294 



BIRDS. 



LEFT SIDE OF SHOULDER AND BREAST-BONES OF A 

 FLIGHTLESS BIRD. 



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 (st), by a metacoracoid (c) ; 

 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 

 short and broad, in other species it 

 is more or less elongated ; and in 

 either case its lower expanded end 



s, blade-bone or scapula ; c, metacoracoid ; h, arm-bone 1S received 111 a groove Oil tlie 

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



the collar - bones, or clavicles, 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 

 (". 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 



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



FRONT SURFACE OF THE 

 LEFT METACORACOID OF 

 A FLYING BIRD. 



a, process for articulation 

 of furcula ; />, c, it, surface 

 for junction with breast- 



