370 



ZOOLOGY 



SECT. 



sb.l 



thymus on each side of the neck. The adrenals (Fig. 990, adr.) are 

 irregular yellow bodies placed at the anterior ends of the kidneys. 



Respiratory and Vocal Organs. The glottis (Fig. 981, gl.\ 

 is situated just behind the root of the tongue, and leads into the 

 larynx, which is supported by cartilages a ventral thyroid, a dorsal 

 cricoid, and paired arytcnoids but does not, as in other Vertebrates, 

 function as the organ of voice. The anterior part of the trachea 

 (tr.) has the usual position, ventral to the gullet, but further back 

 it is displaced to the left by the crop, becoming ventral once more 

 as it enters the body-cavity, where it divides into the right (r. br.) 

 and left bronchi. The rings supporting the trachea are not 

 cartilaginous but bony, as also is the first ring of each bronchus, 



those of the trachea 

 completely surrounding 

 the tube, those of the 

 bronchi incomplete 

 mesially. 



At the junction of 

 the trachea with the 

 bronchi occurs the 

 characteristic vocal or- 

 gan, the syrinx (syr.\ 

 found in no other class. 

 The last three or four 

 rings of the trachea 

 (Fig. 982, tr.), and the 

 first or bony half ring 

 of each bronchus (br.\ 

 are modified to form 

 a slightly dilated cham- 

 ber, the tympanum, the 

 mucous membrane of 

 which forms a cushion- 

 like thickening on 

 each side. At the 



junction jof the bronchi a bar of cartilage, the pessulus, extends 

 dorso-ventrally and supports an inconspicuous fold of mucous 

 membrane, "Hie membrana semilunaris. The membranous inner 

 walls of the bronchi form the internal tympaniform membranes. 

 A pair of intrinsic syringeal muscles arise from the sides of the 

 trachea and are inserted into the syrinx, and a pair of sterno- 

 tracheal muscles arise from the sternum and are inserted into the 

 trachea. The voice is produced by the vibration of the semilunar 

 membrane : its pitch is altered by changes in the form of the 

 tympanum produced by the action of the muscles. 



The lungs (Fig. 981, Ing.) are very small in comparison with the 

 size of the Bird, and are but slightly distensible, being solid spongy 



FIG. 982. Columba livia. The lungs with the posterior 

 end of the trachea, ventral aspect, a. in. aperture of 

 anterior thoracic air-sac ; br. principal bronchus ; br'. 

 br". br"'. secondary bronchi ; p. aperture of abdominal 

 air-sac ; p. a. pulmonary artery entering lung ; p. in. 

 aperture of posterior thoracic air-sac ; p. v. pulmonary 

 vein leaving lung ; sb. b. aperture of interclavicular air- 

 sac ; sp. b. aperture of cervical air-sac ; sy. syrinx ; 

 tr.' trachea. (From Parker's Zootomy.) 



