4 ZOOLOGY. 



2. The cranial bones fuse closely, and the bones of the face 

 are prolonged into the core for the beak (Fig. 216). 



3. The sternum is normally well developed and provided 

 with a keel to which the muscles of flight are attached. 

 Finger-like processes also increase its surface for the attach- 

 ment of muscles and the support of the viscera. 



4. The ribs are double-headed, and each has a process on 

 the posterior margin, joining it to the rib behind. 



5. The pectoral girdle has its clavicles fused ventrally in the 

 flying birds, forming the " wish bone." 



6. In the pelvic girdle the ventral bones (ischium and pubis) 

 both pass backward from the hip joint and support the viscera. 



7. The ankle region of the birds is very characteristic. The 

 proximal tarsals unite with the tibia, and the distal tarsals unite 

 with the fused metatarsals to form the tarso-metatarsus or 

 shank. The joint is between the proximal and distal tarsals 

 (see Fig. 159). 



426. Digestive Organs. The horny beak entirely replaces 

 the teeth in the modern birds. In the early members of the 

 group teeth are known to have been present. The oesophagus, 

 often of great length, is usually expanded into a, non-glandular 

 crop, where the food is stored and softened. The stomach 

 often consists of two portions, the anterior glandular pro- 

 ventriculus and the posterior muscular gizzard. In birds which 

 habitually feed on grains or other hard objects the inner wall 

 of the gizzard is lined with a hard and thickened cuticle which 

 assists in grinding the food. Fragments of rock, sand, etc., are 

 nearly always swallowed by grain-eating forms to assist in 

 the process. These are manifestly devices to do work usually 

 done by teeth. The usual glands are found associated with 

 the digestive tract, excepting the salivary. The tract ends in a 

 cloaca. 



427. Supplementary Studies. What is the exact position 

 of the crop? Advantage of this position? Make a comparative 

 study of the beak in various birds; how adapted to the habits? 



