ANATOMY OF BIRDS 4) 
It has two articular surfaces on its distal extremity which articulate 
with the tarso-metatarsal bone. The tarsus is absent in birds. In 
males the tarso-metatarsus exhibits a slightly curved conical projec- 
tion near the distal third for the support of the spur. 
Four digits articulate with the distal extremity of the tarso-meta- 
tarsus. Three are directed radially cephalad. The internal digit 
has three phalanges; the middle, four and the external, five. The 
fourth digit is directed caudad and has two phalanges succeeding 
a rudimentary first metatarsal bone attached to the metatarsus by 
fibro-cartilaginous tissue. The distal bone of each digit is covered 
by a horny claw. 
MUSCULAR SYSTEM 
In general the muscles of birds correspond to those of mammals, 
with such modifications as are most suited to the mechanism of avian 
structure. The tendency, especially in turkeys and fowls, is to- 
ward extensive ossification of the muscle tendons. This is partic-. 
ularly noticeable in the tendons of the legs but may also be observed 
in the wing and neck in some cases. The fowl and turkey also 
present a marked contrast in the appearance of the muscles of the 
breast and wings as compared with those of the legs. The breast 
and wing muscles are white, due to the relatively small amount of 
sarcoplasm in the muscle fibers. The red muscles are rich in sarco- 
plasm. White muscles are more powerful but have less endurance 
than red ones. Free flying birds have red pectoral or breast muscles. 
The pectoral muscles are two in number, the superficial or great 
and the deep or small pectoral. The former depresses the wing 
and the latter elevates it. These muscles are very massive. The 
great pectoral muscle is attached to the clavicle, to the sternal crest 
near its border, to the posterior lateral process of the sternum and to 
the last ribs. It is inserted into the lateral tuberosity near the head 
of the humerus. It is this muscle that propels the bird in flight. 
The small pectoral muscle originates from the portion of the 
ventral face of the sternum not occupied by the great pectoral and 
also from the clavicle and membrane between the latter and the 
sternum. Its tendon is inserted into the humerus opposite that of 
the great pectoral muscle after passing through the foramen trios- 
seum which acts as a pulley. This foramen is formed by the union 
of the scapula, coracoid and furculum. 
The diaphragm does not exhibit the muscular development of that 
