THE BONES IN BIRDS. 165 



Their superior extremity rests within and opposite to the j^lenoid cavity, against 

 the scapula and coracoid, forming with these hones a remarkal)ie foramen, 

 through which passes the tendon of the elevator muscle of the wing (Fig. HI, 

 A 4, B 6). The forks play the part of an elastic spring, whose office it is to 

 prevent the wings coming towards each other during contraction of the depressor 

 muscles. The conformation of this bone is, therefore, like the sternum, related 

 to the extent and power of flight ; and for this reason it is that, in swift flyers, 

 the two branches of the furculum are thick, solid, widely separated, and curved 

 like a U ; while in those which fly heavily and with difficulty, these branches 

 are thin and weak, and joined at an acute angle. The latter formation greatly 

 diminishes its strength, and lessens, in a singular manner, the reactionary power 

 of the bony arch it represents. 



Bone of the arm. — The humerus (Figs. 109, n ; 111, a 13) offers an articular 

 oval-shaped head, and an air-opening placed beneath this eminence. It is long 

 in Palmipedes, ordinarily so in the GalUnacm projyer, and very short in Pigeons. 



Bones of the forearm (Figs. 109, o, o ; 111, a 15, 16).— The radius is much 

 less voluminous than the ulna. The latter has an extremely short olecranon ; 

 and the two bones are separated from one another in their middle part to meet 

 again at their extremities, where they are united by ligamentous bands in such 

 a way as to render the movements of pronation and supination impossible. This 

 mode of union, which nevertheless does not prevent the two bones from gliding 

 slightly on each other in the direction of their length, has been wisely adopted 

 by nature in order that the wing might strike the air, like an oar, by its inferior 

 face ; otherwise, the resistance of the aerial medium would make these two bones 

 pivot, and cause the wing to present itself to the air in a wrong direction. 



Bones uf the carpus (Figs. 109, p, p' ; 111, a 17, 17).— These are only two, 

 and are distinguished by the names of radius and ulna, in consequence of their 

 corresponding more particularly to these bones in other animals. 



Bones of the metacarpus (Figs. 109, Q, q' ; HI, a, 19, 20). — These also 

 number only two, and are separated at their middle portion, to be consolidated 

 at their extremities. 



Bones of the digital region. — The wing of a bird is composed of three digits 

 One of them, which resembles the thumb and forms the basis of the false wing 

 is composed of a single styloid-shaped phalanx, articulated at the base of a small 

 particular process belonging to the superior extremity of the largest metacai-pal 

 bone (Figs. 109, r' ; 111, a 23). The largest digit comprises two phalanges, 

 which succeed the last bone (Figs. 109, R, r ; 111, a 21, 21'). The third digit 

 is represented by a small rudimentary phalanx (Fig. 109, A 22), which corre- 

 sponds to the inferior extremity of the small metacarpal bone, and lies beside 

 the first phalanx of the large digit in the closest manner. 



It is well to remark that the hand and forearm are longer in proportion to 

 the quality of flight ; those two regions of the wing, for example, are very short 

 in Gallinaceous Birds. 



Posterior Limbs. Coxa, or os iliac. — This is a voluminous and very 

 solid piece, particularly in walking birds, and composed, as in the Mammalia, 

 of an ilium, ischium, and pubis. The ilium (Fig. 109, s), very long, is con- 

 solidated with the last two dorsal, the lumbar, and the sacral vertebrae ; it is 

 excavated on its internal face. The ischium partly incloses the side of the pelvic 

 cavity ; between its internal border and the external border of the ilium is an 

 orifice which replaces the great ischiatic notch. Its inferior border is united to 



