g6 ATLAS AND TEXT-BOOK OF HUMAN ANATOMY. 



Fig. 134. — The right innominate bone seen from in front (?). 



Fig. 135. — The right innominate bone of a five- or six-year-old child seen from the inner surface (|). 

 In Figs. 134 and 135 the ilium is yellow, the ischium green, and the pubis blue. Parts that are still cartilaginous are white. 



Fig. 136. — The same seen from the outer surface ( T ). 



toward the obturator foramen. Between these two tubercles is stretched the upper margin of 

 the obturator membrane (see page 129). 



The inferior ramus of the pubis is flatter and possesses but two surfaces, an anterior and 

 a posterior or pelvic surface. It passes obliquely downward and outward from the symphysis 

 and is connected with the inferior ramus of the ischium at the site of a slight constriction. 



The ischium is shaped very much like the pubis, and in the adult its body is intimately 

 connected with those of the ilium and pubis. It forms the antero-inferior boundary of the 

 great sciatic notch, and in this situation presents a sharp triangular projection, the spine (Fig. 

 132). Below the spine is situated the lesser sciatic notch, which is not so deep as the greater 

 one and is formed entirely by the ischium, its inferior boundary being furnished by the large 

 rough tuberosity of the ischium (Fig. 132), which forms the main portion of the outer surface 

 of the superior ramus. From the tuberosity, the thin flat inferior ramus passes forward and 

 upward (Fig. 134), forming almost a right angle with the superior ramus of the ischium. To- 

 gether with the inferior ramus of the pubis, it forms the lower boundary of the obturator foramen. 



The acetabulum (Fig. 132) is formed by the bodies of the ilium, pubis, and ischium, but 

 dividing-lines between its component portions are visible in youthful individuals only. It is 

 a hemispherical cavity with elevated margins looking directly outward, and only the anterior 

 inferior portion of the margin toward the obturator foramen is incomplete; this gap is known 

 as the acetabular or cotyloid notch. 



The floor of the acetabulum is composed of two differently shaped portions. The larger 

 portion, the semilunar surface (Fig. 132), is smooth and covered with cartilage; it forms the 

 upper and lateral portions of the cavity and extends downward to the borders of the cotyloid 

 notch. The remaining quadrate area commences at the borders of the acetabular notch; it 

 is rough and uneven and is known as the acetabular fossa. 



The obturator foramen (Fig. 134) is a large opening, the shape of which is subject to 

 considerable individual variation; it may be either oval or triangular, its longest diameter being 

 transverse in some cases and vertical in others. Its borders are formed by the rami of the 

 ischium and of the pubis, and are for the most part sharp, being flat only where the obturator 

 groove runs into the foramen. 



The innominate bone is formed from three main centers of ossification, one for the ilium, one for the ischium, 

 and one for the pubis. The center for the ilium appears in the preformed cartilage at the beginning of the third fetal 

 month, that for the ischium in the beginning of the fourth month, and that for the pubis in the fifth month. At birth 

 and even during the first years of life (Figs. 135 and 136) a large portion of the innominate bone is still cartilaginous 

 (the margins of the acetabulum, the crest of the ilium, the tuberosity of the ischium, and the spine of the ischium). In 

 the acetabulum, a Y-shaped cartilage remains until the age of puberty, when the three portions of the bone become united 

 by osseous tissue, the two inferior rami (of the pubis and ischium) becoming united still earlier, in the seventh or eighth 

 year. In addition to the main centers, there appear a somewhat variable number of epiphyseal centers, of which those 

 worthy of special mention are: one along the entire crest of the ilium, one in the tuberosity of the ischium, one in the 



