604 



MAMMALIAN ANATOMY 



FIG. 464. 



Proximal Aspect. 



Ulnar Aspect. 



With Ulna. 



With Radius. 



THE PISIFORM. 



the long diameter is directed obliquely from the radial to the ulnar 

 and proximal side. It is therefore compressed from side to side, so 

 that its proximo-distal diameter is twice as great as its radio-ulnar 

 diameter. Its dorsal surface is occupied by an articular facet which 



is slightly crescentic in out- 

 line, with the arcuate border 

 on the radial side. It is 

 gently convex from side to 

 side, and applied to the facet 

 on the palmar surface of the 

 cuneiform. 



The proximal prominent 



end of the base bears a small convex surface which meets the side of 

 the radius. On the ulnar side of the base, and extending for a dis- 

 tance on the ulnar border of the shaft, is a large triangular concave 

 articular surface for the styloid process of the ulna. This faces upward 

 as well as to the ulnar side. 



The shaft and head are compressed from above downward, and the 

 radio-ulnar diameter is greater than the proximo-distal. The ulnar 

 border of the shaft is encroached upon below by the facet for the 

 ulnar. The proximal, radial, and distal surfaces are convex from side 

 to side and slightly concave from the dorsal to the palmar end. 



The head is the terminal palmar enlargement of the shaft. The 

 enlargement is extended in the form of a hook, upon which is inserted 

 the tendon of the flexor carpi ulnaris muscle. It is divided from the 

 shaft by a more or less marked curved line, so that when seen from 

 behind its outline is oval, and when seen from the sides its shape is 

 crescentic, the concavity facing dorsally. 



Nomenclature. Pisiform is compounded of pisum, a pea, and 

 forma, likeness, from the shape of the human bone. Os pisiforme, 

 le pisiforme, and das Erbsenbein have the same meaning. It is some- 

 times known as os carpi accessorium QT jiexorium. 



Determination. When the proximal side, known by the promi- 

 nent end of the base, is upward, and the dorsal side, with the oval 

 or crescentic facet on the base, is toward the student, the hook on the 

 head points to the side to which the bone belongs. 



Articulation.- -The pisiform articulates with the radius above, the 

 ulna on the inside, and the cuneiform in front. 



