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 skull. 
(Fig. 474). 
The trachea (7'r) is composed of cartilaginous rings with 
intervening membranes, and an external sheath of connect- 
ive tissue, which has been removed at 7r. 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 
(Z), found only in birds, but wanting in the ostrich, ete. 
(Ratite), storks, and certain birds of prey. The trachea 
terminates immediately behind the syrinx in two smaller 
branches, the bronchi (8), 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 inthe 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, Zw) 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. 
