366 



ZOOLOGY 



SECT. 



mtl? 



FIG. 979. Columba livia. 



Part of left foot of an un- 

 hatched embryo (magni- 

 fied). The cartilage is 

 dotted. mil. 2, second ; 

 mtl.S, third; and mil. k, 

 fourth nietatarsal ; ti. tibia'; 

 tl. 1, proximal tarsal car- 

 tilage ; tl. 2, distal tarsal 

 cartilage. (From Parker's 

 Zootoiiiy.) 



belonging respectively to the second, third, and fourth digits. 

 Thus the ankle-joint of the bird is a meso-tarsal joint, occurring, 

 as in the Lizard, between the proximal and distal tarsals, and 



not, as in other Amniota, between the tibia 

 and the proximal tarsals. 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 

 nietatarsal (mtts. 1). The digits have the 

 same number of * phalanges as in the 

 Lizard, the backwardly-directed hallux two, 

 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. 



It will be observed that every part of 

 the Bird's skeleton presents characteristic 

 and indeed unique features. The vertebral 

 column, the skull, the sternum, the ribs, 

 the limb-girdles, and the limbs themselves 

 are all so highly specialised that there is 

 hardly a bone, except the phalanges of the 

 toes and the free caudal vertebrae, which 

 could possibly be assigned to any other vertebrate class. 



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. 975,pn.fr.),})y which, in the entire bird, 

 they communicate with the air-sacs (vide infra}. In the Pigeon 

 the bones of the fore-arm and hand, and of the leg, are non- 

 pneumatic. 



Muscular System. As might naturally be expected the 

 muscles of the fore-limb arc greatly modified. The powerful 

 downstroke of the wing by which the bird rises into, and propels 

 itself through the air, is performed by the pectoralis (Fig. 980, pct.) t 

 an immense muscle having about one-fifth the total weight of the 

 bod}* : it arises from the whole of the keel of the sternum (car. st.), 

 from the posterior part of the body of that bone (cp. st.), and from 

 the clavicle (cl.), filling nearly the whole of the wedge-shaped space 

 between the body and the keel of the sternum and forming what 

 is commonly called the " breast " of the Bird. Its fibres converge 

 to their insertion (pet.") into the ventral aspect of the humerus 

 (hu., 7m'.) which it depresses. The elevation of the wing is per- 

 formed, not, as might be expected, by a dorsally placed muscle, but 

 by the subclavius (sb. civ.), arising from the anterior part of the 



