382 ZOOLOGY SECT. 



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. 1045, pn.fr.}, by which, in the entire 

 bird, they communicate with the air-sacs (vide p. 386). 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 by the pectoralis (Fig. 1050, pet.), 

 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 

 (hu., hu r .) 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 ^he 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 raising 

 the humerus. There are three tensores patagii (tns. lg., tns. br., 

 tns. ace.), the action of which is to keep the pre-patagium tensely 

 stretched when the wing is extended. A similar muscle (tns. m. p.) 

 acts upon the post-patagium. The muscles of the digits are 

 naturally much reduced. 



The muscles of the neck and tail are well developed ; those of 

 the back are practically atrophied, in correspondence with the 

 immobility of that region. In the leg certain of the muscles are 

 modified to form the perching mechanism. The toes are flexed 



