294 



BIRDS. 



LEFT SIDE OF SHOULDER AND BREAST-BONES OF 

 FLIGHTLESS HIHD. 



directed backward parallel with the element termed the ischium; the three 



elements being united together as in Mammals, but differing from them, 



with two exceptions, in that neither the ischium nor the pubis unites with its 



fellow of the opposite side in the middle line. Moreover, the cup in the pelvis for 



the head of the thigh-bone is always open at the base. The firm union of the 



haunch-bones with such a large portion of the backbone is necessary to afford 



a solid basis of support for the rest of the skeleton in flight. 



Equally essential is a solid union 

 between the bones of the shoulder 

 and the breast-bone. Accordingly, 

 we find that, as in the Egg-laying 

 Mammals, the blade-bone or scapula 

 (s) is connected with the breast- 

 bone (sf), by a metacoracoid (<■); 

 the scapula and metacoracoid thus 

 jointly forming the cup for the 

 articulation of the head of the arm- 

 bone or humerus (h). Although in 

 flightless Birds the metacoracoid is 

 sin irt and broad, in other species it 

 is more or loss elongated; and in 

 cither case its lower expanded end 

 is received in a groove on the 



Usually 

 are 



well-developed, and united together to form a V or U-shaped bone, now known as 



the " merry-thought," or furcula (/) ; this furcula generally articulating with a 



process on the metacoracoid (a), and also with the anterior 



end of the breast-bone. The breast-bone in flying Birds is 



provided with a strong keel up the middle of its inferior 



surface, as shown in the figure of the skeleton of a parrot 



on p. 292, in order to afford support for the powerful muscles 



moving the wing ; but in flightless Birds, as in the figure on 



this page, it is smooth and rounded. To the sides of the 



upper part of the breast-bone are attached the lower segments 



of the ribs ; the ribs themselves being few in number, and 



distinguished from those of Mammals by the presence of 



oblique (uncinate) processes projecting from their hinder 



borders. 



With regard to the limbs, the bones of a bird's wing- 

 correspond generally to those of the arm or fore-leg of a 



mammal; the arm-bone or humerus having distinct condyles 



(a, b) for the articulation of the bones of the fore-arm (radius 



and ulna); and being sometimes furnished with a projecting 



process above the outermost of these two condyles. The 



s, blade-bone or scapula; c, metacoracoid; /<, arm-bone 

 or humerus; /, furcula; st, breast-bone or sternum. — After summit of the breast-In HH'. 



the collar-bones, or clavicles 



FRONT SURFACE OP THE 

 LEFT .METACORACOID OF 

 A FLYING BIRD. 



a, process for articulation 

 of furcula ; 6, r, </, surface 

 for junction with breast- 

 two bones of the fore-arm always remain separate from bone 



