ANATOMY OF BIRDS. 167 
_ their circulation and specific lightness to their bodies, from its rich 
organisation enables Birds to live in the coldest atmospheric regions. 
In Fig. 43 the respiratory organs of a Pigeon are represented. The 
trachea or windpipe is composed of many bony rings, varying in 
different species. In the Falcons it is slightly flattened, and tapers in 
a small degree; but in many genera it presents dilatations and con- 
tractions, and in others it is variously curved, two slender muscles, 
which run along its sides towards the 
sternum, serving to contract it. In 
many of the song-birds several pairs 
of small muscles are attached to the 
lower larynx, where the tube bifur- 
cates, by which they are enabled to 
control this organ, which is the pro- 
ducer of their note. 
The trachea carries the air to the 
lungs in a Pigeon, and separates into 
two branches in the breast, where it 
abuts on the aérza/ sacs, and on the 
two lungs (Fig. 44). The air carried 
by the windpipe acts upon the blood 
through the thin substance of the cells 
which constitute the pulmonary tissue, 
which it traverses in an infinity of LY 
minute vessels. \ = 
The lungs are small, and placed Fig. 43. Respiratory organs of a Pigeon. 
in the upper part of the thorax, where 
they are confined on each side by a cavity bounded above by the 
ribs and below by an imperfect diaphragm ; but they are perforated 
by tubes which communicate with membranous cells distributed over 
the thoracic and abdominal cavities, between the muscles, beneath 
the skin, and in all parts of the body. 
The external form of birds is modified so as to be subservient to 
aérial progression. ‘The vertebral column or spine, along the centre 
of which runs the spinal cord, is divided into the cervical, dorsal, 
and sacral regions, terminating in the caudal extremities, the number 
of vertebree or pieces varying much in different genera. The body 
consists of the dorsal, sacral, and caudal parts of the column ; 
laterally, of the ribs and pelvis ; and beneath, of the sternum and the 
_soft parts contained in it. Its anterior part, containing heart, lungs, 
and liver, is named the “orax; the posterior, the pews. The 
_ sternum, with the clavicles and scapule, is perhaps the most curiously 
