PELVIC ARCH. 83 



OS NAVICULARE. 



(Fig. 19. D.) 



The navicular, third sesamoid, or shuttle bone, is an irregular 

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

 the OS coron^e, and behind the os pedis, with both of which it 

 articulates, the articulation of the three forming the so-called coffi^n 

 joint. The superior surface is smooth, with two concavities and 

 a central eminence, which complete the articular surface for 

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

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

 Avhich 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 exto'emities are pointed, 

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



PELVIC ARCH. 



The posterior extremity is united to the trunk by the direct 

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

 The three bones Avhich 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 the 

 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 False Pelvis, the True Pelvis being the posterior part which, 

 by union 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 p)Avic cavity therefore 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 



