OS COSLE THE ILIUM 105 



distal end of the large metacarpal bone. The flexor surface (Fades 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 tertiee) 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 pelvinse) consists of the ossa cox- 

 arum, which unite ventrally at the symphysis pelvis, and articulate with the sacrum 

 dor sally. 



OS COX^ 



The OS coxae 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 glutffia) 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 glutsea), which extends from the middle of the medial border 

 toward the tuber coxse. 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 isohii, and os pubis, but 

 the names given above are sanctioned by common usage. 



