THE SKELETON 47 
The Monotremes are peculiar in possessing a T-shaped 
interclavicle like that of many reptiles, lying upon the sternum, 
and articulating superiorly with the clavicles. 
Brachium and Antebrachium.—The proximal segment of the 
anterior or pectoral limb proper contains a single bone, the humerus, 
and the second segment two bones, the radius and the ulna, placed 
side by side, and articulating with the humerus at their proximal, 
and with the carpus at their distal extremity (Fig. 15). In their 
primitive and unmodified condition these bones may be considered as 
placed one on each border of the limb, the radius being preaxial or 
anterior, and the ulna postaxial or posterior, when the distal or free 
end of the limb is directed outwards, or away from the trunk. This 
is their position in the earliest embryonic condition, and is best 
illustrated among adult mammals in the Cetacea, where the two 
bones are fixed side by side and parallel to each other. In the 
greater number of mammals the bones assume a very modified and 
adaptive position, usually crossing each other in the forearm, the 
radius in front of the ulna, so that the preaxial bone (radius), 
though external (in the ordinary position of the limb) at the upper 
end, is internal at the lower end; and the hand, being mainly fixed 
to the radius, also has its preaxial border internal. In the large 
majority of mammals the bones are fixed in this position, but in 
some few, as in Man, a free movement of crossing and uncrossing— 
or pronation and supination, as it is termed—is allowed between 
them, so that they can be placed in their primitive parallel condition, 
when the hand (which moves with the radius) 
is said to be supine, or they may be crossed, 
when the hand is said to be prone. 
The humerus frequently has a foramen 
piercing the inner border of the distal 
extremity, known as the entepicondylar 2 
foramen, which corresponds with a similar 
one found in the Anomodont reptiles. The 
hollow in the head of the ulna for the recep- ~~" i 
tion of the head of the humerus is known 
as the greater sigmoid cavity, and that for Ai 
the head of the radius as the lesser sigmoid aa Wei 
cavity (Fig. 16). The term olecranon is | iii) i i 
applied to that process of the ulna which Pena 3 
forms the prominence of the elbow. PFW Sed pes ck Wied 
In most mammals walking on four limbs, ane aie Pha Heap (eaus). 
in which the hand is permanently prone, the «, Anterior tubercle; ol, 
ulna is much reduced in size, and the radius pon serene fs een 
increased, especially at the upper end; so ve 
that the articular surface of the latter, instead of being confined to 
the external side of the trochlea of the humerus, extends all across 
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