526 ZOOLOGY. 



adult birds both tubes follow a symmetrical course, but ex- 

 hibit a mock or secondary symmetry with regard to each 

 other. The origin of the two canals is embraced by the 

 hyoidean apparatus, one of the horns (cornua) of which ap- 

 pears at Hy ; the apparatus is too complicated to be de- 

 scribed here ; it closely resembles that of reptiles, and is 

 functionally connected with the rapid thrusting out of the 

 tongue. In some birds, as, for example, the woodpeckers 

 and humming-birds, the horns are so developed as to curve 

 round the back of the cranium on to the top of the skulk 

 (Fig. 474). 



The trachea (Tr) is composed of cartilaginous rings with 

 intervening membranes, and an external sheath of connect- 

 ive tissue, which has been removed at Tr. It extends into 

 the thorax, and is of nearly uniform diameter throughout, 

 except at its lower extremity, where, as shown in Fig. 459, 

 D, it forms an enlargement, the syrinx or vocal chamber 

 (L), found only in birds, but wanting in the ostrich, etc. 

 (Ratitce), storks, and certain birds of prey. The trachea 

 terminates immediately behind the syrinx in two smaller 

 branches, the bronchi (B], each of which passes into the 

 lung (Lu} of the same side. The cartilaginous rings of 

 the bronchi are incomplete, the walls being partly formed 

 by an elastic membrane. The rings of the trachea are pe- 

 culiarly modified in the syrinx, which is furnished with ex- 

 ternal muscles and internal membranous expansions, serving 

 to produce the voice ; the muscles are the sterno-tracheal, 

 furculo- or claviculo-tracheal, and the proper muscles of the 

 syrinx. A true larynx is present in the upper part of the 

 trachea, but is unessential to the formation of the voice. 

 The trachea presents flexuosities in various birds, usually 

 more marked in the male than in the female ; in swans there 

 is a great band which extends into the hollow breast-bone, 

 but the object of this disposition is unknown. 



The lungs (Fig. 459, Lu} are two large sacs, placed dor- 

 sally in the anterior part of the body-cavity, but not suspend- 

 ed freely in a short thoracic sac nor enclosed in a pleura, as 

 in mammals ; they are composed of reddish spongy tissues, 

 and are attached between the ribs by connective tissue. 



