AVES 359 



(3) Bones of strictly aquatic birds are solid, being filled with 

 bony tissue. The presence of air in the bones is believed to aid in 

 oxygenizing the blood and in adjusting the air pressure when a bird 

 descends rapidly from a great height. 



A fossilized wing of a pterosaur recently sent to the U. S. National 

 Museum from Oregon was so perfectly preserved that it was possible 

 to determine the character of the bones. Instead of being hollow 

 as in our modern fliers, the cavities of the bones were filled with light 

 spongy tissue which served to strengthen them. 



Digestive System. — The mouth is without teeth. (Vestigial 

 teeth are present in some parrots.) The tongue is of various types: 

 (i) Pointed, in the pigeon; (2) Long and protrusible, in the wood- 

 pecker; (3) Short, in parrots; (4) Sucking tubes, in the humming 

 birds. 



In birds we find that swallowing consists of violently jerking the 

 head with an accompanying tongue pressure. There is no soft 

 palate, and no epiglottis is present, but the larynx is protected by 

 retroverted papillae at the base of the tongue. Insectivorous birds 

 have a pouch at the base of the throat. In the nutcrackers there is 

 a goitrous swelling in the throat, where the animal stuffs itself with 

 nuts. The pelican's enormous bill holds 10 quarts of water. 



The upper part of the esophagus has buccal glands, sometimes 

 called salivary glands, used to moisten the food. The crop is a non- 

 glandular sac in which the food is softened and macerated. Animal 

 food may remain in the crop for 8 hours, and vegetable food may be 

 retained for from 16 to 20 hours. Fruit and insect eating birds 

 have no crop. The pigeon has a double crop. " Pigeon's milk " is 

 formed in the o'op and consists of proteins and oil, with no casein 

 and no sugar of milk. It is a milky appearing fluid which mixes 

 with macerating grains and is regurgitated for the young. The 

 toucan regurgitates and chews over its food. The indigestible parts 

 of the prey of the owl are regularly " cast " or regurgitated from the 

 stomach. The lower part of the esophagus is a continuation from 

 the crop to the stomach. The proventriculus has glandular walls, 

 its gastric follicles secreting gastric juice. (Figure 202.) 



The gizzard is thick and muscular, and is used to grind food. 

 It corresponds to the pyloric end of the mammalian stomach. 



The triturating agents are hard foreign bodies such as sand and 

 gravel. Pigeons and other gallinaceous birds carry gravel to their 

 young. It is necessary to have such in order to bruise the grains 



