ANATOMY OF BIRDS. 183 



the windpipe acts upon the blood through the thin substance 

 of the cells which constitute the pulmonary tissue, in which it 

 traverses in an infinity of minute vessels, whose thin walls are 

 permeable by the gas. 



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 

 air even penetrating many of the bones when the species are 

 peculiarly aerial in their habits. 



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 sjjinal cord, is divided into three regions — 

 the cervical, dorsal, and sacral regions — terminating in the caudal 

 extremities, the number of vertebrte, or jjieces, 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 piosterior, the pelvis. The sternum, with the clavicles 

 and scapulae, is perhaps the most curiously modified part of the 

 skeleton of birds. 



The sternum, then, is a large expanded plate extended over the 

 whole anterior part of the thorax, and even covering more or less 

 what may be considered the abdomen. It varies greatly in dif- 

 ferent genera ; but in all it is more or less four-sided, and convex 

 externally, forming the basis for the powerful muscles by which 

 the wings are moved. These wings serve as arms by which the 

 bird guides itself, ascending or descending according to the im- 

 pulse given them. " That the anterior form of birds is modified 

 so as to be subservient to the aerial progression for which these 

 animals are intended," says McGillivray, " is obvious and intel- 

 ligible. Their bodies are oval, with the more powerful muscles 

 placed on the breast, so that, when the horizontal position is 

 assumed, the centre of gravity comes between the wings, and is 

 kept near the lower part by the weight of the pectoral muscles. 



