350 



KESPIKATJON IN LOWER ANIMALS 



■which helps it in flight. Its method of breathing differs 

 from ours, there being no diaphragm, and the expansions 



and contractions are 

 produced by the 

 thoracic muscles. The 

 voice box of these 

 birds is at the bottom 

 of the trachea instead 

 of at the top, and is 

 called a syrinx in- 

 stead of a larynx. 



The mammals have 

 lungs practically iden- 

 tical with ours. As 

 we ascend the animal 

 scale, then, we see a 

 constant increase in 

 the absorbent surface 

 of the respu-atory 

 organs, and this in- 

 crease keeps pace 

 with the increased de- 

 mand for oxygen. All lungs are alike in having the blood ca- 

 pillaries separated from the air by a thin membrane through 

 which the exchange of gases may take place, and in having 

 some method of introducing new air into these sacs and 

 removing it at regular intervals, in order to maintain a 

 constant supply of oxygen to the animal. In all verte- 

 brates the oxygen extracted from the air is carried to the 

 tissues by the blood corpuscles which contain haemoglobin 

 for this purpose. The increase in the number of mem- 

 branes in the lirngs is simply an adaptation for protec- 



FiG. 160 — Lungs of a bird; a, trachea; b, bronchi; 

 c, lungs; df apertures communicating with air 

 cells. 



