164 



ANIMAL BIOLOGY 



rough or smooth ? Why ? Is the gizzard tough or weak ? 

 Why are small stones in the gizzard ? Why do not hawks 

 and other birds of prey need a muscular gizzard ? The 

 liver and pancreas empty their secretions into the intestines 

 by several ducts a little way beyond the gizzard. Beyond 

 the mouths of two caeca (Fig. 305) the many-coiled 



intestine empties into the straight 

 rectum, which terminates in a 

 widened part called the cloaca. 

 Not only the intestine, but the 

 two ureters of the urinary system 

 and the two genital ducts of the 

 reproductive system all empty into 

 the cloaca (Figs. 304, 305). 



The lungs have their rear sur- 

 faces attached to the spinal 

 column and ribs (lu, Fig. 304). 

 They are connected with thin- 

 walled, transparent air sacs which 

 aid in purifying the blood. When 

 inflated with warm air, they prob- 

 ably make the body of the bird 

 more buoyant. For the names, 

 location, and shape of several 

 pairs of air sacs, see Fig. 306. 

 The connection of the air sacs with 

 hollows in the humerus bones is also shown in the figure. 

 Many of the bones are hollow; this adds to the buoyancy of 

 the bird. The pulmonary artery, as in man, takes dark 

 blood to the lungs to exchange its carbon dioxide for 

 oxygen. Of two animals of the same weight, which ex- 

 pends more energy, the one that flies, or the one that runs 

 the same distance ? Does a bird require more oxygen 



FIG. 306. POSITION OF Luxes 

 AND AIR SACS (Pigeon). 



Tr, windpipe; P, lungs; Lm, sac 

 under clavicle with prolongation 

 (AA) into humerus; La, sacs in 

 abdomen. 



