372 SAUROPSIDA (BIRDS). 



The bones of the fore-limb or wing of the bird are adapted to 

 its peculiar aijrial life, and form one of the chief characteristics 

 of the bird's skeleton. This anterior limb consists of a single 

 short and strong humerus (fig. 208, JY), an ulna and a radius (0 

 and o) (the former being much the larger bone), two carpal 

 bones (P and P'), the metacarpus (Q and Q'), which consists of 

 three bones united together, but free in the middle ; at the 

 proximal end of the larger metacarpal is attached the " thumb- 

 bone " (P'), which carries the bastard wing. The hind-limb is 

 much as in all animals ; but the chief bone is the tibia ( F), 

 the fibula (X) being very rudimentary. The following joint, 

 tlie metatarsus (Y), is characteristic of birds. Four digits are 



Fia 212. — Pelvis of Fowl (lateral view). 

 11, Ilium ; 7s, ischium ; P, pubes ; A, acetabulum. 



normally present (.v), three pointing forwards and one behind. 

 There is great variation in the disposition of birds' digits. 



T/ie Digestive Si/stem. — As already said, the bill of birds 

 never contains teeth : it is variable in appearance, and is used 

 for eating the prey and food generallj^, for prehension, and in 

 some as an organ of touch. The base of the upper bill is 

 often surrounded by a circle of skin devoid of feathers, the 

 "cere." The tongue is mostly hard and horn)'', and is supported 

 by the hyoid bone. The digestive tract consists of a long 

 gullet or oesophagus (fig. 213, Oe), which has a sac-like dilata- 

 tion in front of the " merry-thought," the croji {C). This crop 

 is seen in carnivorous and graminivorous birds. Here the food 



