The trachea carries the air to the lungs in a Pigeon, and separates into two branches in the breast, where it abuts on the _aërial sacs_, and on the two lungs (Fig. 47). The air carried by 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 aërial in their habits. The external form of birds is modified so as to be subservient to aërial progression. The vertebral column, or spine, along the centre of which runs the spinal cord, is divided into three regions--the cervical, dorsal, and sacral regions--terminating in the caudal extremities, the number of vertebræ, 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 scapulæ, is perhaps the most curiously modified part of the skeleton of birds.