﻿134 PLEISTOCENE MAMMALIA. 



§ 1. j3. First Metacarpal (PL XXI, fig. 1). — The first metacarpal for the hallux or 

 thumb, in its shortness and the obliquity of its articulations, differs from all the rest of its 

 fellows. The proximal articulation with the trapezium occupies the end of the bone, and 

 extends diagonally outwards and downwards on the dorsal surface, forming a shallow, 

 pulley-shaped joint by which the thumb is freely moved in a transverse direction. 

 On the internal edge there is a very small facet (b), which is in apposition with 

 the phacoid bone in the recent Pelidae. Externally also there is a small facet, which rests 

 on the internal edge of the second metacarpal. The united action of these articulations 

 and of the ligaments which bind this bone to the carpus, prevent any rotatory action ; 

 and thus the thumb cannot be opposed, as in man, to the other digits. The shaft is 

 short, stout, and slightly bent towards the outside, and traversed on its dorsal aspect by 

 a groove running from the extero-inferior edge of the proximal articulation diagonally 

 outwards and forwards, and receiving the tendons proper to the borie for the common 

 extensor digitorum. The palmar surface is concave, and much roughened for the 

 attachment of a large number of ligaments. The distal articulation (d) is very oblique, 

 and faces inwards and downwards, so that the action of the phalange is still more 

 transverse than it otherwise would be, and its tearing and grasping power is much 

 increased. Immediately above it in the middle of the palmar surface is a small well- 

 marked tubercle which affords attachment to the ligament, uniting the bone to the first 

 phalange of the second digit, and to the adductor brevis pollicis muscle. The majority of 

 the tendons of the muscles that regulate the motion of this joint are attached, not to the 

 bone itself, but to a large internal sesamoid, occupying the intero-inferior angle (e) of 

 the bone. 



§ 1. y . Second, Third, Fourth, Fifth Metacarpals (Pis. XIX, fig. 6; XX, figs. 6, 7; 

 XXI, figs. 2, 3, 4, 5). The proximal end of the second metacarpal forms a wedge-like 

 mass slightly shifted, as it were, outwards for a quarter of its width. In front it is rec- 

 tangular ; behind, on the palmar surface, it throws out a small tubercle which is the thin 

 end of the wedge. The proximal or trapezoidal articulation (Pis. XIX, fig. 6 ; XXI, 

 fig. 2, a) is triangular in outline ; concave transversely, and nearly flat vertically. On the 

 upper and outer edge of the palmar tubercle is a small flat surface (PI. XXI, fig. 2, b), 

 which articulates with the postero-internal facet of the os magnum, and immediately above 

 is the somewhat triangular surface set at right angles to the trapezoidal articulation which 

 is iu contact with the antero-internal facet of the same bone. At the antero-internal angle of 

 the trapezoidal articulation is a small concave surface (fig. 2, c), which receives the convex 

 external facet of the trapezium, and, extending from it backwards as far as the palmar 

 tubercle, is a broad slightly concave surface for contact with the first metacarpal. Imme- 

 diately under the articulations for the os magnum are two concave surfaces, the anterior 

 being the larger and deeper (fig. 2, d), which overlap corresponding surfaces on the inner 

 side of the third metacarpal. Between them is a large ligamentary cavity. On the dorsal 



