OS COXA—THE ILIUM 105 
distal end of the large metacarpal bone. The flexor surface (Facies flexoria) is 
flattened and oblique; in the fresh state it is covered by a layer of cartilage which 
also fills the interval between the opposed borders of the two bones, and forms a 
smooth groove for the deep flexor tendon. The abaxial surface is concave, and 
gives attachment to part of the suspensory ligament; it is separated from the 
flexor surface by a rough everted border. The base is distal, and furnishes attach- 
ment to the distal sesamoidean ligaments. The apex is proximal and is rounded. 
The distal sesamoid or navicular bone (Os sesamoideum phalangis tertiz) is 
shuttle-shaped, and is situated behind the junction of the second and third pha- 
langes. Its long axis is transverse, and it possesses two surfaces, two borders, and 
two extremities. The articular surface (Facies articularis) faces upward and 
forward; it consists of a central eminence, flanked by concave areas, and articulates 
with the distal end of the second phalanx. The flexor or tendon surface (Facies 
flexoria) is directed downward and backward. It resembles the articular surface 
in form, but is more extensive and not so smooth. In the fresh state it is coated 
with cartilage and the deep flexor tendon plays over it. The proximal border 
(Margo proximalis) is wide and grooved in its middle, narrower and rounded on 
either side. The distal border (Margo distalis) bears in front a narrow facet for 
articulation with the third phalanx. Behind this is a groove, which contains a 
number of relatively large foramina, and is bounded behind by a prominent edge. 
The extremities are blunt-pointed. 
Development.— Each ossifies from a single center. 
THE BONES OF THE PELVIC LIMB 
The pelvic girdle (Cingulum extremitatis pelvine) consists of the os cox, 
which unites ventrally with the opposite bone at the symphysis pelvis, and artic- 
ulates with the sacrum dorsally. 
OS COX 
The os coxe or hip bone! is the largest of the flat bones. It consists primarily 
of three parts, the ilium, ischium, and pubis, which meet to form the acetabulum, 
a large cotyloid cavity for articulation with the head of the femur. These parts 
are fused at about one year of age, but it is convenient to describe them separately.? 
THE ILIUM 
The ilium (Os ilium) is the largest of the three parts. It is irregularly triangu- 
lar and presents two surfaces, three borders, and three angles. 
The wide part of the bone is the wing (Ala ossis ilium). Its gluteal surface 
(Facies glutzwa) faces dorso-laterally and backward. It is wide and concave in 
front, narrower and convex behind. The wide part is crossed by the curved 
gluteal line (Linea glutzea), which extends from the middle of the medial border 
toward the tuber cox. This surface gives attachment to the middle and deep 
gluteal muscles. 
The pelvic surface (Facies pelvina) faces in the opposite direction; it is convex, 
and consists of two distinct parts. The medial triangular part (Pars articularis) 
is roughened for ligamentous attachment, and bears an irregular facet, the auricu- 
lar surface (Facies auricularis), for articulation with the sacrum. The lateral quad- 
rilateral part (Pars iliaca) is in general smooth. It is crossed by the ilio-pectineal 
| line (Linea iliopectinea), which begins below the auricular surface and is continued 
1 Formerly called the os innominatum. 
* The proper terms, strictly speaking, for these bones are os ilium, os ischii, and os pubis, but 
_ the names given above are sanctioned by common usage. 
