THE OS INNOMINA TUM 207 



fication commences in the centre of the shaft about the eighth or ninth week, the centre for the 

 first metacarpal bone being the last to appear; ossification gradually proceeds to either end of 

 the bone: about the third year the distal extremities of the four inner metacarpal bones and the 

 base of the first metacarpal begin to ossify, and they unite with the shaft about the twentieth year. 



The phalanges are each developed from two > entres, one for the shaft and one for the base. 

 Ossification commences in the shaft, in all three rows, at about the eighth week, and gradually 

 involves the whole bone excepting the upper extremity. Ossification of the base commences in 

 the first row between the third and fourth years, and a year later in those of the second and third 

 rows. The two centres become united, in each row, between the eighteenth and twentieth years. 



In the ungual phalanges the centre for the shaft appears at the distal extremity of the phalanx, 

 instead of at the middle of the shaft, as is the case with the other phalanges. The ungual 

 phalanges are the first bones of the hands to begin to ossify. 



THE LOWER EXTREMITY. 



The lower extremity consists of the following bones: Ossa innominata (with 

 the sacrum and coccyx forming the pelvis), the femur (thigh}, the tibia and 

 fibula (the leg), the tarsus, the metatarsus and phalanges (the foot). 



THE OS INNOMINATUM, CALLED ALSO OS COXAE, HIP BONE (Figs. 167, 168). 



The os innominatum is so called from bearing no resemblance to any known 

 object. It is a large, irregularly shaped, flat bone, constricted in the centre and 

 expanded above and below. With its fellow of the opposite side it forms the 

 sides and anterior wall of the pelvic cavity. In young subjects it consists of three 

 separate parts, which meet and form the large, cup-like cavity, the acetabulum, 

 situated near the middle of the outer surface of the bone; and, although in the 

 adult these have become united, it is usual to describe the bone as divisible into 

 three portions the ilium, the ischium, and the pubis. 



The ilium is the superior, broad, and expanded portion which runs upward 

 from the acetabulum and forms the prominence of the hip. 



The ischium is the inferior and strongest portion of the bone; it proceeds down- 

 ward from the acetabulum, expands into a large tuberosity, and then, curving 

 forward, forms, with the descending ramus of the os pubis, a large aperture, the 

 obturator foramen. 



The os pubis is that portion which extends inward and downward from the 

 acetabulum to articulate in the middle line with the bone of the opposite side; it 

 forms the front of the pelvis and supports the external organs of generation. 



The Ilium (os ilium) presents for examination two surfaces, an external and 

 an internal, a crest, and two borders, an anterior and a posterior. 



The external surface (Fig. 167) is divided into two parts an upper or gluteal 

 and a lower or acetabular. The upper portion known as the dorsum ilii is 

 directed backward and outward behind, and downward and outward in front. 

 It is smooth, convex in front, deeply concave behind; bounded above by the crest, 

 below by the upper border of the acetabulum; in front and behind by the anterior 

 and posterior borders. This surface is crossed in an arched direction by three 

 semicircular lines the superior, middle, and inferior curved lines. The superior 

 curved line (linea glutaea posterior), the shortest of the three, commences at the 

 crest, about two inches in front of its posterior extremity; it is at first distinctly 

 marked, but as it passes downward and backward to the upper part of the great 

 sacrosciatic notch, where it terminates, it becomes less marked, and is often alto- 

 gether lost. Behind this line is a narrow semilunar surface, the upper part of 

 which is rough and affords origin to part of the Gluteus maximus; the lower part 

 is smooth and has no muscle fibres attached to it. The middle curved line (linea 



