THE BONES OF THE CARPUS 53 



formed between the two rows of small bones is also of the ginglymoid 

 variety. But the inferior surfaces of the lower row of bones are almost 

 flat, and they form with the metacarpal bones an arthrodial joint which 

 permits only of a slight gliding movement. 



In general the bones of the carpus are very solid and resistant, 

 and this, together with the adequate protection afforded them by the 

 numerous and powerful tendons playing over them, or attached to them, 

 renders fracture of them extremely rare. 



The pisiform, however, differs very much from the remaining 

 bones. It is not weight-bearing, and, as already stated, it is the outer- 

 most bone of the upper row. Unlike the other carpal bones, it is 

 distinctly flattened, presenting two surfaces and four edges. The outer 

 surface is convex and roughened. Running obliquely downwards and 

 forwards across this surface is a groove through which the outer tendon 

 of the flexor metacarpi externus muscle passes to its insertion into the 

 head of the external small metacarpal bone. The inner surface is 

 concave and smooth, and it forms the outer boundary of the carpal sheath. 

 The superior, posterior, and inferior borders are rounded, roughened, 

 and blended with one another. The superior border gives attachment 

 to the tendon of insertion of the flexor metacarpi medius muscle, 

 and also to one of the tendons of the external flexor. The anterior 

 border of the bone presents two small facets, the upper of which is 

 concave and almost circular. This facet articulates with the before- 

 mentioned facet at the back of the inferior extremity of the radius. 

 The lower facet is elongated and convex, and it responds to a corre- 

 spondingly concave facet on the cuneiform bone. These two facets 

 form synovial joints with the facets with which they articulate. In the 

 living animal the tension of the skin at the knee draws the pisiform 

 bone round towards the back of the limb, so that its posterior border 

 forms the projection which is so prominent in the living animal slightly 

 to the outer side of the niedian line. 



Owing to its position, this bone forms a most adequate protection 



