472 



MANUAL OF ZOOLOGY 



Mtl: 



Following the tibio-tarsus is an elongated bone, the tarso- 

 metatarsus (ts. mtts), presenting at its proximal end a con- 

 cave surface for the tibio-tarsus, and at its distal end three 

 distinct pulleys for the articulation of the three forwardly 

 directed toes. In the young bird the proximal end of 

 this bone is a separate cartilage (Fig. 285, ti' 1 ), repre- 

 senting the distal tarsals, and followed by three distinct 

 metatarsals, belonging respectively 

 to the second, third, and fourth 

 digits. To the inner or preaxial 

 side of the tarso-metatarsus, near its 

 distal end, is attached by fibrous 

 tissue a small irregular bone, the 

 first metatarsal {mtts. 1). The back- 

 wardly directed hallux has two pha- 

 langes, the second or inner toe 

 three, the third or middle toe four, 

 and the fourth or outer toe five. In 

 all four digits the distal or ungual 

 phalanx is pointed and curved, and 

 serves for the support of the horny 

 claw. 



A further peculiarity is the fact 

 that the larger proportion of the 

 bones contain no marrow, but are 

 filled during life with air, and are 

 therefore said to be pneumatic. The cavities of the various 

 bones open externally in the dried skeleton by apertures 

 called pneumatic foramina (Fig. 281, pn.for), by which, in 

 the entire bird, they communicate with the air-sacs (vide 

 infra). 



As might naturally be expected, the muscles of the fore- 

 limb are greatly modified. The powerful downstroke of the 



Fig. 285. — Columba livia. 

 Pari of left foot of an un- 

 hatched embryo (magni- 

 fied). The cartilage is 

 dotted. mtl. 2, second; 

 mtl. 3, third; and mtl. 4, 

 fourth metatarsal; ti, tibia; 

 tl. 1. proximal tarsal car- 

 tilage; tl. 2, distal tarsal 

 cartilage. (From Parker's 

 Zootomy.) 



