470 



MANUAL OF ZOOLOGY 



SECT. 



flf 



a small irregular bone, the first metatarsal (Fig. 267, mtts. i). 

 The backwardly-directed hallux has two phalanges, 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. 264, 

 pn. fr.), by which, in the entire 

 bird, they communicate with the 

 air-sacs (vide p. 475). 



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., 269, /<:/), an im- 

 mense muscle having about one- 

 fifth the total weight of the .body ; 

 it arises from the whole of the 

 keel of the sternum (car. sf.\ from 



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

 the clavicle (<:/), 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'.) which it depresses. The 

 elevation of the wing is performed, not, as might be expected, 



mtlf 

 md? 



FIG. 268. Columba livia. 

 Part of left foot of an un- 

 hatched embryo (magni- 

 fied). The cartilage is 

 dotted. mtl.2, second, 

 iittl. 3, third, and mil 4, 

 fourth metatarsal ; ft. tibia ; 

 //./, proximal tarsal car- 

 tilage ; tl.2, distal tarsal 

 cartilage. (From Parker's 

 Zoototny.) 



