PELVIC ARCH. 83 



OS NAVICULARE. 

 (Fig. 19. D.) 



The navicular, third sesamoid, or shuttle bone, is an irreg- 

 "ular bone, situated with its long axis transversely, behind and 

 below the os coronas, and behind the os pedis, with both of which 

 it articulates, the articulation of the three forming the so-called 

 4iofin joint. The superior surface is smooth, with two concavities 

 and a central eminence, which complete the articular surface for 

 the condyles of the os coronse ; the inferior surface is rather 

 rough, and also has two concavities and a central projection, 

 which are covered with fibro-cartilage, and together form a kind 

 of pulley over which plays the tendon of the flexor perforans 

 muscle. The anterior border is divided into two portions, a 

 superior, smooth and triangular, which articulates with the os 

 pedis, and an inferior, elongated, rough, and porous, and attached 

 to the same bone by the inferior navicular ligament. The 

 posterior border is triangular, rough, and porous, and gives 

 insertion to the superior ligament; the extremities are pointed, 

 and attached to the alse of the os pedis by lateral ligaments. 



Pelvic Auch. 



The posterior extremity is united to the trunk by the direcV 

 articulation of the pelvic arch with the femur and sacral vertebrae. 

 The three bones which form the arch become early united by 

 ossification, and the entire arch is called the os innominatum. 

 The two ossa innominata articulate with each other in tbe inferior 

 median line, and at a later period this union becomes also ossified; 

 the complete structure is called the pelvis, and the space which 

 it helps to enclose is the pelvic cavity. On examining the pelvis 

 in situ, we find that the anterior portion bounds superiorly a 

 space which has no bony floor ; this portion has been termed the 

 Palse Pelvis, the True Pelvis being the posterior part which, by 

 Tinion of its right and left segments, forms the floor of the cavity. 

 Each side of the floor is pierced by a very large opening, the 

 obturator foramen, or foramen ovale. 



The|3e7v/c cavity tlierefore is, in the skeleton, included between 

 the sacrum, part of the coccyx, and the two ossa innominata. 

 The pelvis is always much smaller proportionally in the lower 



