20 ELEMENTARY SKETCH OF THE OSTEOLOGY OF BIRDS, 



and median plate or hypocleidium* while, on the other hand, the 

 scapular extremity is likewise expanded, and ossifies separately as 

 an epicleidium.^ The united clavicles of birds appear to be quite 

 peculiar to the class, and their great development and strength 

 is correlated with the development of the power of flight. 

 Their anchylosis mesially operates to counteract the tendency of 

 the chest to collapse by the force of the downward stroke of 

 the wings. 



The fore-limb. — The fore-limb of birds presents the same divi- 

 sions as in man and other mammals. The proximal bone (h. pi. I., 

 fig. I, pi. IV.) or humerusX lies parallel with the long axis of 

 the body, its typically ventral surface looking outwards. Its 

 proximal extremity is expanded laterally into a large head (pi. 

 IV., fig. 1 h) ; its outer (or ventral) surface superiorly is expand- 

 ed into a large pectoral crest (p.c), which affords attachment to 

 the powerful pectoral muscles ; this surface is convex. The upper 

 part of the anterior surface presents a depression for the passage 

 of the tendon of the biceps§ muscle. The inner (or dorsal) surface 

 (right side of figure) is concave, and carries thepneumatic aperture 

 in those birds which have a hollow humerus. The distal extremity 

 of the humerus is also expanded and carries two condyles, — an 

 interiial condyle (i.e.) for the articulation of the ulna and an 

 external condyle (e.e.) for the radius', the latter coudyle forms 

 a convex facet directed obliquely upwards and inwards on the 

 anterior surface. Such are the general features of the humerus 

 of a carinate bird. In the Ratitse, on the other hand, the humerus 

 is much less developed, and is in general a slender, cylindrical, 

 and slightly curved bone. In the Apteryx the humerus, and 

 the other bones of the fore-limb, are practically rudimentary. 



The ulna 1| (pi. I., ul.) forms the larger of the two bones of 

 the forearm ; it is a cylindrical bone slightly expanded at its two 

 extremities, and which is not developed into an olecranon as in 

 mammals. On its outer surface the ulna generally shows a row 

 of tubercles which formed the points of attachment of the secon- 

 dary quill-feathers. The radius (pi. I. r.) T[ is a slender, slightly 

 curved bone, which carries the greater part of the carpus or wrist 

 at its distal exti'emity. The radius and ulna are placed not by 

 the side of one another, but one is placed in front of the other, 



* Hypocleidium; derived from vivo beneath, and KAetj the clavicle, 



t Epieleidium ; a similarly-formed word to the last, substituting £Tt upon, for viro, 

 J Humerus, the shoulder, applied to the bone of the upper arm, 

 § Biceps, from his, double, and ceps, a form of caput, the head. The main flexor 

 muscle of the arm which arises from two distinct heads, 



II Ulna, from wXtvr] the elbow, applied to the outer one of the two bones of the 

 fore- arm. 

 ^ liadius, a spoke, applied to the inner one of the two bones of the fore-arm, 



