92 



restriction of the motion to one plane. Two oblong sesamoids play upon the 

 inferior end of the double trochlea. 



The metacarpal of the middle digit presents a very singular form cmd is 

 of prodigious strength ; the sides of its base are so produced as almost to 

 equal the length of the bone, and give it a resemblance to the letter T. The 

 middle part of this extended base is occupied by two articular surfaces, both 

 adapted to the os magnum : the larger surface traverses the vertical extent of 

 the base, but is narrow from side to side ; it is separated from the smaller facet 

 by a deep cavity with a somewhat rough and perforated surface. The smaller 

 facet is continuous with that of the convex process from the radial side of the 

 base, which is received into the corresponding cavity of the second metacarpal : 

 the ulnar extension of the proximal end of the bone is terminated by the sharp 

 angle formed by the meeting of the articular surfaces for the unciforme and 

 fourth metacarpal. The dorsal surface of the third metacarpal is nearly flat, 

 sUghtly concave on the ulnar process : the under side is smooth, concave length- 

 wise, convex transversely : on the middle of the ulnar side there is a rough 

 prominence. The distal trochlear surface is similar to that of the second meta- 

 carpal, but is larger, and there is no lateral channel on the ulnar side of the 

 median ridge, except for the sesamoid bone ; the upper end of the trochlear 

 ridge is nearer the ulnar side of the joint, and consequently more oblique ; the 

 lower end, which projects into the interval of the two sesamoids, is rather sharper 

 and more produced. 



The fourth metacarpal bone is longer and less thick than the preceding ; its 

 base is slightly expanded, obliquely truncated, and of a triangular form, oc- 

 cupied by a continuous smooth articular surface, divided between the unciform 

 bone and the ulnar process of the middle metacarpal. To the latter it presents 

 a convexity and a concavity ; the surface for the carpal bone is slightly and 

 uniformly convex ; the ulnar side of the base presents a flat triangular surface 

 for articulation with the fifth metacarpal. The upper, radial and under surfaces 

 of the body of the bone are pretty smooth ; the ulnar side is rough, with many 

 vascular perforations and a median tuberosity. A similar tuberosity projects 

 from the upper part of the proximal end : the articular surface is here limited 

 to the middle convex rising of bone, which extends vertically in nearly a straight 

 hne ; it is therefore very narrow in proportion to its length, and only at its 



