744 EVOLUTION OF ANIMALS Part V 



from the lungs received in the left auricle of the heart is mixed with the 

 oxygen-poor blood from the right auricle in the incompletely separated 

 ventricles. For the slow metabolism of the cold-blooded reptiles, this is enough 

 oxygen to supply the needs. This is not true in birds. The bird's heart is com- 

 pletely four-chambered and the two kinds of blood are entirely separated. 

 Except for those that go to the lungs, all arteries carry highly oxygenated 

 blood. Only a rapid and generous supply is adequate for the oxygen-hungry 

 body of a bird. The heartbeat of birds is incredibly rapid. The basal rates of 

 the heartbeat of an English sparrow, a canary, and a hummingbird have been 

 recorded respectively as 350, 500, and 1000 per minute. The adult human 

 heart beats about 70 times per minute. The red blood cells of birds are 

 nucleated like those of lower vertebrates; there are more of them per unit of 

 blood than in any other animal. 



Respiration. The vocal organ or syrinx is ordinarily located where the wind- 

 pipe forks into the bronchial tubes, one to each lung (Fig. 36.15). The lower- 

 most rings of the windpipe fuse to form a tube within which are the membranes 

 and muscles whose vibrations produce the voice. Because of their intense ac- 

 tivity and high temperature, birds have the highest oxygen consumption of all 

 animals. This is satisfied by fast breathing, the rapid passage of air through the 

 small compact lungs, and the extremely swift flow of blood through them. The 

 lungs are expanded by the pull of the ribs to which they are closely fitted. Air 

 goes through them and enters the internally ciliated air sacs by way of the 

 bronchial tubes. The air sacs extend along the neck, beneath the wishbone, and 

 far back among the viscera (Fig. 36.16). Air spaces connected with them 

 reach into the larger bones. Air is forced out of the air sacs by the pressure 

 of muscles; this time, it enters the lungs directly from the sacs. It rushes 

 through them past the blood capillaries from which carbon dioxide is collected 

 and to which oxygen is contributed. The air sacs constitute a cooling system 

 that combats the intense heat of the bird's body produced by the muscles and 

 kept within it by the feathers. When a bird's air sacs are opened experimentally 

 it continues to live, but its temperature rises higher than the usual 100° to 

 110° F. In swimming birds the air sacs are helpful floats. 



Excretion. Birds conserve water and excrete salts. The completed urine of 

 a bird is a semisolid mass of uric acid crystals cast out of the body as whitish 

 material adhering to the darker waste from the digestive tract. There is no 

 urinary bladder. 



Nervous System and Sense Organs. The cerebellum and optic lobes are rela- 

 tively well developed. This indicates that birds have good coordination and 

 sight. The olfactory lobes are small and even buzzards suspect dead flesh by 

 sight rather than smell. As might be expected birds taste very little. They are 

 sensitive to touch in particular places. Woodcocks probe soft earth and feel for 

 worms with the tips of their bills; various ducks have sensitive sifting plates 



