THE SKELETON OF THE POSTERIOR LIMB. 165 
lateral side of the articulation at the base of the second metacarpal. 
As its name implies, it is originally an element interposed between 
the, proximal and distal rows. The hamate is a comparatively 
large element associated with the fourth and fifth metacarpals, but 
extending also to the articulation of the third, where it tends to 
replace the greatly reduced capitate. 
THE METACARPUS AND PHALANGES. 
The metacarpus (Fig. 68) comprises five stout elements, the 
metacarpal bones (ossa metacarpalia), which form the basal 
supports of the digits. Each consists, in addition to a main portion 
or body, of a flattened proximal end, or base, and a rounded distal 
extremity, or head. The four lateral bones are normally developed, 
while the first, which belongs to a reduced digit, is of very small size. 
The phalanges or bones of the digits are distributed according 
to the formula 2, 3, 3, 3,3. They are similar in form to the meta- 
carpals, with the exception, however, of the terminal, ungual 
phalanges, which are laterally compressed, pointed, and cleft at 
their tips for the attachment of the claws. 
SESAMOID BONES. 
Accessory elements, sesamoid bones (ossa_ sesamoidea) 
developed in connection with the ligaments of muscles, are found 
on the volar surface of the foot in association with certain of the 
joints. They occur in transverse pairs at the metacarpophalangeal 
articulations and in linear pairs at the articulations of the second 
with the third phalanges. The pisiform bone of the carpus is also 
a sesamoid, being formed in the insertion tendon of the flexor carpi 
ulnaris muscle. 
THE SKELETON OF THE POSTERIOR LIMB. 
In the posterior limb the proximal or girdle portion comprises 
the paired coxal bones, which are united ventrally at the pelvic 
symphysis, thus forming the pelvic girdle. The distal portion— 
comprising, as in the anterior limb, the supports of the free ex- 
