394 ZOOLOGY 



SEPT. 



directed toes. In the young Bird the proximal end of this bone is a 

 separate cartilage (Fig. 1038, tl.2), representing the distal tarsals, 

 and followed by three distinct metatarsals, 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 Mammals (q.v.), between 

 the tibia and the proximal tarsals. To the inner or pre-axial side 

 of the tarso-metatarsus, near its distal end, is attached by fibrous 

 tissue a small irregular bone, the first rnetatarsal (mtts. f). 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. 1034, pn.fr.), by 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 are greatly modified. The powerful 

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

 itself through the air, is performed "bytlnepectoralis (Fig. 1039,_pc.), 

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

 body ; 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 

 (ku., hu f .) 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 

 body of the sternum, dorsal to the pectoralis, and sending its 

 tendon (sb. civ.) through the foramen triosseum to be inserted 

 into the dorsal aspect of the humerus. In virtue of this arrange- 

 ment, the foramen acting like a pulley, the direction of action 

 of the muscle is changed, the backward pull of the tendon 



