THE INTERNAL SKELETON. Igt 
air into the interior of the humerus, is situated on the hinder 
surface of its head; and even in birds in which this bone is not 
pneumatic—as e.g. the Ostrich and the Penguin—we find a deep 
depression in the place where this orifice in other birds is to be 
found. At the lower, or distal, end of the cylindrical part, or 
shaft, of the humerus are two prominent articular surfaces, or 
condyles, with a median depression. In birds which do not fly 
—the Ostrich, and especially the Apteryx—it is greatly reduced 

Bonss or tue Riagur WrinG oF A Duck, seen from above. 
1. Humerus. 2. Radius. 3. Ulna. 4 indicates all the bones of the manus. 
5. Carpal ossicles—that opposite the end of the radius is the radiale, or 
scapho-lunar bone ; that opposite the end of the ulna is the ulnare or 
cuneiform bone. 6. Pollex, consisting of two phalanges. 7 & 8 indi- 
cate the large bones of the hand, made up of 7, the metacarpal of the 
index digit, and 8, the metacarpal of the third digit. 9. Basal pha- 
lanx of index digit. 10, Single phalanx of third digit. 
in size and the characters above stated are very feebly marked. 
In the Penguin the humerus is in the form of a flattened plate, 
or lamina, of bone. 
The radius and ulna are two elongated cylindrical bones placed 
side by side in the fore-arm. They meet together at either end, 
but diverge more or less from one another medianly. Each bone 
has an articular concavity or cup at its proximal end, into which 
one of the convex condyles of the humerus is received. The 
