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 aerial 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 

 minute vessels. 



The lungs are small, and placed 

 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 

 aerial 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 vertebrae 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 thorax; the posterior, the pelvis. The 

 sternum, with the clavicles and scapulae, is perhaps the most curiously 



Respiratory organs of a Pigeon 



