THE CONNECTIVE TISSUES AND THE SKELETAL SYSTEM. 171 



except in the thumb where it appears at the proximal end. In each phalanx it 

 develops at the proximal end (Fig. 145). 



The skeletal elements of the lower extremities, including the pelvic girdle, are 

 of intracartilaginous origin. Each hip bone (os coxae, innominate bone) is pre- 

 formed in cartilage which, in a general way, resembles in shape the adult bone. 

 The ventral part of the pubic cartilage does not at first join the ischial; but by the 

 eighth week the junction is complete, leaving dorsal to it the obturator foramen. 

 In the earliest stages the long axis of the cartilage is nearly at right angles to the 

 ; vertebral column, and the ilium lies close to the fifth lumbar and first sacral 

 | vertebrae; later (eighth week) the long axis lies nearly parallel with the vertebral 

 l column and the whole cartilage has shifted so that the ilium is associated with 

 I the first three sacral vertebrae (Figs. 146 and 147). 



-Ischium 



Pubic bone ^ _ ,^^^^ 



Acetabulum 



Ilium 



Cartilage 



FIG* 148 Right os coxae (innominate bone) of new-born child. Bonnet. 

 Bone is indicated by darker areas/ cartilage by lighter areas. 



Ossification begins at three centers which correspond to the ilium, ischium 

 and pubis; the center for the ilium appears during the eighth week, the centers 

 j for the ischium and pubis several weeks later (Fig. 148). The process of ossifi- 

 : cation is slow, and is far from complete at the time of birth, for at that time the 

 entire crest of the ilium, the bottom of the acetabulum and all the region ventral 

 to the obturator foramen are cartilaginous. During the eighth or ninth year 

 the ventral parts of the pubis and ischium become partly ossified, but up to the 

 time of puberty the pubis, ischium and ilium remain separated by plates of car- 

 tilage which radiate from a common center at the bottom of the acetabulum. 

 Soon after this, the three bones unite to form the single os coxae, leaving only the 

 crest of the ilium, the pubic tubercle and the sciatic tuber (tuberosity of the 

 ; ischium) cartilaginous. In each of these regions an accessory ossification cen- 



