BIRDS. 



351 



terminated by claws; short in the terrestrial, longer in 

 the arboreal, forms. Claws occur in some cases, espe- 

 cially in young birds, upon the 

 wings. 



In all living birds teeth are ab- 

 sent, and even in the embryos but 

 the slightest trace of their former 

 existence can be found. In certain 

 fossil birds well-developed teeth oc- 

 cur (fig. 150). The tongue is usu- 

 ally slender, stiff, and horny, and 

 in some forms (woodpeckers, etc.) 

 it is very extensible. The oesophagus 

 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 second or muscular stomach 

 (gizzard) is a veritable chewing organ. 

 It is most developed in the grain or 

 seed-eating birds, and in these often 

 contains small stones to assist in 

 grinding the food. 



The lungs are especially well de- 

 veloped, and a peculiarity is that 

 connected with them are air-sacs 

 which extend among the other viscera and even into some 

 of the bones, as those of the wing.* These air-sacs serve 



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

 fossil reptiles (Dinosaurs, p. 349). 



FIG. 147. A 1 i m e n t a ry 

 tract of an eagle, c, 

 crop; m, muscular stom- 

 ach (gizzard); i, intes- 

 tine; p, glandular stom- 

 ach (proventriculus); t, 

 trachea; v, vent. 



