal 

 muscles, but not by Cuvier, since it is not directly attached to 

 any part of the lower larynx, and exists in Birds, as, e. g., the 

 Vulture and Ostrich, in which that larynx is not developed. 



The manifold ways and degrees in which the several parts of 

 the complex vocal organ in Caiitores may be affected, each of the 

 principal bony half-rings, as one or other end may be pulled, being 

 made to perform a slight rotatory motion, are incalculable : but 

 their effects are dehghtfully appreciable by the rapt listener to 

 the singularly varied kind and quality of notes trilled forth in the 

 stillness of gloom by the Nightingale. 



In many of the Volitores there is a single pair of ' broncho- 

 tracheales,' and a single pair of short ventricose ' bronchiales.' In 

 ThamnoiDMlus each sterno-trachealis bifurcates to send a small strip 

 to the lower larynx, and the rest to the side of the trachea, as 

 usual. In Furnaria 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 

 105 together, and occupying the interspace between, the 



last tracheal and first bronchial half-rings. These 

 have each one margin concave, with the concavity 

 turned towards each other, and are moveably joined 

 too'ether 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, a, passes from the side of the trachea to the 

 upper (tracheal) half-ring ; and, by raising it, makes 

 tense the elHptical elastic membrane : a broader ' ten- 

 sor brevis glottidis,' ib. Z>, 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-rings, ib. c, and, by approximating them, 

 relaxes the membrane occupying the elliptical interspace. These 

 membranes, projecting on each side into or below the termi- 

 nation of the air-tube, leave a narrow chink between them, 

 through which the air passes to and from the lungs ; and when, 

 in forcible expiration, the membranes are put into a sufficient 



Lower laryiix^, 

 Parrot, xxx". 



