OS INNOMIXATUM. 149 



OF THE LOWER EXTREMITY. 



The Lower Extremity consists of three segments, the thigh, leg, and foot, which 

 correspond to the arm, forearm, and hand in the upper extremity. It is connected 

 to the trunk through the os innominatum or haunch, which is homologous with 

 the shoulder. 



THE Os INNOMINATUM. 



The Os Innominatum or nameless bone, so called from bearing no resemblance 

 to any known object, is a large irregular-shaped bone, which, with its fellow of 

 the opposite side, 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, situated near the middle of the outer side 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 pubes. 



The ilium, so called from its supporting the flank (ilia), is the superior broad 

 and expanded portion which runs upwards from the upper and back part of the 

 acetabulum, and forms the prominence of the hip. 



The ischium (lazlov, the hip} is the inferior and strongest portion of the bone ; 

 it proceeds downwards from the acetabulum, expands into a large tuberosity, and 

 then, curving upwards, forms with the descending ramus of the pubes a large 

 aperture, the obturator foramen. 



The pubes is that portion which runs horizontally inwards from the inner side 

 of the acetabulum for about two inches, then makes a sudden bend, and descends 

 to the same extent ; it forms the front of the pelvis, supports the external organs 

 of generation, and has received its name from being covered with hair. 



The Ilium presents for examination two surfaces, an external and an internal, 

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



External Surface or Dorsum of the Ilium (fig. 97). The back part of this sur- 

 face is directed backwards, downwards, and outwards ; its front part forwards, 

 downwards and outwards. 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, the shortest of the three, com- 

 mences at the crest, about two inches in front of its posterior extremity ; it is at 

 first distinctly marked, but as it passes downwards and outwards to the upper 

 part of the great sacro-sciatic notch, where it terminates, it becomes less marked, 

 and is often altogether lost. The rough surface included between this line and 

 the crest, affords attachment to part of the Gluteus maximus above, a few fibres 

 of the Pyriformis below. The middle curved line, the longest of the three, com- 

 mences at the crest, about an inch behind its anterior extremity, and, taking a 

 curved direction downwards and backwards, terminates at the upper part of the 

 great sacro-sciatic notch. The space between the middle and superior curved 

 lines, and the crest, is concave, and affords attachment to the Gluteus medius 

 muscle. Near the central part of this line may often be observed the orifice of a 

 nutritious foramen. The inferior curved line, the least distinct of the three, com- 

 mences in front at the upper part of the anterior inferior spinous process, and 

 taking a curved direction backwards and downwards, terminates at the anterior 

 part of the great sacro-sciatic notch. The surface of bone included between the 

 middle and inferior curved lines is concave from above downwards, convex from 

 before backwards, and affords attachment to the Gluteus minimus muscle. Beneath 

 the inferior curved line, and corresponding to the upper part of the acetabulum, 

 is a smooth eminence, sometimes a depression, to which is attached the reflected 

 tendon of the Rectus femoris muscle. 



The Internal Surface (fig. 98) of the ilium is bounded above by the crest, 

 below by a prominent line, the linea ilio-pectinea, and before and behind by the 



