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 Antebrachmm. 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 5 

 foramen, which corresponds with a similar 

 one found in the Anomodont reptiles. The 

 hollow in the head of the ulna for the recep- 

 tion of the head of the humerus is known 

 as the greater sigmoid cavity, and that for 

 the head of the radius as the lesser sigmoid 

 cavity (Fig. 16). The term olecranon is 

 applied to that process of the ulna which 

 forms the prominence of the elbow. 



In most mammals walking on four limbs, 

 in which the hand is permanently prone, the 

 ulna is much reduced in size, and the radius 

 increased, especially at the upper end ; so 

 that the articular surface of the latter, instead of being confined to 

 the external side of the trochlea of the humerus, extends all across 



FIG. 16. Outer aspect of 

 the proximal extremity of the 

 right ulna of a Bear (Ursus). 

 a, Anterior tubercle ; ol, 

 olecranon ; 6, greater sigmoid 

 cavity. ; c, lesser do. 



