404 ZOOLOGY 



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. 161). 



435. 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 esophagus, 

 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 proventriculus 

 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 grind- 

 ing 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, ex- 

 cepting the salivary. The tract ends in a cloaca. 



436. 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 ? 

 Is there any recorded evidence that the character of the gizzard 

 in a given individual may vary somewhat in accordance with 

 the food used ? 



437. Respiration. The trachea corresponds in length to the 

 length of the neck. Its rings are rigid (ossified). It divides 

 into a right and left bronchus which pass to the respective 

 lungs. The lungs are closely applied, and even attached, to the 

 dorsal wall of the thorax and are small in proportion to the 

 size of the animal. Some of the bronchial tubes connect with 

 air spaces (nine in the pigeon) among the viscera and extending 

 even into the hollow bones. They are probably chiefly respira- 

 tory in function. 



