THORACIC LIMBS 641 



The palmar surface is widened proximally and narrowed distally. 

 Its proximal angles and lateral borders are rounded and prominent ; 

 they serve for ligamentous attachment. The greater part of the sur- 

 face is occupied by a deep triangular median depression in which the 

 tendon of the flexor sublimis digitorum muscle is inserted. Distal to 

 this depression the surface is continuous with the palmar surface of 

 the shaft. 



The radial and ulnar surfaces slope from the edges of the palmar 

 surface dorsally to the dorsal border, and face dorsally and to the 

 radial and ulnar sides respectively ; they are convex, and afford attach- 

 ment to the ligaments of the joint. 



The dorsal border is straight and rounded. It begins on the 

 proximal dorsal angle, and is continuous with the dorsal border of the 

 shaft. To the angle is attached the tendon common to the different 

 extensor muscles. 



The Shaft differs from the shaft of the proximal phalanx in being 

 confessed from side to side. It is straight for most of its extent, but 

 turns to the ulnar side at its distal end. It has three surfaces, the 

 palmar, the radial, and the ulnar, separated by three borders, the 

 dorsal, the radial, and the ulnar. 



The radial and ulnar borders are placed on the palmar aspect of 

 the bone, and are nearly parallel. For their proximal half they are 

 emarginate ; for their distal half the ulnar remains emarginate, but the 

 radial border becomes arcuate. 



The dorsal border turns to the ulnar side at the distal end. 



The palmar surface, included between the radial and ulnar borders, 

 is flat from the proximal to the distal end and flat or gently convex 

 transversely. 



The radial and ulnar surfaces slope from the radial and ulnar 

 borders respectively, dorsally, as well as to the ulnar and radial sides 

 respectively. The radial surface is sinuate in the proximo-distal 

 direction and convex from the dorsal to the palmar side. The ulnar 

 surface is flattened or gently convex from the dorsal to the ulnar border 

 and deeply concave from the proximal to the distal end. This con- 

 cavity receives the distal phalanx when retracted. 



The Head is a half-cylinder placed obliquely to the long axis of 

 the bone, in such manner that the ulnar end projects distally and 



beyond the ulnar surface of the shaft, producing an appearance of 



41 



