72 



ELEMENTS OF COMPARATIVE ZOOLOGY. 



in the terrestrial, longer in the arboreal, forms. Claws 

 occur in some cases, especially in 

 young birds, upon the wings. 



In all living birds teeth are absent, 

 and even in the embryos but the 

 slightest trace of their former exis- 

 tence can be found. In certain fossil 

 birds well-developed teeth occur. The 

 tongue is usually slender., stiff, and 

 horny, and in some forms (woodpeck- 

 ers, etc.) it is very extensible. The 

 oasophagus is long, and frequently a 

 part of it in the neck is swollen out 

 to form a reservoir of food or crop. 

 The stomach is divided into two parts. 

 The first of these (proventriculus), 

 which is glandular, appears much like 

 an enlargement of the gullet. The sec- 

 ond or muscular stomach (gizzard) is 

 a veritable chewing organ. It is most 

 developed in the seed-eating birds, 

 and in these often contains small stones 

 to assist in grinding the food. 



The lungs are especially well devel- 



FiG. 28.-Alimentary , & .. . . 



Tract of an Eagle, c, oped, and a peculiarity is, that con- 

 crop; m, muscular stom- . _ .,1,1 



ach (gizzard) ; i, intes- nected with them are air-sacs which 



tine; p, glandular stom- _ ,1,1 



ach (proventriculus); extend among the other viscera and 



t, trachea ; v, vent. . . < n i ,, 



even into some or the bones, as in the 

 wing.* These air-sacs serve to increase the respiratory sur- 

 face, and also to lessen the weight of the bird. They are 



* A similar pneumaticity occurred in the bones of some of the 

 fossil reptiles (Dinosaurs). 



