272 THE SKELETON. 



the sixth year ; in the trapezoid, during the eighth year ; and in the pisiform, 

 about the twelfth year. 



The Metacarpal Bones are each developed by two centres : one for the shaft 

 and one for the digital extremity for the four inner metacarpal bones ; one for the 

 shaft and one for the base for the metacarpal bone of the thumb, which in this 

 respect resembles the phalanges. 1 Ossification commences in the centre of the 

 shaft about the eighth or ninth week, and gradually proceeds to either end of the 

 bone : about the third year the digital extremities of the four inner metacarpal 

 bones and the base of the first metacarpal begin to ossify, and they unite about 

 the twentieth year. 



The Phalanges are each developed by two centres : 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 of the 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. 



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 con- 

 nected to the trunk through the os innominatum, or hip-bone, which forms the 

 pelvic girdle. 



THE HIP. 

 The Os Innominatum. 



The Os Innominatum (in, not ; nomino, I name), or nameless bone, so called from 

 bearing no resemblance to any known object, 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 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 os pubis. 



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

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

 acetabulum and forms the prominence of the hip. 



The ischium (t<T%lou, the hip) is the inferior and strongest portion of the bone ; 

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

 then, curving upward, forms with the descending ramus of the os pubis, a large 

 aperture, the obturator foramen. 



The os pubis is that portion which runs horizontally inward from the inner side 

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

 for about one inch : it forms the front of the pelvis, supports the external organs of 

 generation, and has received its name from the skin over it 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. 207). The back part of this 

 surface is directed backward, downward, and outward ; its front part, forward, 

 downward, and outward. 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 

 gluteal lines. The superior gluteal line, the shortest of the three, commences at 



1 Allan Thomson has demonstrated the fact that the first metacarpal bone is often developed 

 from three centres ; that is to say, there is a separate nucleus for the distal end, forming a distinct epiph- 

 ysis, visible at the age of seven or eight years. He also states that there are traces of a proximal 

 epiphvsis in the second metacarpal bone. Journal of Anatomy, 1869. 



