BONES OF THE THORACIC LIMB 201 
carpal bones. The carpal articular surface formed by them is concave from side 
to side, convex from before backward. The distal ends (Capitula) have articular 
surfaces of the nature of a head, but bear a sagittal ridge on the volar aspect, ex- 
cept the first, which is grooved. Ossification is complete at five or six months of 
age. 
The five digits have three phalanges each, except the first, which has two. 
The third and fourth digits are the 
longest; the first is very short and does 
not come in contact with the ground 
in walking. The first phalanges of the 
chief digits have four-sided shafts, 
which are slightly curved dorsally. 
The proximal end of each has a con- 
eave surface for articulation with the 
metacarpal bone and is deeply notched 
behind. The distal end has a trochlea 
for articulation with the second pha- 
lanx, and depressions on each side for 
ligamentous attachment. The second 
phalanges are about two-thirds of the 
length of the first phalanges. The 
proximal articular surface consists of 
two cavities separated by a sagittal 
ridge. The distal extremity is wider 
and flatter than that of the first. The 
third phalanges correspond in general 
to the forin of the claws. The base has 
an articular surface adapted to the 
second phalanx and is encircled by a_ P.2 
collar of bone (Crista unguicularis). pg 
The volar surface bears a wing or tuber- if 
osity, and on each side of this is a fora- 
men. The ungual part is a curved rod 
with a blunt-pointed free end. It is 
rough and porous. Its base forms with 
the collar previously mentioned a deep 
groove, into which the proximal border 
of the claw is received. The two pha- Fig. 221.—SKeE.eToN oF Distau Part oF RicuT THorRacic 
langes of the first digit resemble in ar- Lime or Doc; Dgrsat View. 
rangement the first and third phalanges The digits are spread. I, Distal end of interosseous 
of the other digits. Ossification ie rears distal ond of ree r.+1., aig yt aad 
carpal, .u., Ulnar carpal; -a@., accessory carpal (very 
complete at five or six months. small part visible); C. 1, C. 2, C. 3, C. 4, first to fourth car- 
Nine volar sesamoids are usually pal bones; Mc. I, metacarpal bone of first digit; P. 1+ 2, 
~~ fused first and second phalanges of same; P. 3, third phalanx 
present. TIwoare found at each meta-  ¢¢ same; Mc. V, fifth metacarpal bone; P. 1, P. 2, P. 8, 
carpo-phalangeal joint of the chief phalanges of fifth digit; S, dorsal sesamoid; C, C, volar 
digits. They are high and narrow, “*™04* 
articulate with the distal end of the 
metacarpal bone in front, and have a small facet on the base for the first phalanx. 
On this joint of the first digit there is usually a single flattened sesamoid, but ex- 
ceptionally two are present. The distal volar sesamoids remain cartilaginous. A 
nodular dorsal sesamoid occurs in the capsule of the metacarpo-phalangeal joints, 
and cartilaginous nodules are found in a similar position in connection with the 
joints between the first and second phalanges. 
