THE SKELETON AND ANATOMY OF BIEDS. 341 



except the first and last pair. There i.s a large breast-bone or 

 sternum (14). In flying birds this sternum has a deep sternal 

 ridge or keel, to which are attached the powerful muscles which 

 move the wings. The pectoral arch (fig. 177) consists of a 

 pair of scapulse {Sc), clavicles (/'), and coracoid bones {Co). The 

 scapula is an elongated simple bone ; the coracoids are distinct 

 and very strong, and articulate with the upper angle of the 

 sternum ; the clavicles form the V-shaped bone popularly called 

 the "merry-thought." 



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

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

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



^11- 



,-I? 



^ i 



Fia. 17S. — Pelvis of Fowl (lateral view). 

 II, Ilium ; 7s, iscliiuiu ; P, i^ubes ; A, acetabulum. 



short and strong humerus (fig. 175, N), 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 " {R'), 

 which carries the bastard wing. The hind-limb is much as in 

 all animals ; but the chief bone is the tibia (F), the fibula (JT) 

 being very rudimentary. The following joint, the metatarsus 

 (7), is characteristic of birds. Four digits are normally present 

 (2), three pointing forwards and one behind. There is great 

 variation in the disposition of birds' digits. 



The digestive system. — As already said, the bill of birds 



