148 



CHARACTERS OF VERTEBRATE ANIMALS 



encountered, for the lungs do not constitute the only goal of the 

 in-breathed air, but communicate with a number of thin-walled 

 air-sacs, and these again with the air-filled cavities in the bones. 

 The only other animals which are so thoroughly permeated by 

 air are insects, and in them, as in birds, one end to be gained 

 is the reduction of the specific gravity of the body as an aid 

 to flight. The small lungs, though spongy, are not freely 

 movable in the thoracic cavity, but are firmly fixed to its roof, 



WINDPIPE 



SUBCLAVIAN A. 



POSTCAVA 



B. 



Fig. 102. Structure of Pigeon 



A, Respiratory organs, seen from vertical side. Air-sacs connected with right lung are drawn in outline. 

 Rods are inserted into two cut air-sacs. B, Diagrams of heart and great blood-vessels, from ventral side. 

 Two halves of heart shown as if separated, and direction of blood-flow indicated by arrows. R.A. and L.A., 

 right and left auricles; R.V. and L.V., right and left ventricles; Pul. Art. and Pul. V., pulmonary artery and 

 vein ; A. and V. elsewhere mean artery and vein. 



and there being no midriff as in a mammal (see p. 46), there 

 is no sharp boundary between thoracic and abdominal regions. 

 The bronchi branch out in their respective lungs, and their 

 larger branches run to the lung surface, where they open into 

 the large thin-walled air-sacs, which are placed in various 

 positions. Some of the anterior air-sacs communicate with the 

 air-spaces contained in certain bones, as, e.g., the humerus and 

 sternum. The mechanical part of breathing is brought about 

 by the movement of the ribs and sternum, much as in a 

 mammal (p. 46), but here there is, of course, no diaphragmatic 

 breathing. The entering air rushes into all the air-sacs, and 

 the work to be done by diffusion is not so great as in a mammal. 

 The reason for this is, to keep the process of blood oxygenation 



